Hyperpigmentation

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In dermatology, hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Hyperpigmentation may be caused by sun damage, inflammation from acne, or other skin injuries.<ref>Hyperpigmentation. Dermatalogic Disease Database. American Osteopathic College of Dermatology. Retrieved on 2006-03-08.</ref> It is also associated with a number of diseases or conditions, including:

Addison's disease and other sources of adrenal insufficiency, in which hormones that stimulate melanin synthesis (e.g. MSH) are frequently elevated.
acanthosis nigricans, or hyperpigmentation of intertriginous areas associated with insulin resistance.
chloasma, or patchy hyperpigmentation often found in pregnant women
linea nigra, a hyperpigmented line found on the abdomen during pregnancy
Peutz-Jeghers syndrome, an autosomal dominant disorder characterized by hyperpigmented macules on the lips and oral mucosa and gastrointestinal polyps.
Certain chemicals such as Salicylic Acid.