A study during second trimester of pregnancy was conducted among 100 women from a population with mild to moderate deficient iodine intake (median 7 ${mu}g$/L)to monitor the changes in serum free thyroxin ($FT_4$), triiodothyionine ($T_3$) and thyrtropin (TSH). Thyroid related hormones were assessed using radioimmunoassay (RIA) and immunoradiometricassay (IRMA). As compared with non pregnant women, mean $FT_4$ levels decreased, $T_3$ levels increased and TSH levels were slightly depressed in pregnant women. Mean $FT_4$ concentration reached its highest level in the $4^{th}$ month and linearly decreased until the $6^{th}$ month. Mean $T_3$ level remained almost constant during $2^{nd}$ trimester of pregnancy. Mean TSH level remained unaltered during the $4^{th}$ and $5^{th}$ months and was slightly lower than that for the control group. However, these differences were statistically insignificant At the end of the second trimester, hypothyroxinemia and $T_3$ levels greater than the upper limit of the laboratory reference range were found in $34\%$ and $38\%$ of women, respectively. These changes in thyroid hormones reflect typical adaptations of the thyroid gland during pregnancy to the limited availability of iodine.