Thyroid Stimulating Hormone Levels Are Related to Fatty Liver Indices Independently of Free Thyroxine: A Cross-Sectional Study.

Introduction: The relationship between metabolic dysfunction-associated steatotic liver disease (MASLD) and thyroid hormones has been established, but the direct effects of TSH on the liver, potentially leading to steatosis, and insulin resistance remain unclear. Objective: To investigate the association of TSH levels with MASLD and insulin resistance. Methods: We conducted a cross-sectional study of 8825 euthyroid individuals. Subjects were volunteers or patients referred to the International Center for Nutritional Status Assessment (University of Milan, Italy) undergoing clinical examination and blood drawing for thyroid function tests and liver indices calculation. Liver outcomes were fatty liver index (FLI), hepatic steatosis index (HSI), and FIB-4. All associations were adjusted for fT4 and confounders. Results: We found a positive association of TSH levels with FLI (β = 2.76; p < 0.001) and HSI (β = 0.58, p < 0.001). This relationship remained significant when stratifying by sex and BMI category, except for HSI in normal weight individuals. No significant association was found between TSH and hepatic fibrosis or insulin resistance. Conclusions: We found a positive association between TSH levels and MASLD in euthyroid individuals independently of fT4, sex, and BMI. Insulin resistance and hepatic fibrosis appear unrelated to TSH, independent of fT4 and BMI. The specific role of TSH in MASLD warrants further investigation.