Hypothyroidism is caused by deficient thyroid hormone production secondary to autoimmune disease or insufficient iodine consumption or as a complication of hyperthyroidism management. Signs and symptoms include fatigue, weight gain, dry skin, constipation, and cold intolerance. The U.S. Preventive Services Task Force found insufficient evidence to recommend for or against screening for hypothyroidism, but some organizations support screening in special populations. If hypothyroidism is suspected, initial laboratory evaluation consists of a serum thyrotropin (TSH) measurement with reflex testing of free thyroxine (T4). Thyroid function tests must be interpreted carefully because acute illness, diet, and drugs may alter values. Overt hypothyroidism occurs when a patient has an elevated TSH level and a low free T4 level with symptoms of hypothyroidism. Management includes thyroid hormone replacement, ideally levothyroxine. Subclinical hypothyroidism is characterized by an elevated TSH level with a normal T4 value. The decision to treat subclinical hypothyroidism should be based on patient characteristics and shared decision-making discussions. Special consideration should be taken in treating patients with high-risk conditions, including heart disease, pregnancy, and myxedema coma, and in patients requiring high-dose levothyroxine. Thyroid hormone should be titrated based on goal TSH values, symptoms, and potential treatment adverse effects.
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