Thyroxine and triiodothyronine uptake by the brain of chickens.

The instant uptake of 125-I-labelled triiodothyronine and 125-I-thyroxine by the brain tissue was measured in sham-operated (SH) controls, thyroidectomized (TX) and thyroidectomized plus iodothyronine supplemented (TX+T3 or TX+T4) young chickens. It was found that thyroidectomy significantly increases the uptake of iodothyronines, and iodothyronine supplementation returns the uptake close to sham-operated levels. The characteristics of the inhibition of the uptake of labelled hormones by non-labelled iodothyronines speak in favour of the presence of specific, separate uptake sites for T3 and T4 in the blood-brain barrier of young chickens.