Triiodothyronine and Thyroxine in the Serum and Thyroid Glands of Iodine-Deficient Rats

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Abstract
Triiodothyronine (T3) and thyroxine (T4) were measured by immunoassay in the serum and thyroid hydrolysates of control (group A), mildly iodine-deficient (group B), and severely iodine-deficient rats (group C). These results were correlated with changes in thyroidal weight, 131I uptake and 127I content as well as with the distribution of 131I in Pronase digests of the thyroid. There was a progressive increase in thyroid weight and 131I uptake at 24 h with decrease in iodine intake. The 127I content of the thyroids of the group B animals was 44% and that of the group C animals 2% of that in group A. The mean labeled monoiodotyrosine/diiodotyrosine (MIT/DIT) and T3/T4 ratios in group A were 0.42±0.07 (SD) and 0.12±0.01, 0.59±0.06 and 0.11±0.03 in group B, and 2.0±0.3 and 1.8±0.9 in the group thyroid digests. Mean serum T4 concentration in the control rats was 4.2±0.6 (SD) μg T4/100 ml, 4.5±0.3 μg/100 ml in group B animals, and undectectable (4/100 ml) in group C animals. There was no effect of iodine deficiency on serum T3 concentrations, which were 44±9 (Mean±SD) ng/100 ml in A animals, 48±6 ng/100 ml n B animals, and 43±6 ng/100 ml in the C group. Thyroidal digest T3 and T4 concentrations were 39 and 400 ng/mg in group A animals and were reduced to 5 and 1% of this, respectively, in group C. The molar ratio of T3/T4 in the thyroid digests of the groups A and B animals was identical to the ratio of labeled T3/T4 and was slightly less (1.0±0.9) than the labeled T3/T4 ratio in the group C animals. The mean ratio of labeled T4 to labeled T3 in the serum of the severely iodine-deficient animals 24 h after isotope injection was 11±1 (SEM). With previously published values, it was possible to correlate the ratio of labeled T4/T3 in the thyroid digest with the labeled T4/T3 ratio in the serum of each iodine-deficient animal. This analysis suggested that the labeled thyroid hormones in the severely iodine-deficient rat were secreted in the ratio in which they are present in the gland. Kinetic analysis of total iodothyronine turnover indicated that two-thirds of the T3 utilized per day by the iodine-sufficient rat arises from T4. If the T4-T3 conversion ratio remains the same in iodine deficiency, then the analysis suggests that about 90% of the T3 arises directly from the thyroid. Therefore, it would appear that absolute T3 secretion by the thyroid increases severalfold during iodine deficiency. The fact that serum T3 remains constant and T4 decreases to extremely low levels, combined with previous observations that iodine-deficient animals appear to be euthyroid, is compatible with the hypothesis that T4 in the normal rat serves primarily as a precursor of T3.