Definition and Clinical Manifestation

Benign nodular thyroid disease is a heterogeneous thyroid disorder that is highly prevalent in iodine-deficient areas. On a very general basis, it can be divided into solitary nodular and multinodular thyroid disease. Histologically, benign thyroid nodules are distinguished (1) as encapsulated lesions (true adenomas) or adenomatous nodules that lack a capsule, and (2) by morphologic criteria according to the World Health Organization (WHO) classification.¹ On functional grounds, nodules are classified as cold, normal, or hot, depending on whether they show decreased, normal, or increased uptake on scintiscan. Approximately 50% to 85% of all nodules are cold, up to 40% are scintigraphically indifferent, and about 10% are hot,^2,3 although the prevalence will vary geographically with the iodine supply and with the clinical setting.

Classification of thyroid nodules on the basis of results from clonality studies implies that most thyroid nodules are “true” thyroid tumors compared with polyclonal hyperplastic nodules. Traditionally, only thyroid adenomas are considered true tumors, on the basis of an exclusive histologic definition—the presence of a capsule and a growth pattern that are different from the surrounding normal parenchyma in an otherwise normal thyroid gland. Strict histologic criteria for an adenoma and its differentiation from hyperplastic thyroid nodules or adenomatous nodules (without a capsule) are, however, difficult to obtain in the frequent presence of goiter or thyroiditis. The biological basis for separating hyperplastic thyroid nodules from true tumors, therefore, should also depend on their clonality.⁴ Because many thyroid nodules without a capsule (adenomatous nodules) are monoclonal, a mixed functional and molecular definition of true thyroid tumors, as outlined in Fig. 16-1, appears objective and consistent. In contrast to solitary nodular thyroid disease, which has a more uniform clinical, pathologic, and molecular picture, multinodular nontoxic goiter (MNG) and multinodular toxic goiter (MNTG) make up a mixed group of nodular entities. Thus a combination of hyper-, hypo-, or normally functioning thyroid lesions usually is found within the same thyroid gland. The overall balance of functional properties of individual thyroid nodules within a multinodular goiter ultimately determines the functional status in the individual patient, which may be seen as euthyroidism (normal thyroid-stimulating hormone [TSH] and free thyroid hormone levels), subclinical hyperthyroidism (low or suppressed TSH and normal free thyroid hormone levels), or overt hyperthyroidism (suppressed TSH and elevated free thyroid hormone levels). The term MNG is applied in the first scenario, and MNTG refers to the latter situations. It is important to emphasize that this functional characterization is not stationary, but patients with MNTG usually have a history of long-standing MNG.⁵ In general, development of MNG proceeds in two phases: global activation of thyroid epithelial cell proliferation (e.g., as the result of iodine deficiency or other goitrogenic stimuli) leading to goiter, and a focal increase in thyroid epithelial cell proliferation causing thyroid nodules. So far, the most common stimulus for local proliferation is a somatic mutation, as detailed in the following section.

Environment Versus Heredity

Until recently, nontoxic goiter was regarded mainly as a consequence of iodine deficiency. However, a number of goitrogens6–8 and cigarette smoking are important environmental risk factors in the origin of nontoxic goiter. The impact of smoking, most likely mediated by thiocyanate, which competitively inhibits the iodide transport into the thyroid, has been extensively studied.^6,9 Additional etiologically important factors consist of gender,^10,11 age,¹² and increased body mass index (BMI).¹³ The effect of a certain goitrogen is influenced by the degree of iodine sufficiency and therefore varies regionally and interindividually. However, it is most likely that interactions between environmental factors and individual genetic determinants ultimately determine the onset of the goiter.¹⁰ Nontoxic goiter appearing early in life, often clustering in families, suggests strong genetic susceptibility, whereas environmental determinants are more likely to have additive or triggering effects. However, in an individual, it may be impossible to evaluate the relative contribution of genetic predisposition and a multitude of potential environmental factors.

In contrast to sporadic goiters, caused by spontaneous recessive genomic variations, most cases of familial goiter present an autosomal dominant pattern of inheritance, indicating predominant genetic defects.14–16 Gene-gene interactions or various polygenic mechanisms (i.e., synergistic effects of several variants or polymorphisms) could increase the complexity of the pathogenesis of nontoxic goiter and offer an explanation for its genetic heterogeneity. A strong genetic predisposition is indicated by family and twin studies. Thus, children of parents with goiter have a significantly higher risk for developing goiter compared with children of nongoitrous parents.¹⁷ The high incidence in females and the higher concordance in monozygotic than in dizygotic twins also suggest a genetic predisposition.¹⁰ Moreover, preliminary evidence reveals a positive family history for thyroid disease in those who have postoperative relapse of goiter, which can occur from months to years after surgery.^18,19

The development of nontoxic goiter is most likely a continuous process that starts with thyroid hyperplasia. Therefore, defects in genes that play an important role in thyroid physiology and thyroid hormone synthesis could predispose to the development of goiter, especially in cases of borderline or overt iodine deficiency. Such defects could lead to dyshormonogenesis as an immediate response, thereby indirectly explaining the nodular transformation of the thyroid as a late consequence of dyshormonogenesis, as a form of maladaptation.¹² Genes that encode the proteins involved in thyroid hormone synthesis, such as the thyroglobulin gene (TG gene), the thyroid peroxidase gene (TPO gene), the sodium-iodide symporter gene (SLC5A5), the Pendred syndrome gene (SLC26A4), the TSH receptor gene (TSH-R gene), the iodotyrosine deiodinase gene (DEHAL1), and the thyroid oxidase 2 gene (THOX2), are convincing candidate genes in familial euthyroid goiter. Originally, several mutations in these genes were identified in patients with congenital hypothyroidism. However, in cases of less severe functional impairment that can still be compensated, the contribution of variants of these genes in the development of nontoxic goiter is possible. Moreover, in case of mutations in the TG gene,^20,21 SLC26A4,^22,23 and SLC5A5,^24,25 patients with nontoxic goiter have also been identified.

