Chapter 223

Hypothyroidism is a condition of insufficient thyroid hormone production that causes slowed metabolism. Table 223-1 lists common causes of hypothyroidism. Hypothyroidism is particularly common in areas of iodine deficiency, particularly inland areas where there is no access to marine foods. In iodine-sufficient areas, chronic autoimmune (Hashimoto) thyroiditis is the leading cause of primary hypothyroidism. Hypothyroidism has also been estimated to occur in 1% to 32% of patients taking amiodarone.2 Individuals who have thyroid peroxidase antibodies and those who have thyroid-stimulating hormone (TSH) values that are in the upper normal range are at increased risk for developing hypothyroidism.

Table 223-1 Common Causes of Hypothyroidism

Thryroxine is the major form of thyroid hormone. The ratio of thyroxine to triiodothyronine released in the blood is about 10:1. Peripherally, thyroxine is converted to the active triiodothyronine, which is three to four times more potent than thyroxine. The half-life of thyroxine is 7 days, and the half-life of triiodothyronine is about 1 day.

Myxedema coma (also called myxedema crisis) is the end spectrum of severe hypothyroidism and is an emergency. It is a clinical syndrome of multi-organ and metabolic dysfunction resulting from severe untreated hypothyroidism, and precipitated by a number of stressors. The condition is a severe, life-threatening decompensation of a hypothyroid patient heralded by mental status changes, hypotension, and hypothermia. Almost 90% of cases occurs in elderly women during the winter.

Subclinical hypothyroidism is more prevalent and can be seen in as many as 15% of older women. In the United States National Health and Nutrition Examination Survey the prevalence of overt hypothyroidism was found to be 0.3%, whereas the prevalence of subclinical hypothyroidism was found to be 4.3%.3

Euthyroid individuals, who have detectable thyroid autoantibodies, are at increased risk for developing overt hypothyroidism. Up to 15% of elderly women have thyroid autoantibodies.4

Primary hypothyroidism is caused by the intrinsic dysfunction of the thyroid gland, and this is the most common type. Secondary hypothyroidism is caused by a deficiency of TSH from the pituitary gland or deficiency of thyrotropin-releasing hormone from the hypothalamus. The distinction between primary and secondary hypothyroidism is important because TSH administration is ineffective in primary hypothyroidism.

The common features of hypothyroidism are listed in Table 223-2...

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