MATURATION OF THYROID FUNCTION IN NORMAL HUMAN FOETUSES

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Abstract
The functional maturation of an independent fetal thyroid activity was investigated in the present study. Serum concentrations of total T4, free T4, TSH and TBG were measured in 23 fetuses between 18 and 31 weeks'' gestational age. Fetal samples were collected by transabdominal needling from the placental cord insertion. TT4, FT4, TBG and TSH levels significantly increased with gestational age. FT4 levels were comparable with the adult range by 28 weeks'' gestation; TBG levels reached adult values at approximately 30 weeks, while TT4 was lower than adult levels throughout the whole period studied. TSH values were, in all cases, higher than the normal adult range. A significant positive correlation was present between TT4 and TBG, TT4 and TSH, and TBG and TSH levels; on the contrary, no correlation was deomonstrated between FT4 and TSH levels. The TSH/TT4 ratio significantly decreased with gestational age. The results suggest an incomplete responsiveness of the fetal thyroid gland to TSH, while the feedback control system between pituitary and thyroid is operating at a different set point from that in post-natal life. The normal range of thyroid parameters established is of clinical relevance for the antenatal diagnosis, and eventual treatment, of thyroid disorders that may seriously damage fetal development and maturation.