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

3,3'-diiodothyronine sulfate (T 2 S) 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 T 3 from fetal serum (disappearance T 1/2 of 0.7 h versus 2.4 h for T 4 in the first 4 h). The metabolites found in fetal serum after labeled T 3 infusion were T 2 S > T 3 > T 3 S; in maternal urine, T 2 S > unconjugated iodothyronines (UI) > T 3 S > unknown metabolite (UM). After labeled T 4 infusion, the metabolites in fetal serum were rT 3 > T 3 >T 2 S > T 4 S in the first 4 h, and rT 3 = T 3 = T 4 S = T 2 S > T 3 S at 24 h; in maternal urine we found T 2 S > UM > UI > T 4 S > T 3 S in the first 4 h and UM > T 2 S > UI in 4-24 h pooled sample. In conclusion, the conversion of T 3 to T 2 S followed by fetal to maternal transfer of T 2 S and other iodothyronines appears to contribute importantly to maintaining low fetal T 3 levels in late gestation.