Triiodothyronine in pregnancy

The radioactive T3 uptake test is a relatively simple laboratory procedure which indirectly measures circulating T3, the active fraction of the thyroid hormone. The present study applies this test to the three trimesters of pregnancy, the postpartum period, and to inevitable and incomplete abortions. The mean of these values shows a slight drop in T3 in the first trimester, a rather marked drop in the second and third trimesters, and a return to euthyroid levels by the sixth postpartum week. There are no significant changes with inevitable or incomplete abortions. Individual readings, however, are so variable and unpredictable that they cannot be used reliably to confirm clinical suspicion of a thyroid disorder. The variations in the mean values of circulating T3 are correlated with the physiologic changes of pregnancy.