Linkage Studies

To identify novel susceptibility loci, as well as to account for the coinheritance of different genomic regions, and to further improve the understanding of the genetic mechanisms that contribute to the development of nontoxic familial goiter, linkage analyses have been performed. A genome-wide linkage analysis has identified a candidate locus, MNG1 on chromosome 14q31, in a large Canadian family with 18 affected individuals.¹⁵ This locus was confirmed in a German family with recurrent euthyroid goiters.²⁶ A dominant pattern of inheritance with high penetrance was assumed in both investigations. Moreover, a region on 14q31 between MNG1 and the TSH-R gene was identified as a potential positional candidate region²⁶ for nontoxic goiter. However, in the earlier study by Bignell et al.,¹⁵ the TSH-R gene was clearly excluded. Furthermore, an X-linked autosomal dominant pattern and linkage to a second locus MNG2 (Xp22) was identified in an Italian pedigree with nontoxic familial goiter.²⁷ To identify additional candidate regions, the first extended genome-wide linkage analysis was performed to detect susceptibility loci in 18 Danish, German, and Slovakian euthyroid goiter families.¹⁴ Assuming genetic heterogeneity and a dominant pattern of inheritance, four novel candidate loci on chromosomes 2q, 3p, 7q, and 8p were identified. An individual contribution was attributable to four families for the 3p locus and to one family for each of the other loci, respectively. On the basis of previously identified candidate regions and established environmental factors, nontoxic goiter can be defined as a complex disease (Fig. 16-2). However, for the first time, a more prevalent putative locus, present in 20% of the families investigated, was identified.¹⁴

The candidate region on 3p¹⁴ suggests a dominant pattern of inheritance for goiter. However, whereas linkage studies are suitable for the detection of candidate genes with a strong effect, it is possible to miss weak genetic defects of first-line candidate gene variants or of novel genes by linkage studies. Moreover, it is conceivable that the sum of several weak genetic variations in different genomic regions could lead to goiter predisposition. Therefore, the widely accepted risk factors such as iodine deficiency, smoking, old age, and female gender are likely to interact with and/or trigger the genetic susceptibility. In the future, loci identified by linkage analysis and/or association studies could reveal important genetic risk factors for familial nontoxic goiter. Further narrowing down the candidate regions and performing association studies with SNP markers in additional families and especially in case control association studies is necessary to identify the specific candidate genes for hereditary nontoxic goiter.

Mutagenesis as The Cause of Nodular Transformation Leading to Multinodular Goiter

Most goiters become nodular with time. From animal models of hyperplasia caused by iodine depletion,²⁸ we have learned that besides an increase in functional activity, a tremendous increase in thyroid cell number occurs. These two events very likely orchestrate a burst of mutation events. It is known that thyroid hormone synthesis goes along with increased H₂O₂ production and free radical formation,²⁹ which may damage genomic DNA and cause mutations. Together with a higher spontaneous mutation rate, a higher replication rate often will prevent mutation repair and increase the mutation load of the thyroid, thereby also randomly affecting genes crucial for thyrocyte physiology. Mutations that confer a growth advantage (e.g., TSH-R or G_sα protein mutations) very likely initiate focal growth. Hence, autonomously functioning thyroid nodules (AFTNs) are likely to develop from small cell clones that contain advantageous mutations, as shown for the TSH-R in “hot” microscopic regions of euthyroid goiters.³⁰

Epidemiologic studies, animal models, and molecular/genetic data outline a general theory of nodular transformation. Based on the identification of somatic mutations and the predominant clonal origin of AFTNs and cold thyroid nodules (CTNs), the following sequence of events could lead to thyroid nodular transformation that occurs in three steps, as outlined in Fig. 16-3.¹⁷ First, iodine deficiency, nutritional goitrogens, or autoimmunity may cause diffuse thyroid hyperplasia. Then, at this stage of thyroid hyperplasia, increased proliferation together with possible DNA damage due to H₂O₂ action causes a higher mutation load (i.e., a higher number of cells bearing mutations). Some of these spontaneous mutations confer constitutive activation of the cyclic adenosine monophosphate (cAMP) cascade (e.g., TSH-R and G_sα mutations), which stimulates growth and function. Finally, in a proliferating thyroid, growth factor expression (e.g., insulin-like growth factor 1 [IGF-1], transforming growth factor β [TGF-β], or epidermal growth factor [EGF]) is increased. As a result of growth factor co-stimulation, most cells divide and form small clones. After increased growth factor expression ceases, small clones with activating mutations further proliferate if they can achieve self-stimulation. They thus can form small foci, which may develop into thyroid nodules. This mechanism may explain AFTNs by advantageous mutations that initiate growth and function of the affected thyroid cells, as well as CTNs by mutations that stimulate proliferation only (e.g., ras mutations, other mutations in the RAS/RAF/MEK/ERK/MAP cascade). Moreover, nodular transformation of thyroid tissue due to TSH-secreting pituitary adenomas³¹ and nodular transformation of thyroid tissue in Graves’ disease³² and in goiters of patients with acromegaly³³ could follow a similar mechanism, because thyroid pathology in these patients is characterized by early thyroid hyperplasia.

