AMNIOTIC-FLUID 3,3'-DIIODOTHYRONINE AND 3',5'-DIIODOTHYRONINE IN FETAL HYPO-THYROIDISM IN SHEEP

The concentration of rT3 in human amniotic fluid (AF) at term is higher than in maternal serum and about 50%that of fetal serum. On the other hand, rT3 concentrations in sheep AF are very low (< 10–20 ng/dl) compared to both maternal (24–85 n/gl) and fetal (400–740 ng/dl) sera. This finding led us to consider the possibility that rT3 in sheep may normally be metabolized to 3, 3′-diiodothyronine (3, 3′-T2) and/or 3′, 5′-T2 and that measurement of T2S in AF may be useful in recognition of fetal hypothyroidism. The concentrations of T2s were measured by specific RIAs in maternal sera and term AF and sera from normal and previously thyroidectomized neonatal (10–60 min of age) sheep; thyroidectomy was performed surgically in utero at 50–93 days gestation (term in sheep is about 145 days). The mean (±SE) serum 3, 3′-T2 concentration of 15 ± 12 ng/dl in 3hypothyroid neonates was significantly lower than the corresponding value of 98 ± 7.4 ng/dl in 10 normal neonates (P < 0.001) and lower (borderline) than that of 24 ± 2.4 ng/dl in 9 maternal specimens (0.05 > P < 0.1). Serum 3′, 5′-T2 concentrations were undetectable (<1.5 ng/dl) in hypothyroid neonates; the corresponding mean values were 14 ± 1.8 ng/dl in normal neonates and 10 ± 0.6 ng/dl in maternal specimens. The concentrations of 3, 3′-T2 [7.5 ± 1.4 (n = 3) vs. 25 ± 1.9 (n = 10) ng/dl; p < 0.001] and 3′, 5′-T2 [4.9 ± 1.1 (n = 4) vs. 14 ± 1.3 (n = 9) ng/dl; P < 0.001] in AF of pregnancies with hypothyroid fetuses were also clearly less than those in normal pregnancies. The various data suggest that 1) 3, 3′-T2 and 3′, 5′-T2 are normally clearly detectable in sheep AF at a time when rT3 (or T3) is very low or undetectable and 2) low AF 3, 3′-T2 and 3′, 5′-T2 are reliable indicators of fetal hypothyroidism. By inference, AF rT3 may be useful in antenatal diagnosis of hypothyroidism in species (e.g. man) where it is normally present in easily measureable concentrations. © 1979 by The Endocrine Society.