3′-Monoiodothyronine Sulfate and Triac Sulfate Are Thyroid Hormone Metabolites in Developing Sheep

We used novel 3′-monoiodothyronine sulfate (3′-T 1 S) and 3,3′,5-triiodothyroacetic acid sulfate (TriacS) RIAs to characterize sulfation pathways in fetal thyroid hormone metabolism. 3′-T 1 S and TriacS levels were measured in serum samples obtained from fetal ( n = 21, 94–145 d gestational age), newborn (NB, n = 5), and adult sheep (AD, n = 5) as well as from fetuses after total thyroidectomy (Tx), or sham-operated twin fetuses controls, conducted at gestational age 110–113 d ( n = 5). Peak levels (expressed as ng/dL) of both 3′-T 1 S and TriacS occurred at 130 d gestation. These levels in fetuses were higher than those in NB and AD. In Tx fetuses, there was a significant decrease in the mean serum level of 3′-T 1 S, but not TriacS. The decrease in 3′-T 1 S in Tx is similar to that observed for thyroxine sulfate (T 4 S) and 3,3′,5′-triiodothyronine sulfate (rT 3 S), whereas TriacS levels were not altered in the hypothyroid state, similarly to 3,3′,5-triiodothyronine sulfate (T 3 S). These data demonstrate that 3′-T 1 S and TriacS are normal thyroid hormone metabolites in ovine serum and that TriacS is likely derived from T 3 S or from the same precursor(s) as T 3 S.