Hypothyroidism Falsely Raises HbA1c and Glycated Albumin Levels

Researchers reported that conditions that influence erythrocyte turnover (including hypothyroidism) can lead to falsely high or low hemoglobin A1c levels, and hypothyroidism is known to decrease albumin degradation.

Dr. Mee Kyoung Kim from The Catholic University of Korea, Seoul, conducted two separate studies: a cross sectional study of 45 nondiabetic patients with thyroid cancer who underwent thyroid hormone withdrawal (THW) in preparation for radioiodine treatment after total thyroidectomy; and a prospective trial of 30 similar nondiabetic patients who underwent THW during radioiodine treatment and were subsequently treated with thyroid hormone replacement.

In the first study, patients with overt hypothyroidism had significantly higher levels of HbA1c (5.54%) than did euthyroid controls (5.34%), despite having lower fasting blood glucose levels.

In the second study, hypothyroid patients treated with thyroid hormone replacement experienced significant decreases in HbA1c level, serum albumin level, and glycated albumin level.

The decreases in HbA1c paralleled the increases in reticulocyte count and mean corpuscular hemoglobin seen with thyroid hormone replacement.

In six patients followed from their baseline euthyroid state through untreated hypothyroidism to thyroid hormone replacement, HbA1c levels tended to increase during hypothyroidism and then return to baseline levels with thyroid hormone replacement.

"These data suggest that thyroid hormone replacement is associated with a decrease in A1c level, which is influenced by increased erythropoiesis rather than by changes in glucose level," the investigators say.

Based on recently endorsed criteria for diagnosing prediabetes in patients with HbA1c levels between 5.7% and 6.4%, the researchers add, "20% of our subjects (6 of 30 patients) were misclassified as having prediabetes."

They say clinicians must consider the effects of thyroid hormone on HbA1c and glycated albumin when interpreting these parameters in patients with thyroid disorders.

Dr. Kim's team notes that their study was limited by patient selection that resulted in inclusion only of iatrogenic hypothyroid patients prior to radioiodine treatment.

Diabetes Care. Posted online September 7, 2010