As an alternative to the increase in cell mass, and as illustrated by those individuals who do not develop a goiter when exposed to iodine deficiency, the thyroid might also adapt to iodine deficiency without extended hyperplasia.³⁴ Although the mechanism that allows this adaptation is poorly understood, data from a mouse model suggest an increase of mRNA expression of TSH-R, sodium iodine symporter (NIS), and TPO in response to iodine deficiency, which might be a sign of increased iodine turnover in the thyroid cell in iodine deficiency.³⁵ Moreover, expansion of the thyroid microvasculature, caused by upregulation of vascular endothelial growth factor and other proangiogenic factors, is an additional mechanism that might help the thyroid to adapt to iodine deficiency.³⁶

Oxidative Stress as The Downside of Thyroid Hormone Synthesis

Because of the slow proliferation rate of thyroid epithelial cells, a long period (tens of years) between initiation of the tumor and appearance of MNG would seem likely. Therefore, the prevalence of thyroid tumors is a paradox that can be explained only by a high frequency of tumor initiation and/or enhanced thyroid epithelial cell proliferation. The origin of tumor formation in the thyroid therefore could be a natural or induced high mutation rate and aberrant growth stimulation in the thyroid gland. The latter very likely involves endogenous growth factors and/or exogenous goitrogenic substances.

Proliferation is very important for the manifestation of mutagenesis, and DNA replication during cell division leads to mispairing of damaged nucleotides, causing fixation of spontaneous mutations into the genome and a certain mutation load of dividing cells. Hence, compared with highly proliferating and therefore tumor-prone tissues—such as the colon, endometrium, skin, prostate, or breast—proliferation of the thyroid is rather low. If mutations occurred in the thyroid at a similar rate as in other tissues, the tumor incidence ought to be much lower than it actually is.

The main function of the thyroid gland is to synthesize the thyroid hormones L-3,5,3′,5′-tetraiodothyronine (T₄) and L-3,5,3′-triiodothyronine (T₃). To do so, the thyroid gland takes up iodine from food and incorporates it into thyroglobulin (Tg)—the precursor of the thyroid hormones. Iodination of tyrosyl residues on Tg requires high concentrations of H₂O₂ and oxidized iodine, which are generated by the enzymes thyroid oxidase 1 (THOX1), thyroid oxidase 2 (THOX2), and TPO.

Besides being a substrate in the hormone synthesis, H₂O₂ could be a major source of free radicals and reactive oxygen species (ROS). Because these molecules can cause substantial damage to a cell and impair normal function, thyroid epithelial cells are likely to have a potent defense mechanism to counterbalance potential damage mediated by free radicals. It has been shown that antioxidant enzymes, such as glutathione peroxidases (GPXs) or TPO, are upregulated during thyroid hormone synthesis.³⁷ GPX3 has been suggested to directly interfere with thyroid hormone synthesis by affecting the concentration of H₂O₂.³⁸ If antioxidant defense is not effective enough, excessive damage (e.g., peroxidation) should be detectable in lipids, DNA, and proteins of thyroid epithelial cells.

In thyrocytes, H₂O₂-mediated cytotoxicity appears to be dose dependent, requiring only low concentrations to result in thyroid cell apoptosis rather than necrosis, which could function as a barrier for tumorigenesis.³⁹ Furthermore, findings in the thyroid glands of male Wistar rats suggest that the predominant cytotoxic response to oxidative stress might differ depending on the functional state of the gland.³⁹ Moreover, ample evidence for excessive oxidative DNA damage has been found in the thyroid gland.⁴⁰ Does this affect the spontaneous mutation rate (SMR) in the thyroid at large? It is most interesting to note that a strikingly high SMR was found in the thyroid gland of mice⁴¹: with an 8- to 10-fold higher number compared with liver, the thyroid stands out from many other tissues. Indeed, the SMR in mouse thyroid glands without any experimental mutagenic challenge shows values that usually are found only in other organs (e.g., liver) of animals treated with mutagens like ethyl nitrosourea or benzo[a]pyrene.⁴² The above data point to a connection between thyroid hormone metabolism, oxidative DNA damage, and SMR in the normal thyroid gland that may represent the basis for frequent tumorigenesis.

On top of this generally high mutation rate in the normal thyroid gland, environmental and lifestyle factors may add to the pool of DNA damage and increase mutagenesis and tumor initiation. Tobacco smoke, as previously mentioned,⁹ because it is one of the prime suspects in thiocyanate-induced blocking of iodine transport into the thyrocyte, could lead to intracellular iodine deficiency.

As outlined in the aforementioned, H₂O₂-mediated ROS generation is very likely to be an important starting point for thyroid tumor development (Fig. 16-4). Because iodine and H₂O₂ act as co-substrates in thyroid hormone synthesis, changes in iodine concentration are very likely to affect the H₂O₂ concentration. In fact, generation of H₂O₂ is inhibited by iodide in vivo and in vitro.⁴³ H₂O₂ generation—which is mandatory for the organification of iodine—is, moreover, stimulated by TSH (in contrast to many other aspects of thyroid hormone synthesis, it is unclarified whether cAMP is the second messenger in H₂O₂ generation), which increases the expression of genes important for thyroid hormone synthesis (e.g., NIS, TPO).²⁹ Low iodine levels and markers of increased thyroid functionality suggest activation of H₂O₂ generation, which could result in DNA damage and somatic mutations.⁴⁴ Consequently, low iodine and high H₂O₂ levels should activate antioxidative defenses, which should be detectable in the cellular regulation of enzymes involved in the defense against oxidative stress.

