A study of the serum 3,5,3'-triiodothyronine sulfate concentration in normal and hypothyroid fetuses at various gestational stages

We have studied T₃ sulfate (T₃S) levels, blindly, in coded plasma samples from 21 normal and 3 hypothyroid fetuses at different stages of gestation (19-42 weeks). Fetal plasma samples were obtained by cordocentesis. T₃S was detectable in all samples studied, with values ranging from 50-294 (mean ± SD, 130 ± 62 pmol/L). Plasma T₃S was low (3S ranged from less than 20 to 130 in another set of 18 control subjects (mean ± SD, 63 ± 32 pmol/L). Fetal T₃S values were positively correlated with gestational age (r = 0.43; P <0.05), but not with free T₄ (FT₄), FT₃, or TSH values. In the 3 hypothyroid fetuses at 31, 38, and 40 weeks gestation, respectively, plasma TSH was elevated (26, 98, and 24 mU/L, respectively), FT₄ was low (10, 6.7, and 7.5 pmol/L, respectively), and FT₃ was normal or high (3.2, 8.2, and 2.2 pmol/L, respectively). However, T₃S values in hypothyroid fetuses (88, 133, and 252 pmol/L, respectively) were similar to those in normal fetuses at corresponding gestational ages. We conclude that 1) T₃S is detectable in fetal circulation from at least 19 weeks gestation, and its concentration increases with fetal age; 2) plasma T₃S concentrations in the fetus at 19-40 weeks gestation are at least comparable to but generally higher than those in the adult; and 3) plasma T₃S levels in hypothyroid fetuses are similar to those in normal fetuses. Recent studies demonstrating the ability of some fetal rat tissues (e.g. cerebral cortex) to desulfate T₃S to T₃ have suggested a possible role of T₃S as a source of T₃. Normal T₃S in fetal hypothyroidism suggests that T₃S may contribute to attenuation of the effects of hypothyroidism during intrauterine life.