Effect of Correction of Hyperthyroidism with Anti-thyroid Drugs on the Glycated Hemoglobin in Non-diabetic Patients with Primary Hyperthyroidism

Background: Glycated hemoglobin (HbA1c) levels are dependent not only on the average blood glucose levels over the preceding 2 - 3 months but also on the turnover of erythrocytes. Hyperthyroidism is known to be associated with an increase in erythrocyte turnover that may falsely lower the HbA1c in relation to the level of glycemia. Objectives: To assess the impact of medical correction of hyperthyroidism on HbA1c, independent of changes in the fasting plasma glucose and 2-hour post-oral glucose tolerance test plasma glucose. Methods: Adult patients with overt hyperthyroidism (n = 36) were tested for their hemoglobin, reticulocyte percentage, HbA1c and fasting and post-oral glucose tolerance test (OGTT) 2-hour plasma glucose, both at baseline and following at least three months of near normalization of serum thyroxin on Carbimazole treatment. Results: Correction of hyperthyroidism in 36 patients was associated with an increase in the hemoglobin (P = 0.004) and a rise in HbA1c (P = 0.025), even though no significant change was observed in both the fasting (P = 0.28) and post OGTT two-hour plasma glucose (P = 0.54). Also, the proportion of patients with HbA1c ≥ 5.7% rose from 3/36 to 10/36; P = 0.016, while the proportion of patients with either abnormal fasting or abnormal post OGTT 2-hour plasma glucose or both did not show any significant change (P = 0.5). The sensitivity of HbA1c to diagnose prediabetes increased from 20% to 50% post- treatment. Conclusions: Glycated hemoglobin is falsely low in relation to glycemia in patients with untreated hyperthyroidism.