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)
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Greer et al. (1968) studied this question.