Maternal thyroid function during pregnancy and puerperal period

It has been noted that hypothyroidism in pregnant women can adversely affect the children's subsequent psychoneurotic development. Also, transient elevation of serum free thyroxine is occasionally seen in the first trimester of normal pregnancy. However, normal thyroid function during pregnancy and the puerperal period has not been clearly defined in Japan. The aim of this study was to assess maternal thyroid function during pregnancy and puerperal period in Japan. The concentrations of thyroid stimulating hormone (TSH), free triiodo-thyronine (free T(3)), free thyroxine (free T(4)) and thyroid binding capacity (TBC) of 522 normal pregnant and puerperal women (119 in the first trimester; 132 in the second trimester; 135 in the third trimester and 136 in the early puerperium) were measured by electrochemiluminescence immunoassay. We compared the measured data with those of healthy nonpregnant control. Twenty-six (21.8%) of 119 women in the first trimester had lower TSH levels and 23 (16.9%) of 136 women in the early puerperium had higher TSH levels than the normal range of healthy nonpregnant controls. Free T(3) gradually decreased during pregnancy, although it remained within the normal control range. Eight (6.7%) of 119 women in the first trimester had high free T(4) levels, which gradually decreased during pregnancy. Sixty (44.4%) of 135 women in the third trimester had low free T(4) levels. The values of TBC in the second trimester increased compared with the first trimester and did not change in the third trimester and decreased after delivery. There were no correlations between maternal TSH and levels of thyroid hormones (free T(3) or free T(4)), except for TSH and free T(4) in the first trimester. In conclusion, we showed that maternal thyroid function, especially TSH and free T(4), changed during the course of pregnancy. In assessing the thyroid function associated with pregnancy, one needs to keep in mind the tendency toward low free T(4) levels in the third trimester and high TSH levels in the early puerperal period.