Fetal-to-maternal transfer of thyroid hormone metabolites in late gestation in sheep

3,3'-diiodothyronine sulfate (T2S) derived from T-3 of fetal origin is transferred to the maternal circulation and contributes significantly to the maternal urinary pool. The present study quantitatively assesses the fetal to maternal transfer of T-4 metabolites compared with those of T-3. Labeled T-4 or T-3 was infused intravenously to four singleton fetuses in utero in each group at gestational age 138 +/-: 3 d. Maternal and fetal serum and maternal urine samples were collected hourly for 4 h and at 24 h (serum) or in pooled 4-24 h samples (urine). Radioactive metabolites were identified by HPLC and by specific antibody in serum and urine extracts and expressed as percentage infusion dose per liter. The results demonstrate a rapid clearance of labeled T3 from fetal serum (disappearance T-1/2 of 0.7 h versus 2.4. h for T, in the first 4 h). The metabolites found in fetal serum after labeled T-3 infusion were T2S > T-3 > T3S; in maternal urine, T2S > unconjugated iodothyronines (UI) > T3S > unknown metabolite (UM). After labeled T-4 infusion, the metabolites in fetal serum were rT(3) > T-3 > T2S > T4S in the first 4 h, and rT(3) = T-3 = T4S = T2S > T3S at 24 h; in maternal urine we found T2S > UM > UI > T4S > T3S in the first 4 h and UM > T2S > UI in 4-24 h pooled sample. In conclusion, the conversion of T-3 to T2S followed by fetal to maternal transfer of T2S and other iodothyronines appears to contribute importantly to maintaining low fetal T-3 levels in late gestation.