Thyroxine dehalogenation by rat pancreatic islets was studied incubating 
isolated islets with labelled T4. [125I]T4 added to the incubation medium was 
deiodinated by the islets with the consequent production of T3, rT3 and iodide. 
This deiodination process showed a clear glucose-dependence, being significantly 
increased in the presence of 16.6 mmol/l glucose. The existence of high and low 
affinity binding sites for T3 was also demonstrated incubating [125I]T3 with 
islets under different experimental conditions. The properties of these binding 
sites were greatly influenced by the extracellular concentration of glucose. 
Addition of T3 to the incubation medium, significantly modified the insulin 
release, but its effect varied according to the glucose concentration in the 
medium, i.e. it enhanced the insulin release at a glucose concentration between 
2 to 8 mmol/l; it has no effect at 12 mmol/glucose, and significantly inhibited 
the secretion of insulin in the presence of 16.6 mmol/l glucose. Our results 
suggest that thyroid hormones might play a direct regulatory effect on insulin 
secretion, probably mediated by its deiodination and interaction with specific 
receptors in the islet cell.