A somewhat embarrassing topic but important for many! Dark underarms can be caused by a number of factors related to medical and lifestyle concerns.
Melanin production and the increase in such levels can be a factor. As with skin on the lips and nipples, hormonal changes and Addison's disease can increase melanin production. A more serious concern is diabetes, where people with type 2 diabetes can find that insulin remaining in the bloodstream can lead to the thickening and darkening of the skin in certain areas, including under the arms.
Deodorants and antiperspirants can also produce an allergic or irritated reaction in the delicate armpit area. Excessive rubbing can have a similar effect. Both situations can lead to hyperkeratosis, a thickening of the skin- a condition designed to protect the skin against rubbing, pressure and irritation. Unfortunately, hyperpigmentation is often the downside of hyperkeratosis.
A more serious problem is erythrasma, a chronic infection in the deeper layers of the skin. Sometimes, the infection can lead to the formation of red, scaly patches. The border of the affected skin is well-defined, with a clear demarcation between the discolored patch and the surrounding normal skin. Over time, the pink or red skin fades to brown.
Shaving can also lead to the appearance of dark armpits – particularly as underarm hair tends to be dark and coarse. This stubble can be visible under the surface of the skin, especially if there are ingrown hairs. Waxing your armpits is a recommended way of hair removal as it removes hair by the root and leaves no stubble.
As with any skin concern, you should consult your doctor especially if you experience persistent itching or tenderness.