Nonalcoholic steatohepatitis (NASH) is an aggressive part of the pathological spectrum of nonalcoholic fatty liver disease (NAFLD) that has evolved into a world-wide epidemic and is the most common chronic liver disease in the United States. NASH is strongly linked to obesity, and type 2 diabetes mellitus and may progress to cirrhosis and liver cancer. Disturbingly, prevalence rates of NASH appear to be greater in the Veteran population compared with the general US population. Thus, the burden of NASH is substantial to Veterans. Despite the recognition of NASH as a major public health problem, only lifestyle modifications aimed at increasing physical activity and reducing caloric intake are currently recommended as treatments with limited success. Thus, there is a great need for effective therapy of NASH. The goal of this study is to test whether low dose thyroid hormone administered to Veterans with biopsy-proven NASH, at a titrated dose (25, 50, or 75 mcg daily) that will maintain thyroid function within normal range (TSH at or above 0.47 mIU/L) can result in histological improvement of NASH, measured by significant reduction in nonalcoholic fatty liver disease score (NAS). The hypothesis is that low dose thyroid hormone causes histological improvement in NASH by increasing mitochondrial FAO in the liver mediated by an increase in mitochondrial trifunctional protein (MTP), the key enzyme in FAO cycle. The preliminary clinical studies in human subjects, including Veterans, support reduced FAO in NASH. In addition, preclinical in vivo animal studies and in vitro cell culture studies support that low dose thyroid hormone can effectively increase mitochondrial fatty acid oxidation and rescue NAFLD. In a randomized double-blinded placebo-controlled clinical trial, Veterans with normal thyroid levels and biopsy-proven NASH will be recruited and randomized to receive 52 weeks of either active treatment (low dose levothyroxine at 25, 50, or 75 mcg daily) or placebo control to accomplish the following Specific Aims. 1) To determine the efficacy of low dose thyroid hormone in improving NASH histological features. To accomplish this aim, NAS will be measured in pre- and post-liver biopsy samples. 2) To determine the effect of low dose thyroid hormone on mitochondrial FAO, MTP, and mitochondrial health markers in the liver compared to control. To accomplish this aim, mitochondrial FAO and the expression of genes supporting mitochondrial health will be measured in the pre- and post-liver biopsy samples. The primary clinical outcome is histological improvement in NASH measured by improvement in NAS and the secondary clinical outcome is improvement in fibrosis stage. The mechanistic outcomes include improvements in mitochondrial FAO, MTP, and quality markers. This project will be conducted in two stages. In stage 1 (proof-of-concept, 2 years), 32 Veterans with biopsy-proven NASH will be recruited and randomized to either the active treatment or placebo groups (16/group). At the end of the second year, interim analysis will be performed based on criteria that will include conditional power analysis for early efficacy to determine the probability that the full study will yield a statistically significant finding in the primary outcome (histological improvement in NASH) in the clinically relevant direction. Other milestones include trend in the secondary outcome, adverse events, and feasibility. The outcome from the proof-of-concept stage will guide the Go or No-Go decision to stage 2 for continuation of the study with recruitment of additional Veterans for a full 6-year clinical trial (total recruitment of 128 Veterans with biopsy-proven NASH: 64/active treatment group and 64/placebo control group). This project will exert a sustained and powerful impact in the field by providing new information and strategies for treatment of NASH that will be highly relevant and impactful to the health of the Veteran population.