Some hyperthyroid patients develop skin hyperpigmentation, mainly in the exposed areas of the body, but not in the oral mucosa, areola and vulva. The degree of hyperpigmentation varies, with severe cases reaching the depth of Addison’s disease. The hyperthyroidism of skin pigmentation may be due to the accelerated renewal of cortisol and increased secretion of adrenocorticotropic hormone, which has a 7 amino acid sequence and is identical to the melanocortin stimulating hormone of the pituitary gland, thus stimulating melanocytes and increasing melanin production, resulting in dull and dark skin.