Ontogenesis of the nuclear 3,5,3'-triiodothyronine receptor in the human fetal brain.

A high-affinity T3 binding site with the binding specificity of the nuclear T3 receptor is present in the brain of the human fetus at midgestation. Its concentration was found to be very low at 10 weeks of gestation, and increased by a factor of 10 up to the 16th week, in coincidence with the period of neuroblast multiplication. Liver, heart, and lung also contained receptor. Both T4 and T3 were present in the brain, as measured by RIA. In other tissues, however, only T4 was detected, suggesting that brain T3 in the fetus arises from local 5'-deiodination of T4. The results suggest that the human fetal brain is a potential target of thyroid hormone at midgestation.