Maternal thyroid function in early and late pregnancy

Thyroid function was investigated during and after pregnancy in 12 healthy euthyroid women. During pregnancy, serum total T₄ (TT₄) levels were significantly elevated and nearly stable, while thyroxine-binding globulin (TBG) levels progressively increased till the 7th month. A slight elevation, though not significant, of free T₄ (fT₄) was recorded in early pregnancy. In the following months, fT₄, free T₃ (fT₃) and the T₄/TBG ratio progressively diminished, reaching a plateau at the 7th month. Serum TSH levels, measured by an ultrasensitive immunofluorometric assay, were comparable to postpartum values during the first trimester and showed a moderate upward trend with the progression of pregnancy. The evaluation of 24-hour TSH profiles was performed in 5 women during the first trimester of pregnancy. In all women, the circadian rhythm of TSH was present with a normal nocturnal surge, though anticipated in 1 case. In summary (1) during the first trimester of pregnancy, the increased thyroid activity does not seem to be only sustained by pituitary TSH which remains unmodified; the negative correlation between TSH and hCG levels might suggest that hCG also stimulates the gland to increase thyroid hormone output, and the presence of a normal TSH circadian rhythm indicates that the central mechanism of neuroregulation of the pituitary-thyroid axis is preserved in early pregnancy, and (2) in late pregnancy, a marked decrease in free thyroid hormone fractions is accompanied by serum TSH levels still in the normal range, indicating a modification of thyroid homeostasis which might recognize various etiological factors.