The recognition of abnormality in thyroid function tests during pregnancy is important for the welfare of the mother as well as fetus. The values of serum tri-iodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH) in nonpregnant women are not applicable during pregnancy and also differ in iodine deficient areas. In the present study, one hundred and twenty-four apparently normal, healthy young primigravidas with no known metabolic disorders and normal carbohydrate gestational intolerance test, consecutively attending the antenatal clinic were included in the study. The serum tri-iodothyronine (T3), thyroxine (T4) and thyroid-stimulating hormone (TSH) in these women were estimated. In the first trimester, the mean T3 values were found to be 1.85 nmol/L, which increased to a mean of 2.47 nmol/L in the second trimester and declined in the third trimester to 1.82 nmo/L. Mean T4 levels were also seen to rise from 164.50 nmol/L in the first trimester to 165.80 nmol/L in the second trimester and then decreased in the third trimester to 159.90 nmol/L. Mean TSH levels were seen to rise progressively through the three trimesters of pregnancy from 1.20 microlU/ ml in the first trimester to 2.12 microlU/ml in the second trimester and further to 3.30 microlU/ml in the third trimester of pregnancy. Three asymptomatic pregnant women (2.5%) were found to have abnormal TSH values with normal T3 and T4 levels and good obstetric outcome. This pilot study also indicates the range to T3 as 1.7 - 4.3 nmol/L in second trimester and 0.4 - 3.9 nmol/L in third trimester, T4 as 92.2 - 252.8 nmol/L in second trimester and 108.2 - 219.0 nmol/ L in third trimester, and TSH as 0.1 - 5.5 microlU/ml in second trimester and 0.5- 7.6 microlU/ml in third trimester of pregnancy.