Studies on the effect of hyperthyroidism on the insulin response to hyperglycemia in the dog.

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We studied the effect of hyperthyroidism on the insulin response to hyperglycemia in the dog during a standard oral (OGTT) and intravenous (IVGTT) glucose tolerance test. The development of the hyperthyroid condition was elicited through chronic subcutaneous 1-thyroxin treatment at one of the following doses: 0.5 and 100 γ/kg body weight/day, for 10-12 days. Blood sugar and serum immunoreactive insulin (IRI) were measured. Normal fasting blood sugar and serum IRI levels remained unchanged in the dog, despite the thyroxin treatment. The rate of disappearance of intravenously injected glucose from blood was normal as well. However, a shorter than normal, 50% diminished and late insulin response to glucose was found in the hyperthyroid group (both thyroxin levels). The blood sugar curve during the OGTT was found to be normal during the mild degree of hyperthyroidism, which was accompanied with an earlier than normal and moderately maintained insulin response to hyperglycemia, whose maximum was 25% above normal. The blood sugar curve during the OGTT was normal in the severely hyperthyroid dogs only for the 60 minute-period following glucose administration. However, the blood sugar was still above control levels 30 minutes later in this group. A definitely reduced (low, late and shorter than normal) insulin response to hyperglycemia was found after thyroxin treatment (high level).