Study of Resmetirom in Children and Adolescents With MASH

Inclusion Criteria: 1. Male or female participants 6 to 17 years of age, inclusive. 2. Parent(s) or legal guardian(s) able to provide written informed consent (as required by local regulations), with participant assent obtained as applicable. 3. Diagnose of MASH with fibrosis stage F1-F3 based on a liver biopsy obtained within 24 months before Screening. 4. Hepatic fat fraction ≥8% by magnetic resonance imaging-proton density fat fraction (MRI-PDFF) obtained during the Screening period. 5. Able to swallow study tablets. 6. Females of childbearing potential must have a negative pregnancy test at screening, must not be pregnant or breastfeeding, and must agree to use a highly effective method of contraception during the study and for at least 30 days after the last dose of study drug. Premenarchal participants and those not of childbearing potential are eligible without contraception. Exclusion Criteria: 1. Previous exposure to resmetirom. 2. Clinically significant liver disease other than MASH or evidence of cirrhosis (F4), decompensated liver disease, or other hepatic conditions that may interfere with study participation or interpretation of results. 3. Clinically significant thyroid disease or use of thyroid replacement therapy, triiodothyronine, thyroxine, or other prohibited thyroid medications. 4. Use of prohibited concomitant medications, including medications known to affect hepatic steatosis or liver function, lipid-lowering therapies, CYP2C8 inhibitors, OATP1B1/OATP1B3/BCRP inhibitors, protease inhibitors, St. John's Wort, or other medications prohibited by the protocol. 5. Use of glucagon-like peptide-1 (GLP-1) receptor agonists unless on a stable dose for at least 24 weeks before screening. 6. Clinically significant alcohol or substance abuse, or regular use of tobacco/nicotine products within 6 months before screening. 7. Active or clinically significant infection, including chronic hepatitis B or hepatitis C infection, HIV infection, or other immunocompromising conditions. 8. History or presence of clinically significant cardiovascular, pulmonary, renal, gastrointestinal, neurologic, hematologic, endocrine, psychiatric, or other medical conditions that, in the opinion of the Investigator, could interfere with study participation or interpretation of study results. 9. History of malignancy within the past 5 years (except adequately treated non-melanoma skin cancer or other protocol-permitted exceptions). 10. History of organ transplantation or known immunocompromised status. 11. Participation in another investigational study within 60 days or 5 half-lives (whichever is longer) before screening, unless permitted by the protocol. 12. Major surgery within 6 weeks before screening. 13. Known hypersensitivity to resmetirom or any excipient. 14. Females who are pregnant or breastfeeding. 15. Any condition that, in the opinion of the Investigator, would compromise participant safety, compliance, or the integrity of the study.