The Association of Serum Thyroid Hormone Levels and Hepatic Gene Expression of Potential Regulators of Intracellular Thyroid Hormone Concentration with Severity of Nonalcoholic Fatty Liver Disease

Abstract

Purpose: Nonalcoholic fatty liver disease (NAFLD) is associated with dyslipidemia, metabolic syndrome, and hypothyroidism. It is unknown if serum thyroid hormone (TH) levels are associated with NAFLD disease severity. We wished to evaluate differences in serum TH levels and liver expression of TH-associated genes between early and advanced NAFLD. Methods: RNA was extracted and microarray performed from liver tissue of 70 patients with biopsy-proven NAFLD (42 early (F:0-1), 28 advanced (F:3-4)). Serum collected on the same day as biopsy was analyzed for thyroid stimulating hormone (TSH), free T4 (fT4), free T3 (fT3), and reverse T3 (rT3). TH levels were compared between early vs advanced NAFLD using Wilcoxon rank-sum analysis. Logistic regression models using a stepwise algorithm were built with histologic severity as the outcome and TH serum markers as the primary predictors, with and without adjusting for potential confounders including: age, gender, BMI, ethnicity, HgBA1C, history of hypertension, and history of hyperlipidemia. Differential gene expression was determined on normalized data by an empirical Bayes method with a 5% False Discovery Rate control. Results: Free T3 was significantly lower (p=0.03) and rT3 significantly higher (p=0.005) in patients with advanced disease as compared to early disease. Free T3 and rT3 remained significant predictors of advanced fibrosis in the final stepwise logistic regression model after adjusting for potential confounders (p=0.034 and 0.032). The fT3/rT3 ratio, previously reported as a positively-associated surrogate test for intracellular fT4 uptake, was lower in those with advanced NAFLD (p=0.001). Both univariate and multivariate logistic regression analysis confirmed that a reduced fT3/rT3 ratio was a strong predictor of advanced fibrosis (p values<0.005). Area under the curve (AUC) for fT3/rT3 ratio as a predictor of fibrosis was 0.72. In addition to serum TH levels, we evaluated gene expression levels of potential regulators of intracellular hepatic TH uptake. Analysis revealed increased expression of THRSP in early NAFLD and increased expression of tissue remodeling genes in advanced NAFLD (IGFBP6, RCAN2, and ITGAV). Spearman's rank correlations indicate a significant positive relationship between serum fT3 and fT3/rT3 ratio, with RAC3 expression (ρ=0.55, ρ=0.46) (p <0.001), a gene important in morphogenesis and tissue remodeling. Conclusion: Advanced NAFLD is associated with lower fT3 and higher rT3 levels vs. those with early NAFLD. Thyroid-specific gene expression differences in early vs. advanced NAFLD suggest that TH levels and/or hepatic responsiveness to TH are potential pathogenic mechanisms contributing to NAFLD disease severity.