Thyroid Hormone Changes in Pregnant and Non-Pregnant Women: A Case-Control Study

Pregnancy is associated with significant but reversible changes in thyroid function tests results, which are among the most profound seen as a result of a normal physiologic state. The present study was carried out to find out alterations in thyroid function tests in each trimester in normal pregnant women as compared to non-pregnant women. Methods: A case-control study designed with two groups of women: (1) 75 normal pregnant women randomly selected from the first (25 samples), the second (25 samples), and the third (25 samples) trimesters, and (2) 75 randomly selected non-pregnant healthy female controls. Thyroid function tests were carried out by measuring the serum levels of thyroid stimulating hormone (TSH), free and total thyroxin (FT4, T4), and free and total triiodothyronine (FT3, T3) by commercially available radio-immunoassay kits. Results: We found that mean T4 increased progressively during pregnancy. Serum levels of T3 increased in the second trimester and then declined during the third trimester compared with nonpregnant women. We also found that FT4 strongly decreased during the third trimester. Free T3 declined in the second and third trimesters. The mean TSH did not significantly differ in the three trimesters compared to the levels of non-pregnant women. Thyroid function tests in pregnancy should be interpreted against gestational age-related reference intervals to avoid misinterpretation of thyroid function during pregnancy.