Acanthosis nigricans is a condition where areas of skin become dark, thick, and velvety, usually in places like the armpits, groin, and neck. It’s often seen in people who are overweight. This condition can be caused by things that change hormone levels, like Cushing’s syndrome, polycystic ovary syndrome, or having a low-functioning thyroid. It can also be caused by taking certain medicines, such as steroids or hormone treatments.