Non-parallel variations in the preferential secretion of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) from dog thyroid.

rT3, T3, and T4 were measured in effluents from perfused dog thyroids and in hydrolysates of the thyroids. rT3 was determined by a newly developed sensitive radioimmunoassay (sensitivity approximately 0.1 pg rT3/tube). The rT3 and T3 contents of thyroid effluent were always much higher than would be expected from the relative amounts of T4, T3, and rT3 in thyroid hydrolysate. In four two-sided thyroid perfusions the T4/rT3 ration (wt/wt) in thyroid hydrolysates was 59.3 +/- 26.3 (mean +/- SD) and in effluents from unstimulated thyroids it amounted to 15.1 +/- 7.8. The T4/T3 ratio in thyroid hydrolysates was 10.3 +/- 1.9 and the ratio in effluents from unstimulated thyroids was 6.0 +/- 0.3. A continuous infusion of TSH, 100 micromicron/ml, enhanced the secretion of all three iodothyronines. During the stimulation a biphasic response was observed in the T4/T3 and T4/rT3 ratios in the effluent. At first the preferential secretion of T3 and rT3 was enhanced. Then the T4/T3 ratio returned to prestimulation values while there was a further increase in the T4/rT3 ratio to nearly twice the prestimulation values. Dog thyroids stimulated and examined in vivo showed a similar preferential secretion of rT3 and T3. Thus, both rT3 and T3 were secreted preferentially to T4 from the thyroid. The preferential secretions of rT3 and T3 did not vary in parallel. During a prolonged TSH stimulation the degree of preferential secretion of T3 was maintained despite a considerable increase in hormone secretion. On the contrary, the preferential secretion of rT3 decreased during prolonged TSH infusion.