Changes in thyroid function in pregnancy and with oral contraceptive use.

We measured T4, T3, T3 uptake, and TSH in nonpregnant patients, pregnant patients in the early and late first, and third trimesters, and patients using oral contraceptives. Mean T4 levels increased and T3 uptake decreased during pregnancy. Concentrations of T3 decreased in early pregnancy, then increased during the rest of pregnancy. Thyroid-stimulating hormone (TSH) values were elevated in the second trimester. Decreased T3 concentration in the early first trimester suggests increased use of T3 by the fetus, placenta, and/or mother not compensated for by increased T4 pool size. Values found in oral contraceptive users were similar to those found in early pregnancy and may reflect changes in response to levels of estrogen found in oral contraceptives. Values provided illustrate changes in thyroid function parameters and are useful in providing a target range of laboratory values for therapeutic manipulation when comparing thyroid function values in nonpregnant patients, pregnant patients, and those using oral contraceptives.