[Diagnostic possibilities of the function of the fetal thyroid gland by amniocentesis (author's transl)]

The assessment of fetal thyroid function by measurement of reverse triiodothyronine (RT3) in amniotic fluid (AF) seems to be a valuable diagnostic tool in detecting fetal hypothyroidism. The concentrations of RT3 were measured by radioimmunoassay in 346 samples of AF which were obtained by transabdominal amniocentesis. Among these, 323 were obtained from normal pregnancies between weeks 8 and 41 of gestation and the remaining 23 from various complications of pregnancy: Rh-isoimmune disease, Down's syndrome, Klinefelter's syndrome (XXY), M. Krabbe, and anencephaly. The highest concentrations of RT3 in AF were observed between weeks 15 to 20 of gestation, followed by a gradual decrease with advancing gestational age. In complicated pregnancies a similar distribution of RT3 values was found. It is suggested that RT3 in AF should be measured in order to assess fetal thyroid function whenever amniocentesis is performed between weeks 16 and 20 of gestation, in particular when hypothyroidism of the fetus is suspected.