Maturation of Hypothalamic-pituitary Control of Thyroid Function in the Human Fetus

Abstract

The human fetal thyroid is capable of concentrating iodine and synthesizing hormone by 12 weeks. Fetal thyroxine (T4) production is thyrotropin (TSH) dependent and the fetal pituitary contains TSH by 10–12 weeks. Little is known, however, about maturation of neuroendocrine control of fetal thyroidfunction. Serum T4, free T4 (FT4) and/or immunoreactive TSH were measured in paired maternal and cord blood specimens obtained at the time of therapeutic abortion or lpremature delivery of 35 pregnancies 11 to 34 weeks in duration and 17 pregnancies at term. Fetal pituitary were obtained when possible for determination of TSH content. Maternal T4 and TSH did not change between 11 and 40 weeks but the mean FT4 between 11 and 18 weeks was higher than the value at term. Fetal T4, FT4 and TSH were low between 11 and 18 weeks. TSH and T4 increased abruptly between 18 and 22 weeks and, with FT4, increased progressively thereafter to term. Fetal pituitary TSH content also increased abruptly between 18 and 22 weeks. There was no correlation between maternal and fetal serum T4, FT4 or TSH values. The data indicate that a) the fetal pituitary thyroid system; b) rapid maturation of the fetal hypothalamic-pituitary control occurs between 18 and 22 weeks; c) fetal T4 secretion increases progressively between 24 weeks and term.