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

G. M. Abrams; P. R. Larsen

doi:10.1172/JCI107443

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 (<0.5 μ₄/100 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.