Pregnancy induces hormonal and metabolic changes that result in profound alterations of thyroid hormone economy and regulation. Adaptation of the pituitary-thyroid axis may be influenced by the iodine supply, especially iodine deficiency.
The aim of the study was to define characteristics of changes in certain biochemical parameters and regulation of thyroid function during pregnancy in a mildly iodine-deficient region of Hungary. Thirty-eight healthy pregnant women were enrolled in the study. The local ethical committee approved the study. Serial TSH, free thyroid hormone, total thyroid hormone, chorionic gonadotropin (hCG) and thyroid autoantibody levels were determined 5 times during gestation and 6 months after delivery. Data of 19 individuals were analyzed. To study the influence of pregnancy on the results of free thyroxin measurement, kits of five manufacturers were compared on 40 samples of women with varying gestational ages.
An increase of total T3 and T4 levels was observed parallel with changes of TBG concentration during the first 4 months of gestation. Serum TSH time-curve showed a transient fall in the first trimester, thereafter it returned to the non-pregnant values. Curves of serum TSH and hCG created clear mirror images. Free T4 concentrations elevated in line with the hCG peak at the beginning of gestation, thereafter it clearly followed the course of serum TSH. Free T3 levels gradually decreased throughout pregnancy.
The negative correlation between hCG and TSH levels, and the clear identity of the hCG+TSH and free T4 curves, suggest that thyroid function in pregnancy is the result of the two glycoprotein hormones, TSH and hCG. In pregnancy, total T3 may not be substituted for free T3 in thyroid function estimation, as total and free T3 levels do not correlate. Manufacturers non-pregnant reference ranges do not apply to pregnancy.