Normal thyroid physiology is discussed in Chapter206, "Hyperthyroidism and Thyroid Storm." Hypothyroidism occurs when there is insufficient hormone production or secretion. A general hypometabolic state is the principle feature of this disease. Hypothyroidism occurs more frequently among women than men. The prevalence of hypothyroidism among women ranges from 0.6 to 5.9 percent.1 The most common etiologies of hypothyroidism are primary thyroid failure due to autoimmune diseases (of which Hashimoto thyroiditis is most common), idiopathic causes, postablative therapy, and iodine deficiency. 2 Hypothyroidism may be transient as in some cases of thyroiditis. The pathophysiology of this entity is unclear but may be viral in origin. Postpartum thyroiditis occurs within 3 to 6 months postpartum and reportedly occurs in 2 to 16 percent of women.1 Secondary (due to pituitary tumors, infiltrative disease, or hemorrhage) or tertiary (hypothalamic disease) etiologies of hypothyroidism are less common (Iab|en207-!). Several medications have been reported to induce hypothyroidism. Amiodarone-associated hypothyroidism has been reported to occur in from 1 percent to as many as 32 percent of patients.3 The mechanism is probably due to the large amount of iodine that is released by the metabolism of the drug, which then inhibits thyroid hormone synthesis and release. Lithium acts similarly to iodine and decreases thyroid hormone release. 4 When initiating lithium therapy, especially in patients with prior hypothyroidism, thyroid function must be carefully monitored.
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