Introduction: In overt hyperthyroidism, although impaired glucose tolerance has long been observed as a frequent complication, the impact of subclinical hyperthyroidism on glucose metabolism is unclear. Therefore, we compared glucose tolerance tests in patients with subclinical hyperth­yroidism and a group of healthy controls. Materials and Methods: Forty- five patients with a drug history of therapy with Levothyroxine  and 14 healthy controls  were enrolled and T4, T3, T3RU, TSH were measured prior to and TSH again after intravenous injection of 250 µg TRH. Oral glucose tolerance test was performed after 7 to 11 days. The data obtained for patients with subclinical hyperthy­roidism and euthyroid healthy controls were compared. Results: Twenty-five patients and one control had subclinical hyperthyroidism based on basal TSH. Basal glucose concentrations were not significantly different between the subclinical hyperthyroidism and euthyroidism group. Plasma glucose levels (milligrams per dl ± SEM) at 90 and 120 minutes after ingestion of glucose were significantly higher (p=0.005 and p=0.06 respectively) in the subclinical hyperthyroid (SHR) patients (162 ± 17, 146 ± 16 respectively) compared to euthyroid (EU) normal subjects(101±7 and 94±5 respectively). SHR patients showed higher postprandial glucose concentrations (area under the curve, AUC90, 120 18072±4779 and 18130±1758 mg/dlmin ± SEM respectively) versus EU normal subjects (AUC90, 120 1045±516, 13360 ± 643 mg/dl min ± SEM, P= 0.026, 0.018 respectively). Conclusion: Subclinical hyperthyroidism results in glucose intolerance despite normal fasting plasma glucose, findings that recommend oral glucose tolerance tests for SHR patients.