Insulin Secretion and Action in the Hyperthyroid Rat

Summary

To gain insight into the mechanism of the altered carbohydrate metabolism in thyrotoxicosis, intravenous glucose tolerance tests (IVGTT) and pancreatic suppression tests (PST) were performed in hyperthyroid rats (0.1 mg/kg T₄ × 5 days) to assess insulin secretion and action in vivo. Thyroid hormone injections significantly increased T₄ levels (182.8 nM ± 11.6 (SEM) versus 50.2 ± 6.4; P < 0.001) and baseline glucose concentrations (9.3 mM ± 0.2 versus 7.1 ± 0.2; P < 0.001). Body weights, basal insulin concentrations, glucose concentrations during IVGTT, glucose disappearance rates and steady state plasma glucose levels (SSPG) were normal. Insulin concentrations during the glucose tolerance test and during the PST were significantly decreased. The metabolic clearance rate of insulin (ml/min/kg ± SEM) was significantly (P < 0.01) increased (54.4 ± 3.5 versus 41.6 ± 2.3) in the hyperthyroid rats. If the different baseline glucose values were subtracted from the glucose concentrations achieved during the 2 tests, both the glucose disappearance rate and the fall in SSPG levels were significantly enhanced in the T₄-injected animals. Thus, in the hyperthyroid rat, insulin secretion is decreased, the clearance of insulin is increased and insulin sensitivity is either normal or possibly enhanced.

Key-Words:

Hyperthyroidism - Thyrotoxicosis - Rat - Insulin Secretion - Insulin Action