We report two patients in whom hypothyroidism was considered to cause renal dysfunction. Case 1 was a 65-year-old woman who stopped taking levothyroxine sodium for hypothyroidism. After 6 months, she developed proteinuria, edema, weight gain, and renal dysfunction. Renal biopsy revealed focal segmental proliferative glomerulonephritis. After re-administration of levothyroxine sodium, thyroid function and renal function both recovered. Case 2 was a 51-yearold man who presented with edema, difficulty in swallowing, muscular weakness, and fatigue. We diagnosed hypothyroidism, and focal segmental proliferative glomerulonephritis was revealed by renal biopsy. After administration of levothyroxine sodium, his symptoms resolved and his thyroid function and renal function both improved. Our experience suggests that hypothyroidism should be taken into consideration as one of the causes of renal dysfunction.