Qualitative Changes in the Secretion of Thyroid Hormones Induced by Iodine Deficiency

The relative proportion of total radioactivity secreted by the thyroid gland as triiodothyronine (T3) or as thyroxine (T4) in rats made iodine deficient to varying degrees was investigated using single-pass perfusion of 131I prelabeled thyroid glands in situ with nonradioactive blood. The T3/T4 ratio of the thyroid effluent increased progressively with the length of time rats were iodine deficient and averaged > 3 at 11 months. T3/T4 was almost invariably higher in the thyroid effluent than in the thyroid gland itself. No significant secretion of iodotyrosines was found, even in severely iodine-deficient rats. It is concluded that iodine deficiency results in at least as great an increase (and possibly greater) in the T3/T4 ratio of iodothyronines secreted as in those synthesized. This is an important adaptive mechanism in maintaining a euthyroid state in the face of inadequate iodine substrate. (Endocrinology83: 1193, 1968)