Indeed, a higher expression of mRNA for superoxide dismutase 3 (SOD3)—the extracellular SOD isoform that preferentially acts in the lumen, where H₂O₂ is generated—is detected during experimental iodine deficiency in mice.³⁵ Moreover, oxidative stress and antioxidant defenses are enhanced in hyperplastic and involuting glands.⁴⁵

Differential expression of additional antioxidant enzymes⁴⁰ underlines the importance of the antioxidant defense in the iodine-deficient thyroid gland. Moreover, glutathione peroxidases are selenium proteins. It therefore is very likely that selenium deficiency could impair the antioxidant defense and exacerbate oxidative stress (see Fig. 16-4). An increased oxidative burden in the thyroid gland through iodine deficiency is also suggested by results of the comet assay with repair-enzyme protocols to detect oxidative DNA damage.³⁵ As a consequence, early molecular conditions for nodular and tumor transformation in the thyroid gland consist of a sequence of molecular events that include oxidative stress and DNA damage as the triggers for somatic mutations. The oxidative burden is already detectable in the normal thyroid gland and is very likely to be linked to hormone synthesis and H₂O₂ production. Additionally, environmental conditions (e.g., iodine deficiency) have the potential to aggravate this situation. In general, any external factor (e.g., smoking) that increases oxidative stress, causes DNA damage, or increases the SMR might aggravate the risk for tumor genesis. Also, any factor that increases proliferation (e.g., goitrogens) in all likelihood shortens the time to development of a detectable thyroid tumor.

Hot Thyroid Nodules

Somatic point mutations that constitutively activate the TSH-R were first identified by Parma and coworkers in hyperfunctioning thyroid adenomas.⁴⁶ However, in different studies, the prevalence of TSH-R and G_sα mutations in autonomously functioning thyroid nodules has been reported to vary from 8% to 82% and from 8% to 75%, respectively.^46–49 Available studies differ in the extent of mutation detection and in screening methods. A comparison with respect to the obvious differences between studies has been done elsewhere.^50,51 A comprehensive study using the more sensitive denaturing gradient gel electrophoresis^52,53 revealed a frequency of 57% TSH-R mutations and 3% G_sα mutations in 75 consecutive, autonomously functioning thyroid nodules.⁵⁴ These results raise the question of the molecular origin of TSH-R and G_sα mutation-negative nodules. A possible answer is given by clonal analysis of these AFTNs, which demonstrates a predominant clonal origin of thyroid nodules and implies a neoplastic process driven by genetic alteration.

In addition to the intracellular signaling network that is connected to the TSH-R, the extracellular action of different growth factors enhances the complexity of the signal flux into the thyroid cell. Growth factors like IGF-1, EGF, TGF-β, and fibroblast growth factor (FGF) stimulate growth and dedifferentiation of thyroid epithelial cells.⁵⁵ Studies focused on insulin and IGF show a permissive effect of insulin and IGF-1 on TSH signaling^56,57 as well as a cooperative interaction of TSH and insulin/IGF-1.⁵⁸ Other studies suggest inactivation of TGF-β signaling in AFTNs due to constitutively activated TSH-R (e.g., resulting from TSH-R mutations).⁵⁹ This assumption is supported by the finding of decreased expression of TGF-β₁ mRNA after TSH stimulation of thyrocytes.⁶⁰ Because TGF-β₁ has been shown to inhibit iodine uptake, iodine organification, and thyroglobulin expression,⁶¹ as well as cell proliferation in different cell culture systems,^62,63 these findings suggest that inactivation of TGF-β signaling is a major prerequisite for increased proliferation in AFTNs.⁶⁴ Signal modulation of the TSH-R that would define the cause of AFTNs and the clinical phenotype therefore could take part at a number of stages and very likely involve genetic/epigenetic, gender-related, and environmental factors.

Cold Thyroid Nodules

The term “cold” indicates reduced uptake on scintiscan. Because a histologic diagnosis typically is employed to exclude thyroid malignancy, many investigations of thyroid nodules refer only to the histologic diagnosis of thyroid adenoma. This histologic entity should not be confounded with the scintigraphically characterized entity “cold nodule,” which, like AFTNs or “warm nodules,” can appear histologically as thyroid adenomas or adenomatous nodules according to the WHO classification.¹ In contrast, focal hyperplasia is not very well explained on the molecular level and has been discussed in detail elsewhere as the cause of thyroid tumors.^65,66 A monoclonal origin has been detected for most cold thyroid nodules, which implies nodular development from a single mutated thyroid cell.⁶⁷

With reference to their functional status (i.e., reduced iodine uptake), failure in the iodide transport system and failure of the organic binding of iodide were detected as functional aberrations of cold thyroid nodules long before the molecular components of iodine metabolism were known. Subsequently, decreased expression of the Na⁺/I⁻ symporter (NIS) in thyroid carcinomas and benign cold thyroid nodules were suggested as the molecular mechanisms underlying the failure of iodide transport (reviewed in references 68 and 69). However, a defective cell membrane targeting the NIS protein is a more likely molecular mechanism accounting for the failure of iodine uptake in CTNs.^68,70 The ultimate cause of this defect is currently unknown.

