Ontogeny of thyroid hormone effect on tissue 5'-monodeiodinase activity in fetal sheep.

Most of the thyroxine (T4) in fetal mammals is deiodinated to the inactive metabolite, reverse triiodothyronine (rT3), via an iodothyronine 5-monodeiodinase in fetal tissues. Maturation of the tissue 5'-monodeiodinase (MDI) enzymes required for conversion of T4 to active triiodothyronine (T3) in the rat, an altricial species, occurs in the postnatal period. To characterize fetal maturation of the enzymes for active T3 production in a precocial species, 5'-MDI activities were measured in liver, kidney, and brain tissue homogenates of ovine fetuses 13 days after total thyroidectomy (Tx) conducted at gestational ages of 99-107 or 129-132 days. Sham-operated twin fetuses served as controls. Hepatic type I 5'-MDI activity was not significantly lowered by Tx in group I but was significantly lower after Tx in group II fetuses. Renal type I 5'-MDI was not affected by Tx in either group. Type II 5'-MDI activity in cerebral cortex was significantly elevated after Tx in both groups I and II fetuses. Tissue sulfhydryl contents were similar in liver, kidney, and cerebral cortex from control and Tx fetuses in group I. These data indicate that hypothyroidism induced early in the third trimester is associated with increased brain type II 5'-MDI activity without significant change in liver or kidney type I 5'-MDI. Late third trimester hypothyroidism is associated with decreased type I 5'-MDI activity in liver homogenates as well as increased type II 5'-MDI activity in brain tissue.(ABSTRACT TRUNCATED AT 250 WORDS)