This study will evaluate the safety, pharmacokinetics (how the body absorbs, distributes, metabolizes, and eliminates the drug), and pharmacodynamics (how the drug affects the body) of resmetirom in children and adolescents with metabolic dysfunction-associated steatohepatitis (MASH) and liver fibrosis. Participants will receive oral resmetirom once daily for approximately 14 days at one of several dose levels. The information from this study will help determine appropriate dosing and further evaluate the safety and biological effects of resmetirom in pediatric participants with MASH.