Compared with iodine transport, the organic binding of iodine is a multistep process with a number of protein components that still awaits final characterization.⁷¹ mRNA expression of enzymatic components (e.g., TPO, flavoproteins) and the substrate of iodination (i.e., Tg) have been quantified in CTNs without significant differences compared with normal follicular tissue.^72,73 TPO, Tg, and thyroid-specific oxidases (THOX) have been successfully screened for molecular defects, especially in congenital hypothyroidism.⁷⁴

Although CTNs could be regarded as a form of focal hypothyroidism, somatic mutations in enzymes that catalyze organic binding of iodine would need to exert a growth advantage on the affected cell to cause the development of a thyroid nodule. At least in the case of inactivating mutations in the TPO or THOX genes, growth advantage could result from a lack of enzyme activity, which would reduce not only thyroid hormone synthesis but also follicular iodide trapping in organic iodo compounds. Because these compounds have been shown to inhibit thyroid epithelial cell proliferation,⁷⁵ reduced synthesis could have a proliferative effect. Therefore, somatic TPO or THOX mutations could be a molecular cause of CTN. However, mutations in the TPO gene have not been detected.⁷⁶ A study of 40 cold thyroid adenomas and adenomatous nodules detected ras mutations in only a single case.⁶⁷ Moreover, in the same set of CTNs, no point mutations in the mutational hot spots of the BRAF gene were detected.⁷⁷ This is in line with the lack of BRAF mutations in benign follicular adenomas in other studies.^78,79 So far, only one study has detected a single BRAF mutation in a set of 51 follicular adenomas.⁸⁰ Moreover, the gene expression for approximately 10,000 full-length genes was compared between CTNs and their corresponding normal surrounding tissue.⁸¹ Increased expression of histone mRNAs and of cell cycle–associated genes like cyclin D1, cyclin H/cyclin-dependent kinase (CDK) 7, and cyclin B most likely reflects a molecular setup for increased proliferation in CTNs.⁸² In accordance with the low prevalence of ras mutations in CTNs,⁶⁷ reduced expression of ras-MAPK (mitogen-activated protein kinase) cascade–associated genes was found, which might suggest minor importance of this signaling cascade. Furthermore, gene rearrangements unique to thyroid adenomas have recently been the focus (reviewed in reference 83). These studies led to the identification of the thyroid adenoma–associated gene (THADA) that encodes a death receptor–interacting protein.⁸⁴ Although also reported for thyroid follicular carcinoma,⁸⁵ the finding of loss of heterozygosity (LOH) at the TPO locus is characteristic for some CTNs (about 15%) but rather points to defects in a gene near TPO on the short arm of chromosome 2. Although the frequency of each of these DNA aberrations is rather low, together these chromosomal changes need to be considered in further elucidation of the molecular origin of CTNs.

Occurrence

Epidemiologic studies of multinodular goiter are hampered by problems such as selection criteria (age and gender), influence of environmental factors (e.g., iodine intake, smoking habits), evaluation of size and morphology (palpation, ultrasound, or scintigraphy), and determination of thyroid function, and whether subjects with subclinical hyperthyroidism are categorized as euthyroid or hyperthyroid. Only thyroid nodules of at least 10 mm can be identified reliably by palpation.⁸⁶ With the use of ultrasound, nodules as small as 2 mm are readily detected. It therefore is not surprising that the prevalence of nodules is increased several-fold if sonographic examination is applied, because 70% of thyroid nodules disclosed by sonography are smaller than 10 mm in diameter.^86,87

Most studies have focused on middle-aged women and the elderly, whereas only a few have documented the prevalence of multinodular thyroid disease in a cross-sectional investigation of the adult population in a community. Longitudinal studies covering many years are necessary to give valid figures on incidence, etiologic risk factors, and the natural history. Such studies that take the above-mentioned problems into consideration are not available. These limitations therefore should be borne in mind when the available data are considered.⁸⁷ Iodine deficiency is still the most frequent single cause of multinodular endemic goiter worldwide. Considerable regional variation exists even in nonendemic goiter areas. In the Whickham survey, 16% of the cohort had simple goiter.⁸⁸ In men, the prevalence declined with age from 7% in those younger than 25 years to 4% in those older than 65 years. Among women, the frequency declined from 31% in those younger than 45 years to 12% in those older than 75 years. This finding fits the observation that lean body mass, known to decline with age, is the major determinant of thyroid size.⁸⁷ Illustrating the influence of iodine intake on the epidemiology of sporadic goiter, 31 of 423 (7.3%) 68-year-olds had goiter in Jutland, Denmark (low iodine intake area), versus 2 of 100 (2%) in Reykjavik, Iceland (high iodine intake area).⁸⁹

A cross-sectional study of the community in Whickham found a prevalence of hyperthyroidism of 25 per 1000 women and 2 per 1000 men in an adult population.⁸⁸ Others have reported similar figures.⁸⁸ The yearly incidence of hyperthyroidism (all types) varies between 0.1 and 0.2 per 1000 men and between 0.3 and 1.3 per 1000 women. As with nontoxic goiter, iodine intake is of paramount importance. In Denmark, a country with a borderline sufficient iodine intake, multinodular toxic goiter accounts for 50% of patients with hyperthyroidism, whereas Iceland, with a high iodine intake, has a lower proportion of multinodular goiter (6%) and a greater number of cases of Graves’ disease.⁸⁹

