Thyroid dysfunction and nonalcoholic fatty liver disease: we need new larger and well-designed longitudinal studies

Dear Editor, As reported in Table 1, in the last decade several observational studies have investigated the potential association between primary (subclinical and overt) hypothyroidism and risk of developing nonalcoholic fatty liver disease (NAFLD), as detected by imaging or liver biopsy. Such association, indeed, may be supported by some specifc actions of the thyroid hormones on the liver. For example, free triiodothyronine (FT3) and free thyroxine (FT4) may regulate the lipid metabolism in the liver by several mechanisms, including the thyroid hormone receptor β. When hypothyroidism occurs, the decreased activity of hepatic lipases may promote the development and progression of NAFLD by decreased hepatic triglyceride clearance and the increased hepatic triglyceride accumulation. In addition, the insulin resistance, occurring under some hypothyroidism conditions, may also concur to the development of NAFLD by inducing de novo lipogenesis and generating a fux of free fatty acids from adipose tissue to the liver.