To analyze the correlation between subclinical hypothyroidism (SCH), serum vitamin D levels, and the severity of non-alcoholic fatty liver disease (NAFLD) in patients with obesity.

Clinical data of 120 obese patients with NAFLD who visited the Department of Weight and Metabolism Surgery of the Second Affiliated Hospital of Anhui Medical University from November 1, 2021 to March 31, 2022, were retrospectively analyzed. Patients were divided into SCH group and control group based on the presence of SCH, and further subgroup analysis was performed based on the vitamin D levels: SCH-Vitamin D deficient group, SCH-Vitamin D insufficient group, control-Vitamin D deficient group, and control-Vitamin D insufficient group. Liver ultrasonography was used to diagnose NAFLD, QCT was used to evaluate liver fat content, and the Fatty Liver Index (FLI) and Fibrosis Index Based on the 4 Factor (FIB-4) score were used to predict NAFLD liver fat degeneration and liver fibrosis.

The FLI, FIB-4, and liver fat content were significantly higher in the SCH group compared to the control group (P<0.05), whereas vitamin D levels were lower in the SCH group (P<0.05). Subgroup analysis revealed that patients with both SCH and vitamin D deficiency had higher FLI, FIB-4, and liver fat content (P<0.05). Correlation analysis demonstrated a positive correlation between thyroid stimulating hormone (TSH) and FLI, FIB-4, as well as liver fat content (P=0.030, r=0.199; P<0.001, r=0.387; P=0.004, r=0.261), while vitamin D exhibited a negative correlation with FLI, FIB-4, and liver fat content (P=0.012, r=-0.229; P=0.249, r=-0.106; P<0.001, r=-0.363). Additionally, the correlation analysis between vitamin D levels and TSH demonstrated a negative correlation (P<0.001, r=-0.326). Furthermore, vitamin D (P=0.034, OR=0.615) was identified as an independent risk factor for severe NAFLD.

Our findings suggest a close association between SCH, vitamin D deficiency, and NAFLD in obese patients. Vitamin D deficiency emerges as an independent risk factor for severe NAFLD.Therefore, for obese patients with NAFLD, it is recommended to improve thyroid function and serum vitamin D levels to facilitate timely intervention.