The influence of hypo- and hyperthyreosis on insulin receptors and metabolism

Changes in thyroid status affect metabolism not only directly, but influence it also by alterations in insulin secretion and action. Despite several investigations, these effects are, however, poorly characterised or even controversial. The aim of the studies was to investigate the effect of hyperthyreosis (HT) and hypothyreosis (HPT) on insulin binding by rat liver membranes. Some metabolic parameters reflecting insulin and thyroid hormones action were also determined. HT and HPT were developed by daily administration for 3 weeks of thyroxine (T (4) ) and thiouracil (TU), respectively. Experimental hyperthyreosis and hypothyreosis caused deep changes in metabolism. The greatest alterations were observed in body and thyroid glands weight, blood triiodothyronine (T (3) ), T (4), glucose, and insulin levels, liver glycogen amount and number of insulin receptors. HT reflected in rats in slower rate of growth and in smaller thyroid glands weight. In comparison to controls, T (4) concentration in HT was almost doubled and it was reduced by about 30% in HPT. Also, T(3), insulin and glucose levels in HT were heightened. Simultaneously, binding of insulin to liver membranes was elevated in HT and reduced in HPT. In HT the number of high affinity insulin receptors (HAIRs) and low affinity insulin receptors (LAIRs) was increased, whereas in HPT the amount of HAIRs was diminished. HT caused a drastic reduction of glycogen concentration in liver, but no changes were observed for muscle glycogen. Considering lipid metabolism, only free fatty acids (FFA) level in blood was changed (in HPT), but no differences were observed in serum concentration of triglycerides and cholesterol. Several metabolic changes observed in HT and HPT seem to be the dire ct consequence of alterations of thyroid hormone concentrations. These disturbances, together with the direct effect of HT or HPT on insulin secretion, binding and action lead, in turn, to changes in the other metabolic parameters. As a result of these disturbances the adaptive mechanisms appear. One of them is change in the number of insulin membrane receptors taking place even against the well known "down-regulation" theory.