What causes underarm darkening?

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.

Human skin is alive with a host of microorganisms. One recent study published in the journal Science found that at least 18 different phyla of bacteria dwell in 20 different skin habitats. The underarm is one of those habitats. There, in the dark, swampy environment, the bacterium Corynebacterium minutissimum makes a nice home. Normally, it goes unnoticed, but if your body chemistry changes -- if you sweat -- the bug can cause problems. The least serious problem is body odor, which results when the bacteria break down sweat. 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.

Doctors can diagnose erythrasma by shining a Wood's lamp on the affected area. A Wood's lamp produces ultraviolet light, which can be used to detect several conditions affecting the skin. If you have the infection, the patches will fluoresce in the presence of the UV light and glow a magenta color. The treatment, as with any bacterial infection, requires a course of either topical or oral antibiotics.

If you have underarm darkening and all other hormonal and bacterial causes have been eliminated, you might evaluate your shaving habits. Underarm hair tends to be dark and coarse, which means that stubble -- the hair left just beneath the surface of the skin after shaving -- is darker and coarser than stubble on your legs. This stubble can be visible under the surface of the skin, especially if there are ingrown hairs. Over time, your underarm skin can look dark and shadowy, much as a man's face may appear to have a "five o'clock shadow" just a few hours after shaving.

Waxing your armpits can help you avoid this problem. Waxing removes hair by the root and leaves no stubble. But the technique hurts, especially in the nerve-rich skin under your arm. It's also more expensive, especially if you go to a salon. A good compromise is to get your first waxing treatment done in a salon to watch how the professionals do it. After that, you can do your own hair removal with waxing kits available in drugstores.

Of course, always consult your doctor if your underarm darkening persists or seems to be related to other symptoms, such as itching or tenderness. Even the lowly armpit can be an effective warning sign of larger, more serious problems.

Keep reading for more links on skin care and skin disorders.