Overt hypothyroidism in pregnancy, defined as an elevated serum thyroid-stimulating hormone (TSH) and reduced serum free thyroxine or a TSH >10 mIU/L, is known to have adverse effects on pregnancy. Subclinical hypothyroidism is typically defined as an elevated TSH and normal FT4 levels. There remains much controversy on the benefit of starting levothyroxine for mothers diagnosed with subclinical hypothyroidism. Recent studies are redefining the normal range for TSH in pregnancy, and the data on whether treatment of subclinical hypothyroidism improves outcomes for the mother and fetus are unclear. One confounding variable is the presence of thyroid peroxidase antibodies, as it may be a surrogate marker for other autoimmune disorders detrimental to pregnancy. If levothyroxine treatment is initiated, the dosing and monitoring strategy is different from nonpregnant individuals. Randomized clinical trials are underway that may better elucidate whether treatment of subclinical hypothyroidism is warranted.

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