Limited effect of coexisting hypothyroidism and iron deficiency on HbA1c in non-diabetic adults: A matched cross-sectional study

Aim: To assess whether coexisting hypothyroidism and iron deficiency are associated with glycated hemoglobin (HbA1c) in adults without diabetes, after accounting for glycemic status, erythrocyte indices, and hematinic markers.

Materials and Methods: In this single-center, retrospective, matched cross-sectional study, 143 patients with hypothyroidism who had no diabetes and had received no relevant endocrine or hematinic treatment were matched 1:1 with 143 euthyroid controls on age and sex using propensity scores. Participants were stratified into four combined exposure groups according to thyroid status and iron deficiency. The association with HbA1c was examined using multivariable robust linear regression models with sequential covariate adjustment.

Results: Median HbA1c increased gradually from the euthyroid group without iron deficiency to the hypothyroid group with iron deficiency (p = 0.049), although the absolute difference between the highest and lowest exposure groups was only about 0.15 percentage points. Combined exposure was associated with HbA1c after adjustment for age and sex (β = 0.142; 95% CI, 0.039 to 0.245; p = 0.007), and after further adjustment for fasting plasma glucose and homeostatic model assessment of insulin resistance (HOMA-IR) (β = 0.131; 95% CI, 0.029 to 0.233; p = 0.012). After adjustment for hemoglobin, MCV, RDW, vitamin B12, and folate, the association was no longer significant (β = 0.067; 95% CI, -0.041 to 0.175; p = 0.223).

Conclusion: In adults without diabetes, coexisting hypothyroidism and iron deficiency showed only a limited association with HbA1c. The small observed gradient appears to reflect erythrocyte and hematinic status rather than a synergistic or clinically meaningful metabolic effect.