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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.

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Table 223-1 Common Causes of Hypothyroidism 
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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.

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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.

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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

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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

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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.

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The common features of hypothyroidism are listed in Table ...

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