The lobes of the thyroid lie on the front and sides of the trachea and larynx at the level of the 5–7th cervical vertebrae (Fig. 20.1). They are connected by a narrow isthmus, which overlies the second and third tracheal rings. The thyroid normally weighs 15–30 g and is invested by the pre-tracheal fascia, which binds it to the larynx, cricoid cartilage and trachea (Fig. 20.2). The strap muscles (sternohyoid and sternothyroid) lie in front of the pretracheal fascia and must be separated to gain access to the gland. It is difficult to feel the normal thyroid gland except at puberty and during pregnancy, when physiological enlargement occurs.

Fig. 20.1 Anatomy of the thyroid gland.

The middle thyroid vein has been divided to allow forward rotation of the left lobe of the gland.

Fig. 20.2 Transverse section of the neck at the level of the 7th cervical vertebra to show the arrangement of the deep cervical fascia.

The superior thyroid artery runs down to the upper pole of the gland as a branch of the external carotid artery, whereas the inferior thyroid artery runs across to the lower pole from the thyrocervical trunk (a branch of the subclavian artery). As it nears the gland, the inferior thyroid artery usually passes in front of the recurrent laryngeal nerve, but may branch around it. Blood drains through superior, middle and inferior thyroid veins into the internal jugular and innominate veins. Lymphatics pass laterally to the deep cervical chain and downwards to pretracheal and mediastinal nodes. The recurrent laryngeal nerve, a branch of the vagus, passes upwards in the groove between the oesophagus and trachea to enter the larynx and supply all of its intrinsic muscles except the cricothyroid. The superior laryngeal nerve (also a branch of the vagus) runs with the superior thyroid vessels and supplies the cricothyroid muscles (external branch), which tense the vocal cords. The recurrent nerve also supplies sensation to the larynx below the vocal cords. The internal branch of the superior laryngeal nerve provides sensation above the cords. Normal sensory and motor function within the larynx is necessary for speech and coughing. Both nerves are at risk of damage during thyroid surgery and the consequences, if permanent, can be disabling.

Thyroid function

Histologically, the gland is made up of follicles containing colloid. The follicles are spheroids lined by cuboidal epithelium (thyrocytes). The parafollicular or C cells may be seen between follicles. The gland has a rich blood supply. The thyrocytes secrete triiodothyronine (T₃) and thyroxine (T₄). T₃ is the active hormone, and T₄ is converted to T₃ in the periphery.

Secretion of T₃ and T₄ is controlled by thyroid-stimulating hormone (TSH), which is secreted by the anterior pituitary. TSH release is in turn controlled by thyrotropin-releasing hormone (TRH) from the hypothalamus. Circulating levels of T₃ and T₄ exert a negative feedback effect on the hypothalamus and anterior pituitary. Calcitonin, which is released by the parafollicular cells following stimulation by ingested food, lowers the serum calcium but it is not an essential hormone and does not require replacement after total thyroidectomy.

Assessment of thyroid disease

Measurement of T₃, T₄ and TSH gives a biochemical estimation of thyroid function. TSH is totally suppressed in thyrotoxicosis and elevated in hypothyroidism. Pregnancy or oestrogen administration increases the level of thyroid-binding globulin, so that estimation of the ratio of free to bound hormone may be needed. TRH and TSH stimulation tests may be required to determine the site of failure of production of thyroid hormones.

The thyroid can be imaged by ultrasonography (Fig. 20.3) or radioisotope scanning (^99mTc-sodium pertechnetate behaves like iodine and is ‘trapped’ by the gland). The main value of scanning is to differentiate between ‘hot’ (actively functioning), ‘cool’ (normally functioning) and ‘cold’ (non-functioning) thyroid nodules. Total isotope uptake also reflects thyroid activity.

Fig. 20.3 Ultrasound demonstrating thyroid nodule (arrowed).

Magnetic resonance imaging (MRI) and computed tomography (CT) provide excellent means of determining the extent of goitre. Fine-needle aspiration cytology is used to determine the nature of thyroid nodules. Thyroid antibodies detected in significant titre may indicate autoimmune thyroid disease.

Enlargement of the thyroid gland (goitre)

Clinical features

Goitre is a visible or palpable enlargement of the thyroid (Fig. 20.4). The swelling appears in the lower part of the neck and retains the shape of the normal gland (thyreos – Greek for shield). The swelling characteristically moves upwards on swallowing because of the gland’s attachment to the trachea. Patients may have a dry mouth, and when asking them to swallow, water should be provided.

Fig. 20.4 Massive goitre.

This common disease occurs endemically in areas of iodine deficiency, but can be sporadic or a reaction to drugs. It occurs much more commonly in females. In the past, lack of iodine in the diet was a common cause of thyroid enlargement, but ‘endemic goitres’ in populations where iodine was deficient are now rare because table salt is iodized.

