Thyroid testing during pregnancy should be performed on symptomatic women or those with a personal history of thyroid disease. Overt hypothyroidism complicates up to 3 of 1,000 pregnancies and is characterized by nonspecific signs or symptoms that are easily confused with complaints common to pregnancy itself. Physiologic changes in serum thyroid-stimulating hormone (TSH) and free thyroxine (T(4)) related to pregnancy also confound the diagnosis of hypothyroidism during pregnancy. If the TSH is abnormal, then evaluation of free T(4) is recommended. The diagnosis of overt hypothyroidism is established by an elevated TSH and a low free T(4). The goal of treatment with levothyroxine is to return TSH to the normal range. Overt hyperthyroidism complicates approximately 2 of 1,000 pregnancies. Clinical features of hyperthyroidism can also be confused with those typical of pregnancy. Clinical hyperthyroidism is confirmed by a low TSH and elevation in free T(4) concentration. The goal of treatment with thioamide drugs is to maintain free T(4) in the upper normal range using the lowest possible dosage. Postpartum thyroiditis requiring thyroxine replacement has been reported in 2% to 5% of women. Most women will return to the euthyroid state within 12 months.