A 43-year-old woman with a huge portal-systemic shunt accompanied by myxedema showed slow speech and behavior. Several imaging studies revealed a bold portal-systemic shunt from the splenic vein to the left renal vein. In addition, hypothyroidism caused by chronic thyroiditis was diagnosed, and synthesized thyroxine replacement was effective for the symptoms. However, the serum ammonia and indocyanin green retention remained in the abnormal range, nevertheless the portal vein pressure was normal and findings of liver cirrohsis were not recognized histologically. Surgical shunt closure was performed, resulting in normalized serum ammonia levels and serum branched chain amino acids /aromatic amino acids ratio, and improvement of the ammonia tolerance test.