3,3 '-Diiodothyronine sulfate excretion in maternal urine reflects fetal thyroid function in sheep

We have shown that there is significant fetal-to-maternal transfer of sulfa ted metabolites of thyroid hormone after fetal infusion of a pharmacologic amount of 3,3 ' ,5-triiodothyronine (T-3) or sulfated T3 in late pregnancy in sheep (Am J Physiol 277:E915, 1999). The transferred iodothyronine sulfo conjugate, i.e. 3,3 ' -diiodothyronine sulfate (T2S), of fetal origin appea rs in maternal sheep urine. The present study was carried out to assess the contribution of T2S of fetal origin to the urinary pool in ewes. Eighteen date-bred ewes (mean gestational age of 115 d) and their twin fetuses were divided into four groups. In group I (control, n = 5), both ewes (M) and th eir fetuses (F) were sham operated for thyroidectomy (Tx). In group II, the ewes (MTx, n = 4) and, in group III, the fetuses (FTx, n = 4) were subject ed to Tx. In group IV (MTx . FTx, n = 5), both the ewe and fetus had Tx. Af ter 10-12 d, fetal and/or maternal hypothyroidism were confirmed by serum t hyroxine (< 15 nmol/L) measurements. In addition, we infused radioactive T3 without disturbing the T3 Pool in three singleton near-term fetuses and as sessed the amount of radioactive iodothyronine that appeared in maternal ur ine (MU). After infusing [I-125-3 '],3,5-T-3 via fetal vein to the near-ter m normal fetuses, radioactive T2S was identified as the major metabolite in MU by HPLC and T2S-specific antibody. MU T2S excretion (pmol/mmol creatini ne) was significantly reduced by FTx and MTx . FTx but not by MTx. In addit ion, positive correlations (p < 0.01) were found between MU T2S excretion a nd fetal serum thyroxine and T-3 concentrations but not with maternal serum thyroxine or T-3 levels. T2S of fetal origin contributes significantly to the MU pool.