Effect of thyroid hormone levels upon pancreatic islet function.

The effect of thyroid hormone deprivation upon insulin secretion was studied in the rat, in relation to glucose oxidation, calcium uptake, insulin content and morphology of pancreatic islets. These parameters were also studied after the administration of a substitutive hormone therapy. The deficit in circulating thyroid hormone levels was followed by a smaller glucose-induced insulin secretion, glucose oxidation rate and calcium uptake. At the ultrastructural level a large amount of secretory granules with a marked enlargement of their clear halo, as well as signs of hyperactive granular endoplasmic reticulum were depicted. These changes only affected the B cell islet population. The number, volume and insulin content of the pancreatic islets were not affected. The metabolic and secretory changes, as well as the ultrastructural modifications, were corrected when circulating T3 and T4 levels return to normal values, after thyroid hormone administration. On account of our results, we might suggest that insulin secretion is regulated, at least in part, by the circulating levels of thyroid hormones. This control might be exerted through the effect of these hormones upon the glucose oxidation and calcium uptake rates, as well as on the features of B cell organelles.