Thyroid function in fetus and mother during the second half of normal pregnancy

Thyroid hormones, thyroxine-binding globulin (TBG) and thryrotropin (TSH) concentrations were measured in 46 paired fetal and maternal blood samples collected between 17 and 36 weeks of gestation. The samples were selected retrospectively from fetuses that had undergone cordocentesis for prenatal diagnosis, had been found to be unaffected and confirmed healthy at birth. In maternal serum, total thyroxine (TT4) and triiodothyronine (TT3) concentrations were high, but free thyroxine (FT4) and free triiodothyronine (FT3) were within normal adult ranges; reverse T3 (RT3) increased and TSH levels decreased towards term. Fetal TT4, FT4, TT3, FT3, TBG and TSH levels significantly increased whereas RT3 sharply decreased with gestational age. The ratio of fetal TSH/FT4 significantly decreased, suggesting that the set point for negative feedback of pituitary TSH secretion is changing while the sensitivity of the thyroid gland to TSH increases throughout gestation. There was no significant correlation between the maternal and fetal TBG, TSH, TT4 and FT4, whereas maternal TT3 was positively correlated with fetal TT4, FT4, TT3 and FT3. Normal reference values for maternal and fetal iodothyronines, TBG and TSH throughout the second half of gestation provide insight into fetal thyroid development and may be useful for prenatal diagnosis.