Preferential secretion of triiodothyronine in man

Minireview: Defining the roles of the iodothyronine deiodinases: current concepts and challenges

As is typical of other hormone systems, the actions of the thyroid hormones (TH) differ from tissue to tissue depending upon a number of variables. In addition to varying expression levels of TH receptors and transporters, differing patterns of TH metabolism provide a critical mechanism whereby TH action can be individualized in cells depending on the needs of the organism. The iodothyronine deiodinases constitute a family of selenoenzymes that selectively remove iodide from thyroxine and its derivatives, thus activating or inactivating these hormones. Three deiodinases have been identified, and much has been learned regarding the differing structures, catalytic activities, and expression patterns of these proteins. Because of their differing properties, the deiodinases appear to serve varying functions that are important in regulating metabolic processes, TH action during development, and feedback control of the thyroid axis. This review will briefly assess these functional roles and others proposed for the deiodinases and examine some of the current challenges in expanding our knowledge of these important components of the thyroid homeostatic system.

Basal and thyrotropin-stimulated secretion rates of thyroglobulin from the human thyroid gland during surgery

The secretion rates of thyroglobulin were measured under basal conditions and after exogenous and endogenous thyrotropin stimulation in 23 patients during surgery. In 11 patients with normal thyroid glands the median secretion rate of thyroglobulin was 0.7 pmol/24 h. Higher secretion rates were observed in 8 patients with nontoxic nodular goiter (median 22 pmol/24 h; p < 0.001) and in 4 patients with thyroid malignancy (median 2.2 pmol/24h; p < 0.05). Within 10 min after administration of highly purified human thyrotropin into the thyroid artery an increase of the secretion rates was observed in the 9 patients studied with normal thyroid glands and 5 patients with nontoxic nodular goiter. TRH administration to 5 patients induced a slight but nonsignificant increase of the secretion rate of thyroglobulin after 20 min. These results show that thyroglobulin is secreted directly into the thyroid venous blood. They also confirm that thyrotropin is an important regulator of the thyroglobulin secretion.

Deiodination of T4 to T3 and rT3 by microsomes from normal human thyroid tissue

An enzyme system capable of monodeiodinating T4 to T3 and to rT3 in a microsomal fraction of normal human thyroid tissue is characterized. The pH optimum for 5'-monodeiodination of T4 to T3 was around 7.0 The Km (apparent) for the conversion of T4 to T3 was 5.7 mumoles/l. PTU, ipodate and rT3 inhibited T4 deiodination to T3. The rT3 inhibition was of the partial competitive type. rT3 accumulation could not be detected at pH 7.4 or lower. At higher pH values rT3 could be measured in the incubates; however, the concentration was much lower than that of T3. The pH optimum was 9.0. Added rT3 disappeared rapidly at pH 7.4. The results suggest a high affinity of the 5'-deiodinase in human thyroid tissue for rT3, as has previously been described in the rat, but in contrast to our previous findings in dog thyroid and liver. Using tissue from 3 highly differentiated thyroid medullary carcinomas, deiodination could not be demonstrated, while it was normal using paraneoplastic thyroid tissue from the same patients. This suggests that the deiodinases studied originated from the follicular and not the C-cells of the thyroid.

Thyroid hormone secretion rates: response to endogenous and exogenous TSH in man during surgery

The thyroid hormone secretory response to TSH was studied in twenty-eight patients undergoing thyroid and parathyroid surgery. Eighteen patients received bovine or human exogenous TSH by injection into a thyroid artery, and 10 received TRH to stimulate endogenous TSH secretion. Thyroid secretion rates of T4, T3, and rT3 were determined directly from measurement of blood flow and the hormone gradient across the gland. A significant secretory response was seen for all three hormones following TSH increase. T3 secretion accelerated more rapidly than that of T4 and rT3, thus reducing the T4/T3 and rT3/T3 ratios. The T4/rT3 ratio fell during the first 30 min but then increased. The responses correlated with the area under the curve of the TSH serum concentration, and were similar after administration of bovine and human exogenous TSH, and TRH. Conclusions regarding preferential secretion ought to be made by comparing ratios of thyroid hormone secretion with those of the hormone content of the gland, but our results indicate that TSH induces preferential secretion of triiodothyronines in man.