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

Abstract

A B S T R A C T 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, 'I uptake and "2I content as well as with the distribution of 131I in Pronase digests of the thyroid. There was a progressive increase in thyroid weight and '1I uptake at 24 h with decrease in iodine intake. The PI 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) Ag T4/100 ml, 4.5+0.3 Ag/100 ml in group B animals, and undectectable (<0.5 tL/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 Ts and To 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 Ta/Ti in the thyroid digests of the groups A and B animals was identical to the ratio of labeled Ts/T4 and was slightly less (1.0±0.9) than the labeled T3/T ratio in the group C animals. 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 Ts utilized per day by the iodine-sufficient rat arises from T4. If the Tr-Ts 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 Ts 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 iodinedeficient animals appear to be euthyroid, is compatible with the hypothesis that T4 in the normal rat serves primarily as a precursor of T3. INTRODUCTIONThe thyroidal response to iodine deficiency has been an area of active investigation for many years. Previous studies in the rat reviewed by Studer and Greer indicate that when iodine intake is severely restricted there is an increase in thyroid weight, a decrease in protein-bound iodine (PBI)1 and an altered pattern of tracer iodine distribution in the thyroid gland (1). The last-mentioned changes include an increase in labeled monoiodotyrosine (MIT) and decrease in labeled diiodotyrosine (DIT) as well as a progressive increase in the ratio of labeled triiodothyronine (T3) to labeled thyroxine (T4). ...

Tumor Promoting Effect of Goitrogens on the Rat Thyroid

“…It strongly suppressed thyroid hormones, increased TSH, induced typical morphologic changes and promoted tumorigenesis of the thyroid as previously reported by others (1,3,9).…”

Specificity of Co-Promoting Effects of Caffeine on Thyroid Carcinogenesis in Rats Pretreated with N-Bis(2-hydroxypropyl)nitrosamine

“…In rats maintained on a low-iodine diet, serum T 4 levels decrease to very low values, whereas the T 3 level is relatively well maintained (Abrams and Larsen, 1973;Riesco et al, 1977). The maintenance of serum T 3 levels in ID rats involves increase in thyroidal biosynthesis of T 3 relative to T 4 (Abrams and Larsen, 1973). In the present study, both ID and PB alone slightly increased serum T 3 and T 4 , in addition to TSH levels.…”

Sequential deiodination of thyroxine in rat liver homogenate

“…It has become evident that under normal conditions the products of the thyroid, 3,3',5,5'-tetraiodothyronine (thyroxine) and 3,3',5-tri-iodothyronine (tri-iodothyronine), are secreted in a ratio that far exceeds the ratio of their turnover rates (Abrams & Larsen, 1973;Chopra & Solomon, 1973). This means that most of the circulating tri-iodothyronine is of extrathyroidal origin, which is in line with previous observations on plasma tri-iodothyronine concentrations in athyreotic subjects treated with thyroxine (Braverman et al, 1970).…”