Nivolumab, a blockade of programmed cell death 1, is now administrated for advanced malignant melanomas. Nivolumab-associated adverse events include organ-specific autoimmune disorders; autoimmune thyroid disease, vitiligo and insulin-dependent diabetes. However, predisposed persons are currently unknown. Here, we report serological aggravation of autoimmune thyroid disease in two cases receiving nivolumab: one with Hashimoto disease and another with probable subclinical Hashimoto disease. We should verify if nivolumab-related hypothyroidism and hyperthyroidism are predisposed to occur in euthyroid individuals with subclinical autoimmune thyroid disease.