Free triiodothyronine as determinant of non-alcoholic fatty liver disease in euthyroid subjects: The lifelines cohort study

Background: Non-alcoholic fatty live disease (NAFLD) is becoming the leading cause of chronic liver disease in de Western world. The liver plays a crucial role in the metabolism of cholesterol and triglycerides and thyroid hormones interact on hepatic lipid homeostasis. Given the importance of variations in thyroid function within the euthyroid range for a considerable number of health issues, including (subclinical) atherosclerosis and biochemical markers of increased cardiovascular risk, it is relevant to examine the relationship of NAFLD with thyroid function parameters in an euthyroid population. Methods: The study was conducted in the LifeLines Cohort Study (N=167,729), a population-based cohort study examining the health and health-related behaviors of participants living in the North of The Netherlands. Only euthyroid subjects (TSH 0.5-4.0 mU/L, FT4 11-19.5 pmol/L and FT3 4.4-6.7 pmol/L) older than 18 years were included. Exclusion criteria were participants with missing data, excessive alcohol use, known hepatitis or cirrhosis, liver functions ≥ three times the upper limit, current cancer, non-white ancestry, previous or current use of thyroid medication and current use of lipid and glucose lowering medication. A priori defined liver biochemistry, thyroid function parameters and metabolic syndrome (MetS) were studied. NAFLD was defined by using the validated Fatty Liver Index (FLI); FLI ≥60 was categorized as NAFLD. A p