Abstract
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. 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. 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. HbA1c is not a reliable diagnostic test for diabetes in the presence of hypothyroidism.
Highlights
The American Diabetic Association has recently approved the use of glycated hemoglobin (HbA1c) as a screening as well as diagnostic test for diabetes mellitus
Patients were categorized on HbA1c alone into euglycemia (HbA1c < 5.7%), prediabetes (HbA1c≥ 5.7 but < 6.5) and diabetes mellitus (HbA1c ≥ 6.5%) based on diagnostic cutoffs of HbA1c advocated by the American Diabetes Association [2]
The American Diabetes Association has recommended the use of HbA1c for the diagnosis of diabetes mellitus [2]
Summary
The American Diabetic Association has recently approved the use of glycated hemoglobin (HbA1c) as a screening as well as diagnostic test for diabetes mellitus. The glycated hemoglobin represents the fraction of hemoglobin that undergoes non-enzymatic glycation over the circulatory life span of the erythrocytes (usually 120 days) [3,4]. It depends on the ambient le vel of glycemia over the preceeding 2-3 months [5], and on the average period of exposure of the circulatory red blood cells (RBCs) to this glycemia i.e on the erythrocyte turnover in circulation [3,5]. Conditions which are associated with a low RBC turnover, with a predominance of older cells (and a paucity of younger RBCs and reticulocytes) in circulation are associated with a falsely elevated HbA1c. Proven examples include iron [6,7] and vitamin B12 deficiency [6], and renal failure [8]
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