Thyroid function in early normal pregnancy: transient suppression of thyroid-stimulating hormone and stimulation of triiodothyronine.

In order to determine the effect of gestation on thyroid function in healthy subjects, we have prospectively evaluated thyroid function in pregnant individuals undergoing termination of pregnancy, and repeated the tests 2-3 months later. Venous blood was tested for human chorionic gonadotropin (hCG), thyroid-stimulating hormone (TSH), free thyroxine (FT4) and total triiodothyronine (TT3). Early pregnancy thyroid function tests showed a significant decrease (p < 0.001) in TSH and a significant increase (p < 0.001) in TT3 as compared to the nonpregnant state; FT4, however, did not change significantly. In 8 (11.2%) pregnant subjects, TT3 levels were above the normal range for nonpregnant controls. Elevated thyroid function in early pregnancy is transient, and does not usually warrant antithyroid treatment. Thus, any conclusion regarding thyroid function in early pregnancy should be based on pregnant controls rather than general population controls.