Thyroid Disorders

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167 Thyroid Disorders

Sarah Stewart de Ramirez, Frederick Korley

image      Key Points

• Hypothyroidism secondary to iodine deficiency is the most common endocrine disorder worldwide.

• Primary hypothyroidism results from dysfunction of thyroid tissue. Secondary hypothyroidism is a component of panhypopituitarism that results from pituitary disease.

• Severe hypothyroidism is a rare condition characterized by altered mental status, hypothermia, and primary thyroid dysfunction. Myxedema is seen in most cases; true coma is not.

• Patients with suspected severe hypothyroidism should receive empiric, low-dose intravenous levothyroxine.

• Thyroid storm, the most extreme form of thyrotoxicosis, is a rapidly fatal condition.

• Antithyroid medication should be given at least 1 hour before the administration of iodine for the treatment of thyroid storm.

• Emergency department testing for thyroid disease includes serum thyrotropin, or thyroid-stimulating hormone, and free thyroxine measurements.

Perspective

Hypothyroidism is a deficiency of thyroid hormones resulting in a hypometabolic state. Iodine deficiency accounts for most cases of hypothyroidism and goiter in underdeveloped countries; Hashimoto thyroiditis is a more commonly recognized cause in westernized societies.

Thyrotoxicosis describes any condition characterized by an excess of free thyroid hormones in the circulation. The term is frequently used interchangeably with hyperthyroidism; however, hyperthyroidism refers only to disease states in which the thyroid gland produces an excess of thyroid hormones.

Epidemiology

Abnormal thyroid function is by far the most common endocrine disorder worldwide and is second only to diabetes mellitus in the United States. Thyrotoxicosis is a rare condition that is 10 times more prevalent in women than in men (2% versus 0.2%).1 It is uncommon before the age of 15 years.2 Although most manifestations of thyrotoxicosis do not represent a true emergency, the extreme case of so-called thyroid storm does. Early detection and treatment of this condition may prevent progression to shock and death.

Anatomy

The thyroid gland derives its name from the shape of the nearby thyroid cartilage (from the Greek, meaning “shield”). Although it varies, the isthmus is usually centered over the third tracheal ring. Normal adult thyroid dimensions are a height of 5 cm, thickness of 1.5 cm, isthmus thickness of 0.5 cm, volume of 10 cm, and weight of 12 to 20 g. Thyroid thickness greater than 2 cm is considered abnormal.

The thyroid gland is palpable on physical examination in nonobese patients; thyroid nodules and some cancers are sometimes noted on palpation. Rarely, ectopic thyroid gland tissue is found at the base of the tongue.

Physiology

Thyrotropin-releasing hormone (TRH) is synthesized in the hypothalamus and regulates the production of thyroid-stimulating hormone (TSH), or thyrotropin, in the anterior pituitary gland. Principal cells in the thyroid bind TSH, which activates the production and release of thyroid hormones.

The thyroid is highly efficient in its absorption and extraction of iodine. Iodine is added to tyrosine to make up diiodotyrosine (DIT); two molecules of DIT combine to form thyroxine (T4), much of which is stored in the thyroid gland. T4 is released into the circulation largely bound by thyroid-binding globulin (TBG). Even though levels of TBG vary widely, this protein is seldom involved in a disease process.

Triiodothyronine (T3) is formed by peripheral conversion of T4 in tissues. When compared with T4, T3 is four to six times more active, exhibits less affinity for TBG, and has a much shorter half-life (12 hours versus 6 days). T3 and T4 exert feedback on the hypothalamus and TRH levels, thereby completing a regulatory loop.

T3 and T4 stimulate metabolic activity in tissues throughout the body. Both hormones are associated with growth and development early in life.

Goiter

Goiter refers to a visible enlargement of the thyroid gland that may result from euthyroid, hyperthyroid, or hypothyroid states. The presence of a goiter mandates a detailed review of systems and appropriate thyroid testing to determine the functional status of the tissue. The most common cause of goiter is iodine deficiency. Other causes include Hashimoto thyroiditis, Graves disease, nodules, cancer, and lithium therapy.

