HbA1c in the diagnosis of diabetes and abnormal glucose tolerance in patients with Graves’ hyperthyroidism

Objective To assess the suitability of HbA1c as a criterion for the diagnosis of diabetes in patients with Graves’ disease. Methods This study enrolled 310 patients with untreated newly diagnosed Graves’ disease, 208 patients with euthyroid goiter and 329 age-matched (control) subjects without thyroid disease from Fuzhou, China. The performance of HbA1c against the OGTT for diagnosing diabetes was determined. The Framingham risk score was used to assess general cardiovascular disease (CVD) risk. Results The percentage of patients with abnormal glucose metabolism as classified by HbA1c levels was lower than by OGTT criteria in patients with Graves’ disease—33.2% vs. 41.3% for pre-diabetes and 4.5% vs. 11.3% for diabetes, respectively. The sensitivity of HbA1c for diagnosing diabetes in patients with Graves’ disease was lower than in patients with euthyroid goiter and subjects without thyroid disease (34.9%, 63.2% and 60.6% respectively), while the specificity was similar (99.3%, 98.6%, 97.4%). Approximately 7.4% of patients with Graves’ disease diagnosed with diabetes according to OGTT criteria were misdiagnosed as not having the disease by HbA1c, much higher than that for the other two groups. Patients with Graves’ disease with diabetes not diagnosed with the disease by HbA1c showed a high risk for CVD. Conclusions The low sensitivity of the HbA1c criterion underestimated the percentage of diabetes in patients with Graves’ disease. Patients with diabetes who were misdiagnosed as not having the disease by HbA1c were at high risk for CVD.