Pregnancy is associated with significant, but reversible changes in thyroid function studies, 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 in Tabriz-Iran. A case-control study designed with 229 normal pregnant women that randomly selected from the first (64 samples), the second (92 samples), and the third (73 samples) trimesters and 250 randomly selected non-pregnant healthy female controls. Age range in both groups was 16-40 years. Thyroid function tests carried out by measuring serum levels of thyroid stimulating hormone (TSH), free and total thyroxin (FT(4), TT(4)), and free and total triiodothyronine (FT(3), TT(3)) by commercially available radio immunoassay kits. We found that mean TT(4) increased progressively during pregnancy. Our study showed increasing in serum levels of TT(3) in the second trimester and then declining during the third trimester compared with non-pregnant women. We showed that FT(4) strongly decreased during the third trimester. Free T(3) showed declining in the second and third trimesters. Mean TSH did not show significant difference in each trimester compared with non-pregnant women. The thyroid function tests in pregnancy should be interpreted against gestational age-related reference intervals to avoid mis-interpretation of thyroid function during pregnancy.