Levels of triiodothyronine and reverse triiodothyronine in the thyroid glands of man and swine at different stages of development]

3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) were measured by radioimmunoassay in saline extracts of neonates and human adult thyroid tissues and of fetuses, Piglets and adult Swine thyroid tissues. In all these extracts, T3 content was higher than rT3 content whatever the period of development. Both triiodoamino acids represent a small percentage of the iodinated protein in thyroid tissues.