Effects of Thyroid Hormone Replacement on Glycated Hemoglobin Levels in Non Diabetic Subjects with Overt Hypothyroidism

Objective: Glycated hemoglobin (HbA1c) may not accurately reflect the level of glycemia in conditions of altered erythrocyte turnover. Hypothyroidism is one condition associated with sluggish erythropoesis. To assess changes in HbA1c, independent of changes in plasma glucose after initiation of thyroxine replacement in patients with overt hypothyroidism.

Materials And Methods: In this prospective longitudinal study carried out in a tertiary care centre, adult non-diabetic patients with overt hypothyroidism recruited between March 2012 to August 2013 were rendered euthyroid on thyroxine. They underwent testing for hemoglobin, HbA1c, reticulocyte count, thyroxine, thyrotropin and a standard oral glucose tolerance test, both before and at 3 months after restoration to the euthyroid state. Main outcome assessed was the change in HbA1c independent of the change in glucose parameters.

Results: Thirty eight patients (35 female and 3 male) aged 37.8 ± 10.2 years with overt hypothyroidism (thyroxine 12.6 ± 13.4 ng/mL and thyrotropin -98.1 ± 63.7 µIU/mL respectively) were recruited. While HbA1c fell from 5.8 ± 0.7% to 5.6 ± 0.5% (p = 0.009) at 3 months following the correction of hypothyroidism, there were no changes in the fasting and the 2 hr post oral glucose tolerance test glucose (p = 0.67 and 0.56 respectively). The number of patients with dysglycemia diagnosed by HbA1c (i.e HbA1c ≥ 5.7%) fell from 25 (65.78%) to 17 (44.7%) after treatment (p = 0.008). There were 7 (18.4%) patients with HbA1c ≥ 6.5% at baseline, but this fell to just 4 (10.5%) (p < 0.001) after 3 months of euthyroidism.

Conclusion: HbA1c is not a reliable diagnostic test for diabetes in the presence of hypothyroidism.