3,3',5'-Triiodothyronine (Reverse T3) in amniotic fluid and cord serum.

3,3',5'-triiodothyronine (reverse T3, rT3) was measured in 122 samples of amniotic fluid obtained between the 29th--40th weeks of gestation, and in the blood of 37 newborn and their mothers. The mean rT3 concentration in amniotic fluid was 0.769 +/- 0.47 ng/ml, with a slight decrease from the 29th--40th weeks of gestation which was not statistically significant. Because of the great scatter of rT3 concentrations in amniotic fluid, its estimation does not seem to be useful in prenatal diagnosis of hypothyroidism. The mean rT3 concentration in cord blood (2.62 ng/ml; range 1.4--4.9 ng/ml) was greatly elevated in comparison to the mean maternal level at delivery (0.34 ng/ml; range 0.08--0.69 ng/ml). The possible use of rT3 measurements in cord blood as a screening test for congenital hypothyroidism is discussed.