Bidirectional Association between Thyroid Function and Glycemic Status: A Cross-Sectional Study

Aim: Diabetes and thyroid dysfunctions have similar mechanisms, and they may be overlooked or undiagnosed. This study aimed to evaluate the effect of thyroid hormone levels on glycosylated hemoglobin (HbA1c) and the thyroid-diabetes relationship. Material and Methods: HbA1c, glucose, free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) values measured in the same samples of patients aged 18 and 60 years were obtained. Normoglycemic individuals were divided into subgroups as normal, hypothyroidism, and hyperthyroidism according to their TSH levels (Model 1), and non-thyroid dysfunction patients were divided into normoglycemic, prediabetic, and diabetic subgroups according to their HbA1c levels (Model 2). Correlations and differences of the obtained data were analyzed. Results: In Model 1, the median HbA1c level was 5.44 (range, 5.08-5.68) in the hyperthyroidism group, which was significantly higher than normal (5.32; range, 4.96-5.69) and hypothyroidism (5.33; range, 5.02-5.65) groups (p=0.021). In Model 2, fT3 levels were found significantly higher in normoglycemic patients (3.11; range 1.95-3.75 pg/ml) than prediabetic (3.01; range, 2.13-3.82 pg/ml), and diabetic (3.00; range, 2.02-3.69 pg/ml) patients (p=0.001), while fT4 levels in diabetic patients (1.29; range, 0.56-1.74 ng/dl) were significantly higher compared to normoglycemic (1.23; range 0.72-1.89 ng/dl), and prediabetic (1.24; range, 0.73-1.66 ng/dl) patients (p=0.003). Conclusion: The results showed that HbA1c levels could be a marker for thyroid function in non-diabetic individuals. In diabetic patients, thyroid hormone balance and ratios are changing. For better glycemic control and the prevention of complications, regular thyroid function screenings are recommended, especially in individuals with a predisposition to diabetes.