Pathology

In iodine deficiency, the gland initially enlarges diffusely as the follicles fill with colloid. Later, multiple nodules develop, some of which contain abundant colloid; others show degenerative changes, with the formation of cysts, areas of old and new haemorrhage, and even calcification. The goitre varies greatly in size, from little more than normal to weighing several hundred grams. The whole gland may be involved, or the changes may be confined to one lobe.

Clinical features

Most multinodular goitres are asymptomatic. Others cause tracheal compression and dyspnoea, particularly when they extend behind the sternum (retrosternal goitre). Oesophageal compression can cause dysphagia. Very rarely, bleeding into a nodule may cause pain and rapid enlargement and, for retrosternal goitre, respiratory distress. The thyroid is visibly enlarged and multiple nodules are usually palpable. Sometimes only one nodule is palpable, giving the erroneous impression of a solitary nodule.

Investigations

In the case of retrosternal goitre, plain films of the thoracic inlet may reveal tracheal deviation (Fig. 20.5A) and CT may show tracheal compression (Fig. 20.5B) The presence of stridor indicates compromise of the tracheal lumen. T₃, T₄ and TSH are usually normal and that being the case isotope scans are not indicated.

Fig. 20.5 Retrosternal goitre images showing marked tracheal deviation.

A Plain X-ray. B CT.

Slow-growing and painless clinically ‘solitary’ nodules are common, although 50% of them are really part of a multinodular goitre. Of the true solitary nodules, half are benign adenomas and the rest are cysts or differentiated cancers. The pivotal diagnostic test is fine-needle aspiration cytology, complemented by ultrasonography, isotope scans and thyroid function tests (Fig. 20.6). Cysts can be aspirated and, provided that they do not refill and that the cytology is negative for neoplastic cells, they need not be removed. Very rarely, a cyst contains a carcinoma (often papillary) within its wall, and blood-stained aspirate or a residual swelling after aspiration should raise this possibility. A cytopathologist cannot distinguish between a follicular adenoma and follicular carcinoma; this can only be achieved on definitive histopathology by looking for capsular or vascular invasion. Diagnostic surgery is needed if aspiration reveals a follicular neoplasm. Intraoperative frozen section does not always provide a definitive diagnosis, but the demonstration of carcinoma by whatever means indicates that more extensive surgery may be needed (e.g. complete total thyroidectomy).

Fig. 20.6 Algorithm for the management of a patient with a suspected solitary thyroid nodule.

Other forms of neoplasia

All forms of thyroid cancer can produce goitre. Lymphoma and anaplastic tumours may cause diffuse thyroid swellings. Medullary and follicular tumours are often solitary swellings.

Hyperthyroidism

Thyrotoxicosis results from the overproduction of T₃ and T₄ and, because of the feedback mechanism, serum TSH levels are reduced or undetectable. The three conditions that may produce thyrotoxicosis are primary thyrotoxicosis (Graves’ disease), toxic multinodular goitre and toxic adenoma.

Primary thyrotoxicosis (Graves’ disease)

Pathophysiology

This condition accounts for 75% of cases. It is an autoimmune disease in which TSH receptors in the thyroid are stimulated by circulating thyroid receptor antibodies (TRAbs). The gland is uniformly hyperactive, very vascular and usually symmetrically enlarged. Histologically, there is marked epithelial proliferation, with papillary projections into follicles devoid of colloid. TRAbs can cross the placental barrier, so that neonatal thyrotoxicosis can occur.

Clinical features

The patient is usually a young female (male:female ratio 1:8) and the condition can be familial. The thyroid is usually moderately and diffusely enlarged and soft, and because of its vascularity a bruit may be audible. High circulating levels of T₃ and T₄ increase the basal metabolic rate and potentiate the actions of the sympathetic nervous system.

Metabolic effects

The patient feels hot at rest and heat intolerant. The skin is moist and warm because of peripheral vasodilatation and excess sweating. Weight loss is the rule, despite an increased appetite. Cardiac output is increased to meet the metabolic demands.

Sympathetic effects

Tachycardia is present, even during sleep. Palpitations can be troublesome, and cardiac irregularities and arrhythmias (especially atrial fibrillation) are common in older patients. The hands exhibit a fine tremor. The upper eyelids are retracted (the levator palpebrae superioris has some non-striated muscle which is innervated by the sympathetic nervous system) and there is lid lag. Gastrointestinal motility is increased. There is general hyperkinesia; anxiety and psychiatric disturbance may occur.

Other features

Exophthalmos is usual but not invariable (Fig. 20.7). Ophthalmoplegia, pretibial myxoedema, proximal muscle myopathy and finger clubbing are sometimes present. Menstrual irregularity and infertility can occur.