There are three common causes of underarm skin discoloration. The simplest cause is dark hair growing under the skin that shows through your thin skin layers -- this isn't actually underarm hyperpigmentation.
Another common cause of underarm skin discoloration occurs when dead cells build up beneath the skin. As new cells grow, they typically push the old cells to the surface where they fall off, but sometimes these old cells can get caught in tiny crevices under your skin [source: Cosmopolitan]. If you have too many cells in one spot, you also have more melanin in that spot, which can make skin appear darker.
The third cause of underarm skin discoloration is documented in a study performed in Southeast Asia [source: James, et al.]. Scientists found that darker underarms can be caused by a chain reaction that begins when sensitive underarm skin becomes irritated. The irritated skin makes melanocytes produce more melanin, which causes skin to darken. Shaving, waxing, plucking and using certain deodorants and antiperspirants can all irritate the skin and stimulate melanin production [source: Skincare-news].
Avoiding skin irritants can help you prevent underarm skin discoloration, but what do you do if your underarms are already dark? Read on to learn about treatment options.