Normal gastric inhibitory polypeptide response to oral glucose in hypothyroidism

Although the action of gastric inhibitory polypeptide (GIP) on the beta cells of the pancreas is well documented, the effect of this hormone on insulin secretion in patients who are hypothyroid has not been studied. Hypothyroid patients demonstrate increased serum immunoreactive insulin levels in response to oral glucose when compared with euthyroid subjects. We postulated that a delayed and exaggerated response of GIP could account for the hyperinsulinaemic response. Nine thyroidectomized patients (aged 44.9 +/- 3.6 years) who were otherwise healthy but undergoing re-evaluation for recurrence of thyroid carcinoma, were given a 75 g oral glucose tolerance test (OGTT). These subjects were studied 6 weeks after thyroid hormone replacement had been stopped and while on hormone treatment. The serum glucose and plasma GIP responses to oral glucose were similar in the euthyroid and hypothyroid states. The serum insulin response as well as the areas under the curve for insulin following OGTT were significantly elevated (P < 0.01) during hypothyroidism. Solid-phase gastric emptying times studied in six patients who were euthyroid and hypothyroid were not different (35 +/- 12 versus 36 +/- 14 min respectively). None of the subjects had detectable levels of serum thyroglobulin, microsomal or parietal cell antibodies. In summary, we have confirmed a hyperinsulinaemic response to an OGTT and normal solid-phase gastric emptying rates in this form of hypothyroidism. We did not find significant differences in serum glucose or GIP responses and postulate this as evidence of resistance to the effects of endogenous insulin. The mechanism of alterations in carbohydrate tolerance in the hypothyroid state continue to remain unknown.