It is one of the symptoms of prediabetes and diabetes, and a skin condition that is characterized by dark patches. It occurs in the armpits, neck, groin, and the joints of the fingers and toes. The texture of this pigmentation is generally velvety or leathery and may be itchy. It occurs due to obesity, underactive thyroid, and diabetes, and occurs with the use of corticosteroids and oral contraceptives. It also occurs due to certain tumors. It is common in people with insulin resistance (prediabetes).