Studies on the effect of thyroxine onin vivo insulin secretion as modified by hypophysectomy

Summary Blood sugar and serum immunoreactive insulin (IRI) concentrations were measured in uninjected and thyroxine-injected hypophysectomized dogs as well as in untreated normal dogs. Thyroxine was used at the following two levels: 0.5 and 100 /kg body wt./day, for 10 days. Serum IRI levels in the dog in the post-absorptive conditions were unaffected by both treatments. Mildly subnormal blood sugar levels were detected, in the hypophysectomized dog in the post-absorptive condition and were fully corrected by thyroxine therapy (both levels). The rate of disappearance of glucose from the blood was normal in the hypophysectomized control dogs and was unaffected by thyroxine therapy. The moderate increase in the general mean of the blood sugar throughout the test found after hypophysectomy was corrected by thyroxine (low level); thyroxine therapy (high dose) not only failed to correct the increase but in fact induced a further increase. The typical slow, low and maintained insulin response to hyperglycaemia was found in the hypophysectomized control dogs, and it remained unchanged despite the thyroxine treatment (both levels). We concluded that the presence of the pituitary gland is needed so that the thyroxine-induced inhibition of the insulin secretory response to hyperglycaemiain vivo may be exerted.This paper was partially presented at the Meetings of the VII Congress Intern. Diabetes Fedn, Buenos Aires, Argentina, 23–8 August, 1970.