Natural History

The natural history of multinodular goiter, with respect to goiter growth and function, varies and is difficult to predict in a given patient. The spontaneous growth rate in selected populations has been estimated to be up to 20% yearly⁹⁰ but usually is much lower. No specific parameter exists that can predict the growth potential of multinodular goiter, which can be accurately assessed by serial yearly measurements of the size of the goiter and individual nodules by ultrasonography.⁹¹

Painful nodules are usually the result of hemorrhage into a nodule or a cyst in the goiter. The diagnosis is readily made by ultrasonographic examination and fine-needle aspiration biopsy. Such a growing painful nodule may represent thyroid malignancy and should be investigated accordingly. Multinodular goiter is not usually associated with a significantly increased risk for the development of thyroid malignancy. The risk of malignancy in thyroid nodules occurring within a multinodular goiter has not been completely clarified, but most authors find a similar frequency in uninodular and multinodular goiters.⁹²

Patients with nontoxic multinodular goiter can become hyperthyroid or, less commonly, hypothyroid. Hyperthyroidism in such patients often develops insidiously, in contrast to that of Graves’ disease. It often begins with a prolonged period of subclinical hyperthyroidism characterized by low serum TSH and normal serum free T₄ and triiodothyronine (T₃) concentrations.⁸⁷ This hyperthyroid state is the consequence of goiter growth and an associated increase in the mass of autonomously hormone-producing thyroid cells. Hyperthyroidism can also be the result of an increase in iodine intake from iodine-containing drugs such as disinfectants and amiodarone or from radiographic contrast agents, which, in a goiter with increased autonomous iodine metabolism, leads to the production of excessive amounts of thyroid hormone. Little is known of the incidence and the time frame for this progression from the nodular nontoxic goiter toward the nodular toxic goiter. In a large population-based cross-sectional study in an iodine-deficient area, nodular autonomy increased with age and reached 15% in elderly people.⁹³ It appears from a few longitudinal studies⁸⁷ that within 5 years, hyperthyroidism will emerge in approximately 10% of patients with a nodular goiter. In a few cases, autonomy of some of the thyroid nodules may return.

Development of hypothyroidism in a patient with multinodular nontoxic goiter is rarer. This observation is difficult to explain, but the situation is probably caused by coexisting autoimmune thyroiditis.

Diagnosis

1. Large goiter or progressive growth of the entire gland or individual nodules

2. Signs of cervical compression

3. Overt or subclinical hyperthyroidism

4. Marked neck disfigurement

5. Cosmetic complaints

6. Suspicion of thyroid cancer

Antithyroid Drugs

Antithyroid drugs are indicated if the nodular goiter is complicated by coexisting hyperthyroidism. These drugs normalize thyroid function, but remission is very rare and lifelong treatment should be anticipated. Also, further goiter growth may be seen, possibly as a result of using these drugs. Antithyroid drugs are indicated before thyroid surgery to lower the operative risk and can be stopped during the immediate postoperative period.¹¹⁷ To reduce the risk for exacerbation of hyperthyroidism, it has been recommended to render the patient euthyroid with antithyroid drugs prior to ¹³¹I treatment. Usually, the antithyroid drug is discontinued at least 4 days before and is resumed no sooner than 3 days afterward.¹¹⁷ A meta-analysis, based mainly on studies of Graves’ disease, found that the use of methimazole and propylthiouracil in conjunction with ¹³¹I therapy results in a decreased remission rate.¹¹⁸ Whether this applies also to toxic multinodular goiter patients is unknown. One study has shown attenuation of goiter reduction if methimazole is resumed after ¹³¹I therapy despite a neutral effect on thyroid function.¹¹⁹

Iodine Supplementation

Iodine supplementation for treatment of goiter is used by some clinicians, particularly in Europe.⁹⁹ In a placebo-controlled trial, the median volume of diffuse goiters was reduced from 29 to 18 mL, but thyroid dysfunction and antibodies appeared in 10% of patients.¹²⁰ The efficacy of iodine supplementation, once a nodular goiter has developed, has only very scarcely been evaluated. In nodular goiter, iodine is no better than L-T4 suppression therapy for goiter reduction in comparative trials. But the major hindrance involving the use of iodine is the fact that a sudden increase of intake may induce thyrotoxicosis in predisposed individuals.⁸⁷

Thyroid Hormone Treatment

Thyroid hormone therapy for suppression of pituitary TSH secretion has been much used in the patient with nontoxic multinodular goiter.99,100 Although L-T4 suppressive therapy is effective in reducing the volume of diffuse nontoxic goiters by up to 30%,¹²¹ few controlled studies have examined nontoxic multinodular goiter^90,122 by employing sonography for objective size monitoring. In one study,⁹⁰ 58% of patients had a significant (>13%) decrease in thyroid volume, but regrowth was seen after discontinuation of therapy. Wesche et al.¹²² in a randomized trial found a median reduction of goiter volume in the ¹³¹I-treated group of 38% and 44% after 1 and 2 years, respectively; corresponding values in the L-T4–treated group were 7% and 1%, respectively, and these were nonsignificant.

