3:5:3′-Triiodothyronine. 2. Physiological activity

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Abstract
3,5,3[image]-Triiodo-L-thyronine was found to possess 5 times the activity of L-thyroxine when assayed by the goiter prevention test in rats. Orally administered L-triiodothyronine possessed about 86% of the activity of the same dose given subcut.; oral L-thyroxine only had about 40% of the activity of the same dose given subcut. D-Triiodothyronine had about 7% of the activity of L-triiodothyronine after subcut. admn. in the goiter prevention assay; DL-triiodothyronine had 59% of the activity of the L form. L-Triiodothyronine (1 [mu]g. daily) maintained a rate of growth in the thyroidectomized rat equivalent to that obtained with L-thyroxine (5 ug. daily). A large dose of triiodothyronine (20 [mu]g. daily) reduced the rate of growth and resulted in enlargement of the kidney cortex, heart and possibly the adrenal glands. L-Triiodothyronine (0.64 [mu]g. daily) was more effective than L-thyroxine (1 [mu]g. daily) in preventing the enlargement of the basophil cells and degranulation of the acidophil cells in the pituitary gland of the thiouracil-treated rat. L-Triiodo-thyronine (1 [mu]g. daily) also prevented an enlargement of the pituitary in the thyro-parathyroidectomized rat. From the above evidence it is suggested that triiodothyronine is the peripheral thyroid hormone and that thyroxine is its precursor.