Hypothyroidism is the most common clinical disorder of thyroid function. It is the clinical syndrome that results from inadequate secretion of thyroid hormones from the thyroid gland. The vast majority of hypothyroid patients have primary gland failure, whereas rare patients have pituitary or hypothalamic failure. Most studies define hypothyroidism based on a serum TSH level above the upper limit of the laboratory reference range. In adults, 1.4% of women and 0.1% of men are biochemically hypothyroid. However, the incidence is highly age-dependent. In the Colorado Thyroid Health Study, by age 64, 12% of women and 5% of men were hypothyroid, and in the over 74 year age group, the incidence in men approached that of women. Most epidemiologic studies of hypothyroidism in the elderly show a prevalence of 6% to 12%. There is a strong correlation between the presence of anti-TPOAb or anti-TGAb and the risk of developing hypothyroidism. In patients with subclinical hypothyroid-ism and positive anti-TPOAb, 5% per year will progress to overt hypothyroidism. Other risk factors for the development of hypothyroidism include postpartum state, family history of autoimmune thyroid disorders, a previous history of head and neck or thyroid surgery, head and neck irradiation, other autoimmune endocrine disorders such as type 1 diabetes and Addison's disease, other nonendocrine autoimmune diseases such as celiac disease and pernicious anemia, prior history of treatment for hyperthyroidism, treatment with amiodarone or lithium, and an iodine-deficient diet.
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