L-T4 dose is often targeted toward a partially suppressed serum TSH level.99,100 The consequence is subclinical hyperthyroidism that adversely affects the skeleton and the cardiovascular system.⁹⁵ Because lifelong therapy probably is needed to avoid goiter recurrence, and because the natural history of the disease involves progression toward hyperthyroidism due to autonomous function of the thyroid nodules,⁸⁷ L-T4 treatment in fact is not feasible in most patients.¹²³ Based on the aforementioned, L-T4 treatment should be abandoned on this indication.^87,124,125

Surgery

The goal of surgery is removal of all thyroid tissue with a nodular appearance, usually by a unilateral hemithyroidectomy and subtotal resection of the contralateral lobe. A bilateral subtotal resection cannot be recommended. Only extremely rarely is a thoracic approach necessary. Further resection is not usually recommended if final pathologic evaluation incidentally reveals a unilateral cancer that is smaller than 1 cm. This not uncommon finding accounts for most cancers found in surgical series, a majority of which are of little if any clinical significance.¹²⁶ Macroscopically normal perinodular tissue often harbors microscopic growth foci, which explains the relatively high risk for recurrence in these patients.¹²⁷ In the case of a toxic nodular goiter, thyroid function is normalized more rapidly after surgery than after ¹³¹I therapy on the assumption that antithyroid drugs are not used postoperatively.⁸⁷

Surgery leads to rapid decompression, resulting in improved respiratory function if affected presurgically.¹²⁸ Not all patients are surgical candidates, but among those undergoing surgery, the surgical mortality rate is less than 1% in experienced centers. Disadvantages include the general risks and side effects of a surgical procedure. Specific risks include transient (6%) or permanent (2%) vocal cord paralysis, transient (6%) or permanent (5%) hypoparathyroidism, and postoperative bleeding (1%).¹²⁹ Others have reported lower figures.¹³⁰ Complications are related to increasing goiter size and extent of the resection.^130,131 Novel techniques may reduce the operation time, the postoperative pain, and the length of the hospital stay.^132,133 Postoperative tracheomalacia, necessitating intubation, may ensue in approximately 5% of patients operated for large goiters.⁸⁷ A matter of concern is the apparently high prevalence (7% to 17%) of thyroid carcinomas in substernal goiters,^87,134 but this seemingly high frequency probably is influenced by selection bias.

The long-term risk for hypothyroidism after subtotal resection of multinodular goiters is insufficiently described but is approximately 10% to 20%, as reported for toxic multinodular goiters,⁸⁷ and is related to the extent of the resection. Recurrence of the nontoxic multinodular goiter is seen in 15% to 40% of patients on long-term follow-up.⁸⁷ Postoperative use of L-T4 is preferred by many clinicians^99,100 to avoid recurrence; however, based on results from randomized trials,⁸⁷ the use of L-T4 generally cannot be recommended,⁸⁷ nor can the use of iodine.¹³⁵ A reoperation for recurrent goiter results in a threefold to 10-fold increase in risk for permanent vocal cord paralysis or hypoparathyroidism.^{130,131 131}I therapy seems to be a favorable alternative in these cases. Goiter recurrence can be completely avoided, if a total thyroidectomy is carried out initially, in some centers with the same low rate of complications as was reported with subtotal thyroidectomy.¹³⁶

Radioiodine Therapy

¹³¹I treatment is considered safe and appropriate in nearly all types of hyperthyroidism, especially that occurring in elderly patients.¹¹⁷ Generally, ¹³¹I is thought to carry a lower rate of complications and a lower cost than surgery.¹¹⁷ This fact has led a number of centers to offer ¹³¹I as the first choice of therapy for most patients. In contrast to surgery, which cures nearly all patients and normalizes hyperthyroidism within a few days,¹³⁷ only 50% become euthyroid within 3 months of ¹³¹I therapy, given that antithyroid drugs are not administered.¹⁰¹ Twenty to 40% need additional ¹³¹I therapy, and even up to five treatment sessions will not cure all patients.¹⁰¹ In contrast, persistence of hyperthyroidism after surgery is rare.¹³⁷

Besides being able to cure hyperthyroidism, it has long been recognized that use of ¹³¹I results in shrinkage of the thyroid gland. During the past 25 years, ¹³¹I therapy for symptomatic multinodular nontoxic goiter therefore has been introduced in a number of mainly European centers as a nonsurgical alternative to L-T4 therapy.87,99 Thyroid volume reduction is of the same magnitude in toxic and nontoxic multinodular goiters, that is, approximately 40% after 1 year^{101,122,138–140} and 50% to 60% after 2 years, without further reduction.^101,138 Sixty percent of this decrease is seen within 3 months of initiation of therapy.^101,138 In addition to relief of compressive symptoms, ¹³¹I therapy results in less tracheal compression, which improves pulmonary function, particularly the inspiratory component.^139,140 Because of the often large goiters, normalization of thyroid volume, as is seen in diffuse toxic and nontoxic goiters, can rarely be achieved,⁸⁷ but symptoms in most cases are considerably improved, and patient satisfaction is high.^140,141 If a secondary increase in thyroid volume is seen, this should raise the suspicion of malignancy. Generally, ¹³¹I doses of 100 µCi (3.7 MBq) per gram of thyroid tissue corrected for 100% 24 hour ¹³¹I uptake have been given.^{101,122,138–140} However, whether such dose adjustment is worthwhile has been questioned,^117,142 and fixed doses are given in a number of centers.¹¹⁷ Treatment can be repeated if further goiter reduction is required in a euthyroid patient.^87,138

Radiation thyroiditis, seen in 3% within the first months of ¹³¹I therapy,¹⁰² is easily treated with salicylates or corticosteroids. Another complication is a Graves’-like autoimmune hyperthyroidism, which is seen in 5%. Rare cases of ¹³¹I-induced Graves’ ophthalmopathy have also been reported.⁸⁷ Pretreatment presence of anti-TPO antibodies confers a significantly increased risk for this complication,⁸⁷ which most likely is triggered by ¹³¹I-related release of thyroid antigens and is associated with the appearance of TSH receptor antibodies typically 3 to 6 months after ¹³¹I therapy. It also can be seen after surgical manipulation of the thyroid or after subacute thyroiditis.⁸⁷ The condition is often self-limiting but may necessitate therapy.

