Insulin secretion provoked by glucose in the pancreas of the rat is not modified by addition of thyroxine to the incubation media. Secretion of insulin is reduced (by 34 per cent) by prior thyroidectomy and restored to normal after administration of thyroxine (15 μg./kg. body weight) in vivo to thyroidectomized rats. These changes occur despite normal insulin content of the pancreatic tissue. Treatment of normal rats by higher doses of thyroxine (2,000 μg./kg.) causes marked reduction (ca 40 per cent) of both pancreatic insulin content and secretion. These alterations suggest that thyroid hormones may exert a delicately balanced influence upon islet function in a chronic process.

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