Goiters are usually benign, but thyroid malignancy must be considered in each case. Three rare but potentially life-threatening emergencies can result from continued enlargement of a malignant goiter. First, partial or complete obstruction of the jugular veins may occur, especially in hypercoagulable patients. Clot extension from a partially obstructed jugular vein can impede venous drainage from the brain. Second, invasive or extremely large goiters have been reported to cause airway compromise. Third, malignant involvement of the carotid sheath structures can result in devastating morbidity.

Hypothyroidism

Pathophysiology

Primary hypothyroidism implies a condition of thyroid tissue dysfunction. Causes include Hashimoto thyroiditis, surgical ablation, iodine I 131 ablation, and iodine deficiency. Idiopathic cases are frequently observed as well. The most common cause of hypothyroidism in industrialized countries is Hashimoto thyroiditis, an autoimmune disease of unclear cause.

Secondary hypothyroidism refers to pituitary dysfunction that results in a low TSH level and subsequent poor stimulation of otherwise normal thyroid tissue. Pituitary dysfunction generally affects more than one endocrine axis (panhypopituitarism); hypothyroidism is never the only resultant condition. Secondary hypothyroidism is seen with Sheehan syndrome and space-occupying lesions (adenomas) of the pituitary gland.

Hypothyroidism is 3 to 10 times more common in women than in men. Its incidence increases with age and obesity; 1% of young girls are affected as compared with 6% of older women. Smoking has been identified as an independent risk factor for hypothyroidism, but the reason for the observed association is unknown. Hypothyroidism has no racial or ethnic predilection. Symptoms of hypothyroidism are more apparent during the winter months in moderate and cold climates.

Iodine deficiency is the major worldwide cause of both hypothyroidism and cretinism; the latter is characterized by growth and mental retardation. Selenium deficiency appears to worsen the cretinism. Iodine deficiency is rare in the United States but affects approximately one in three persons worldwide, particularly in mountainous regions. Iodine deficiency has been decreasing because of supplementation programs sponsored by the World Health Organization. Excessive supplementation and diets high in seafood may contribute to thyroiditis.

Clinical Presentation

Hypothyroidism is a deficiency of thyroid hormones that results in decreased metabolic activity. It mimics many conditions commonly encountered in the emergency department (ED) and is accompanied by a myriad of indolent symptoms (Boxes 167.1 and 167.2).

Box 167.1 Symptoms of Hypothyroidism

Fatigue

Cold intolerance

Weakness

Weight gain

Depression

Decreased mental function

Constipation

Decreased libido

Irritability

Coarse hair

Dry skin

Myalgias

Arthralgias

Brittle fingernails

Myxedema

Menstrual irregularity

Peripheral neuropathy

Box 167.2 Signs of Hypothyroidism

Prolonged reflexes

Hypothermia

Narrow eyebrows

Peripheral neuropathy

Hoarse voice

Bradycardia

Facial edema

Periorbital edema

Pale, dry skin

Sparse axillary and pubic hair

In a recent study of the incidence of newly diagnosed primary overt hypothyroidism in adults seen in a Taiwanese ED, the most common symptoms were fatigue (50%), dyspnea (45%), chest tightness (20%), constipation (14%), and cold intolerance (9%). The majority of these patients were seen during winter months. In only 21% was hypothyroidism diagnosed by the emergency physician.3

Clinicians should maintain a high index of suspicion for hypothyroidism in patients with new-onset depression.

Severe Hypothyroidism (Myxedema Coma)

Patients with severe hypothyroidism may have mild to moderate hypothermia, depression, lack of energy, and altered mental status. Myxedema coma is a misnomer often used for severe hypothyroidism; not all patients with severe hypothyroidism are truly comatose, whereas most (but not all) patients with hypothyroid coma have myxedema. Myxedema coma best describes a patient in extremis secondary to a severe hypothyroid state (Box 167.3).

Box 167.3

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