Objective: The objectives of this study were to examine the effects of hyperthyroidism on glucose tolerance, insulin secretion, and insulin sensitivity.
Material and Method: Thirty-eight patients with hyperthyroidism and twenty-six healthy volunteers with matching age and body mass index were included. Patients with conditions known to affect glucose metabolism were excluded. An oral glucose tolerance test was performed after the diagnosis of hyperthyroidism and again when they achieved euthyroid state. Areas under the glucose and insulin curves were used to assess plasma glucose and insulin responses, respectively. β-cell function was determined by the corrected insulin response (CIR) and homostatic model assessment model 2 (HOMA2-%B). Peripheral insulin sensitivity was determined by the insulin activity (IA) and HOMA2-%S.
Result: The prevalence of glucose intolerance in hyperthyroid state was 39.4% [impaired glucose tolerance (IGT) 31.5% and diabetes mellitus (DM) 7.9%]. This was significantly higher than that of 30.7% [IGT 19.2% and DM 11.5%] in healthy volunteers (p < 0.05). Glucose intolerance was associated with higher systolic blood pressure, higher mean arterial pressure, lower CIR, and higher T4 levels but not with the levels of T3. IA and HOMA2-%S significantly improved when achieving a euthyroid state despite the increase in body mass index.
Conclusion: In conclusion, glucose intolerance is common in hyperthyroidism. Both impaired insulin secretion and decreased peripheral insulin sensitivity are the factors contributing to the development of abnormal glucose tolerance in the hyperthyroid state.

Keywords: Hyperthyroidism, Oral glucose tolerance, Diabetes mellitus, Insulin resistance