Although early goiter enlargement caused by radiation therapy may be seen, on average ¹³¹I therapy is not followed by any significant acute thyroid enlargement.139,143 The risk for permanent hypothyroidism after ¹³¹I therapy in multinodular goiters ranges from 14% to 58% within 5 to 8 years.^87,101,138 It occurs more commonly in patients with a small goiter and when anti-TPO antibodies are present.^{87 131}I therapy, given for Graves’ disease for decades, until recently was not thought to be followed by any clinically significant increased risk for cancer death.^144,145 However, a recent study questions these findings.¹⁴⁶ Data regarding ¹³¹I therapy in multinodular goiter are sparse, and in the case of nontoxic goiter, nonexistent. In the study by Ron et al.,¹⁴⁴ 1089 patients were treated for a toxic nodular goiter, and these individuals had a 31% increase in overall cancer mortality, nearly exclusively attributable to thyroid malignancy. However, a similar pattern was seen in patients with the same disease but not treated with ¹³¹I. Hence, the disclosure of a thyroid cancer in a nodular goiter after ¹³¹I therapy raises the question, whether malignancy in a nodule was overlooked at the time of therapy.

In some European countries, ¹³¹I therapy has replaced surgery as the treatment of choice in most patients.87,99 However, the optimum treatment remains to be established, ideally through comparative randomized trials, including data on effects, side effects, costs, and patient satisfaction.

The efficacy of ¹³¹I therapy in multinodular goiter is hampered by irregular ¹³¹I uptake in the gland, and the relative goiter reduction is inversely correlated with initial goiter size.¹³⁹ A high dietary iodine intake diminishes efficacy. However, recombinant human TSH (rhTSH) has the potential of increasing the 24-hour ¹³¹I uptake more than fourfold,^147–149 and the effect is inversely correlated to the initial thyroid ¹³¹I uptake.^147–149 Moreover, pretreatment with rhTSH causes a more homogeneous distribution of ¹³¹I within the nodular gland.¹⁵⁰ These properties of rhTSH are ideal in the context of ¹³¹I therapy for multinodular goiter. Indeed, several studies, including two randomized, double-blinded trials,^151,152 have confirmed that rhTSH, in doses from 0.1 mg to 0.9 mg and administered 24 hours before ¹³¹I therapy, improves goiter reduction by 35% to 55% within a year, when compared with conventional ¹³¹I therapy (Fig. 16-5). The impact of rhTSH pre-stimulation is most pronounced in patients with a low baseline thyroid ¹³¹I uptake.^149,151,152 rhTSH-augmented ¹³¹I therapy also results in reduced tracheal compression and enhancement of the inspiratory reserve, as shown in a randomized trial.¹⁵³ We have speculated that the effects of rhTSH might be mediated through factors beyond the increase in thyroid iodine uptake.^151,152

As an alternative to aiming at increased thyroid irradiation and goiter shrinkage, rhTSH allows a reduction in ¹³¹I activity while still retaining a mean goiter reduction of approximately 40% within the first 12 months.¹⁵⁴ With this approach, extrathyroidal irradiation is diminished,¹⁵⁵ which may render ¹³¹I therapy more attractive in young patients.

rhTSH per se, in doses from 0.3 mg to 0.9 mg, results in a transient 25% to 35% enlargement of the thyroid gland156,157; this may be potentially dangerous in patients with goiter. However, rhTSH-augmented ¹³¹I therapy is generally well tolerated, particularly when a dose of 0.1 mg or lower is used. With a higher rhTSH dose, the patient may experience cervical pain and a temporary rise in thyroid hormones within the first week after ¹³¹I therapy.^{149,151,152,158,159} In parallel with improved goiter reduction, a higher frequency of hypothyroidism is encountered after rhTSH-augmented ¹³¹I therapy is provided.^151,152,159 When safety aspects have been further evaluated, rhTSH-augmented ¹³¹I therapy may well become a routine option in many patients with symptomatic multinodular goiter who decline surgery.

Percutaneous Interventional Therapy

Percutaneous ethanol injection therapy (PEIT) has been used for longer than a decade in solitary hot, toxic, and even cold thyroid nodules.87,108 The most convincing effect is seen in solitary thyroid cysts.¹⁰⁸ Theoretically, PEIT can be used in multinodular goiter. Drawbacks are related to pain, risk for recurrent laryngeal nerve damage, and the possibility of extrathyroidal fibrosis complicating subsequent surgery. Interstitial laser photocoagulation was introduced recently and seems to have the same effect as PEIT with possibly fewer side effects.¹⁰⁷ No controlled studies in multinodular goiter have examined either technique.

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