We've already mentioned how hormonal changes related to pregnancy and Addison's disease can increase melanin production. But there's another common hormone disorder that can lead to underarm darkening. The disorder is diabetes, and the hormone is insulin, the chemical that helps to keep blood glucose in the normal range. In people with type 2 diabetes, cells become resistant to insulin over time. As a result, insulin remains in the bloodstream, and insulin levels become elevated. Normal insulin levels are about 5 to 20 micro units per milliliter while fasting [source: Rennert]. People with insulin resistance can have levels as much as four times higher that.
You probably know that elevated insulin levels can increase the risk for kidney failure, blindness and heart disease. But did you know that too much insulin can overstimulate skin cells? This leads to thickening and darkening of the skin in certain areas, including under the arms. In severe cases, the skin can take on a thick, velvety appearance. Dermatologists refer to this condition as acanthosis nigricans, and they see it most often in people of African descent.
If you have acanthosis nigricans related to diabetes, you should treat the diabetes first. As you bring the underlying hormonal disorder under control, you should see the dark patches fade. If your underarm skin remains dark, try applying over-the-counter topical creams containing lactic acid or glycolic acid, which can help to exfoliate the dark patches.
Another cause of underarm darkening, especially in women with brown skin, relates to external assaults on the skin. For example, chemicals in underarm products, such as deodorants and antiperspirants, can produce an allergic or irritated reaction. Excessive rubbing can have a similar effect. Both situations can lead to hyperkeratosis, a thickening of the skin that's meant to provide protection against rubbing, pressure and irritation. Unfortunately, hyperpigmentation often accompanies hyperkeratosis.
Dermatologists may treat this condition, known as post-inflammatory hyperpigmentation, with prescription creams or lotions. Many of these lotions contain tretinoin, the drug found in the acne medication Retin-A. Other treatment options include oral medications, such as isotretinoin, and dermabrasion, which may help to reduce the thickness of the affected skin.
Other, less serious conditions can result in underarm darkening. We'll look at those next.