Abstract

The effects of thyroxine and triiodothyronine on the adrenal cortex have been studied in rats; ascorbic acid and liver glycogen concentrations and adrenal and thymus weights were employed as measures of adrenocortical activity. The most important results can be listed as follows:

1. Thyroxine and triiodothyronine stimulate adrenocortical function, as indicated by loss of adrenal ascorbic acid after single injections, and by adrenal hypertrophy after chronic administration; in these tests, the relative potency of thyroxine and triiodothyronine is approximately 1 to 5 on a weight basis.

2. Thyroxine and triiodothyronine modify thymus weight and liver glycogen concentration by a dual mechanism: by acting directly on the thymus and the liver, respectively, and by an indirect action mediated through an increased secretion of 17-hydroxy-11-oxysteroids from the adrenals. Whereas in intact animals thyroxine, but not triiodothyronine, markedly decreases thymus weight, in adrenalectomized rats both hormones produce a noticeable enlargement of this organ. Similarly, after single injections of thyroxine and triiodothyronine liver glycogen content is increased or decreased, depending on which of the two actions prevails, the direct (glycogenolytic) or the indirect (glycogenetic) action.

3. The observation that the potency ratio of thyroxine and triiodothyronine varies with the test used is considered contributive evidence to the assumption that each of the two thyroid hormones has a specific and different effect on the hypophyseal-adrenocortical system with the result that there is a difference in the rate of release of the various types of adrenal steroids.