15. Skin darkening
Formally called acanthosis nigricans, this symptom involves darkening and thickening of the skin, especially in folds. Sometimes the darkened skin will be slightly raised and velvety in texture. Most often it is found in the armpits, neck, groin, or under breasts.
Doctors consider acanthosis nigricans to be an indication of diabetes and is thought to be related to insulin resistance.