Key Points

• Hashimoto's thyroiditis is an autoimmune disorder that results in fibrosis of the thyroid gland.

• Hashimoto's thyroiditis is the most common cause of hypothyroidism in the United States.

• Diagnosis of Hashimoto's thyroiditis is based on an elevated TSH, low FT4, and antithyroid peroxidase antibodies (TPO Abs).

Box 35-4 Causes of Hypothyroidism

Insufficient intake of dietary iodine (uncommon in U.S.) Autoimmune disease (primarily Hashimoto's thyroiditis) Surgery (thyroid surgery) Radiation exposure (head and neck) Viral infection

Central disease (primary pituitary failure)

The most common cause of hypothyroidism worldwide is inadequate dietary intake of iodine. Because of the addition of iodine to table salt, however, this is a rare cause in the United States. In the U.S. and the rest of the developed world, the most common cause of hypothyroidism is chronic autoimmune thyroiditis (Hashimoto's disease). CAT is caused by the development of antithyroid antibodies that attack the thyroidal struma, causing progressive fibrosis; TPO Abs and Tg Abs appear to be responsible, with TPO Abs considered the primary cause (Dayan and Daniels, 1996). Diagnosis is based on elevated sTSH (>10.0 mIU/L), low-normal or low FT4, and presence of TPO Abs.

Hashimoto's thyroiditis occurs most often in women, with a female/male ratio of 10:1 to 14:1. CAT is usually diagnosed in the fifth decade and is progressive (Vanderpump, 2005). As it progresses, more functioning thyroid gland will become fibrotic, and less endogenous T4 will be produced. After diagnosis, replacement doses of T4 should be used. In adults, average replacement dose of L-thyroxine is 1.6 ^/kg daily. Serum TSH is followed annually to ensure adequate control.

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