Dark underarms result from a skin condition called acanthosis nigricans (AN). The underarms are usually the same shade as the rest of our skin, but AN causes the skin to darken and thicken around folds of the body, such as the armpits, knees and elbows.
Up to 74% of people may experience some form of AN, according to a 2014 overview of the condition, so this isn't a rare issue. However, dark underarms can be a source of discomfort and embarrassment for many people, even hindering their daily life.
What causes dark underarms? There are a few main factors:
AN can be hereditary, so a person with a family history of the condition is more likely to have it. People with darker skin tones are also more susceptible to underarm darkening.
Shaving and Plucking
Regular shaving and plucking may contribute to dark underarms. There is evidence to suggest that these hair removal practices may increase melanocyte production, which can darken the skin tone if production is excessive.
Certain deodorants can irritate the skin. Often, this irritation takes the form of a rash. However, in some cases, the irritation may cause darkening of the skin. This condition is called post-inflammatory pigment alteration.
Underlying Health Issues
Dark underarms can be a result of an underlying health issue. For example, obese, diabetic or insulin-resistant people are more likely to experience darker underarms. Those with polycystic ovary syndrome or Cushing syndrome are also more prone to darker underarms.