Impact of subclinical hypothyroidism on glycemic markers and insulin resistance in nondiabetic Iraqi patients: a cross-sectional study

Background. Subclinical hypothyroidism is a common endocrine problem in Iraq. It is diagnosed through high serum levels of thyroid-stimulating hormone (TSH) and normal serum free thyroid hormones. Glycated hemoglobin (HbA1c) is used as a marker for long-term glycemic control, but its reliability in the context of subclinical hypothyroidism remains unclear. This study purposed to evaluate glycated hemoglobin levels and insulin resistance in nondiabetic persons with subclinical hypothyroidism. Materials and methods. The study involved 200 participants, a hundred nondiabetic persons with subclinical hypothyroidism, and a hundred healthy matched controls. History was obtained from the participants, complete general and systemic examinations were performed, and weight, height, and blood pressure were recorded. Blood tests for hormonal assessment include free triiodothyronine (fT3), free thyroxine (fT4), TSH, and serum insulin. Glycated hemoglobin was measured, and HOMA-IR was used to evaluate insulin resistance. Results. No significant differences were found in the levels of fT3 and fT4 in both groups. The mean fT3 was 1.59 ± 0.20 nmol/L in the case group and 1.72 ± 1.20 nmol/L in the control group (p = 0.28). The mean fT4 was 76.8 ± 2.7 nmol/L in the case group and 77.21 ± 0.10 nmol/L in the control group (p = 0.13). Significant differences were shown in the level of TSH between the two groups (p < 0.0001). There was a substantial difference in HbA1c in both groups; the mean was 4.87 ± 0.50 % in the case group and 4.12 ± 0.80 % in the control group (p < 0.0001). Likewise, HOMA-IR shows a significant difference between the two groups; the mean was 2.65 ± 1.70 in the case group and 1.31 ± 2.80 in the control (p = 0.0001). Conclusions. Elevated HbA1c and increased insulin resistance in subclinical hypothyroidism patients suggest a potential link between thyroid dysfunction and glucose metabolism dysregulation. These findings highlight the need to carefully interpret HbA1c levels in subclinical hypothyroidism when assessing glycemic status.