The inflammatory response of hyperthyroid and hypothyroid rats. Role of adrenocortical steroids.

The capacity to respond to inflammatory stimuli was tested in hyperthyroid and hypothyroid rats when thyroid defects, induced by hormone administration or thyroparathyroidectomy, respectively, were fully established. Whereas hyperthyroid rats presented consistently depressed inflammatory responses, hypothyroid rats responded in a normal fashion. Decreased reactions to intracutaneously injected histamine and serotonin, inhibited swelling reaction to carrageenin, injected into one of the hind paws, and depressed primary and secondary reactions to adjuvant (heat-killed M. tuberculosis), only occurred in the hyperthyroid group. In addition, only in this group of animals enlargement of the adrenal glands, reduced content of adrenal ascorbic acid, and decreased number of circulating eosinophils, which characterize a circumstance of adrenal cortical hyperactivity, were observed. A spontaneous reversal of the acute inflammatory response of hyperthyroid animals to carrageenin occurred 3-4 days after interruption of hormone administration, and this was coincidental with the return to normal of the previously enlarged adrenal glands. Similarly, specific inhibition of adrenal cortical steroid biosynthesis in hyperthyroid rats with aminoglutethimide, restored the previously depressed response to carrageenin, without interference with the increased levels of seric thyroxin, thus suggesting that the inhibitory effects of thyroid hormones on inflammatory responses are likely to be indirect. It is concluded that an excess of circulating thyroid hormones, but not their deficiency, can impair the development of inflammatory reactions, and that this effect, at least partially, depends on an increased secretion of adrenal corticosteroids.