Dark skin under the arms and on the thighs is a symptom of a condition called acanthosis nigricans. The groin and neck are other areas typically affected, with the dark skin tending to appear in and around creases and folds of skin, explains MayoClinic.com. Dark skin is generally the only symptom associated with acanthosis nigricans, though it does in rare instances present with some itching. Uncontrolled, the discoloration will slowly spread over the course of months or years.
See your doctor for a medical workup if you aren't overweight and/or don't have diabetes or a condition causing insulin resistance, advises the American Osteopathic College of Dermatology, or AOCD. Most cases of acanthosis stem from these conditions. In other cases, malignancy needs to be ruled out. While it is rare, dark skin can by symptomatic of cancer, usually in the stomach, according to AOCD. It may also result from hormonal imbalances associated with disorders such as endocrine problems, states MayoClinic.com.
Review the listed side effects of any medication you take. In some instances, darkened skin is a known effect, and discontinuing use is the only way to get rid of the discoloration, points out the New Zealand Dermatological Society. Acanthosis nigricans is most often associated with niacin, corticosteroids, insulin, nicotinic acid, oral contraceptives and other hormone treatments.
Cut back on starch and sugar consumption and follow your doctor's instructions for managing diabetes or other insulin-related condition. High starch and sugar consumption can trigger insulin resistance, a leading cause of acanthosis nigricans, explains AOCD.
Lose weight, as being overweight is the other significant factor besides insulin resistance that causes darkened skin on the underarms, thighs and elsewhere. Consult with your doctor about a safe and healthy plan that includes better dietary practices and incorporates more physical activity into your schedule. Remember to always begin with a light exercise regimen and slowly build up to longer, more strenuous workouts.
Talk to your doctor about supplements or medications that may help diminish your discoloration while the underlying causes are addressed. Fish oil supplements and oral isotretinoin, a vitamin A-related retinoid prescription, may help, according to MayoClinic.com. Topical skin lighteners may be useful, too. Hydroquinone creams bleach the skin, and are available over the counter or in higher concentrations by prescription. Other effective topical skin lightening ingredients include kojic acid, tretinoin, azelaic acid and glycolic acid, notes DermaDoctor.com. Topical treatments made with salicylic acid, urea or lactic acid may also help, says AOCD.
Consult with a dermatologist about cosmetic procedures that remedy skin discoloration. Dermabrasion, a procedure using a rotating blade or brush to abrade away discolored skin, or laser skin resurfacing are possibilities.