Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Causes Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris. People with darker skin tones are more prone to hyperpigmentation, especially with excess sun exposure. Many forms of hyperpigmentation are caused by an excess production of melanin. Hyperpigmentation can be diffuse or focal, affecting such areas as the face and the back of the hands. Melanin is produced by melanocytes at the lower layer of the epidermis. Melanin is a class of pigment responsible for producing color in the body in places such as the eyes, skin, and hair. The process of melanin synthesis (melanogenesis) starts with the oxidation of -tyrosine to by the enzyme tyrosine hydroxylase, then to -dopaquinone and dopachrome, which forms melanin. As the body ages, melanocyte distribution becomes less diffuse and its regulation less controlled by the body. UV light stimulates melanocyte activity, and where concentration of the cells is greater, hyperpigmentation occurs. Another form of hyperpigmentation is post-inflammatory hyperpigmentation. These are dark and discoloured spots that appear on the skin following acne that has healed. Diseases and conditions Hyperpigmentation is associated with a number of diseases or conditions, including the following: Addison's disease and other sources of adrenal insufficiency, in which hormones that stimulate melanin synthesis, such as melanocyte-stimulating hormone (MSH), are frequently elevated. Cushing's disease or other excessive adrenocorticotropic hormone (ACTH) production, because MSH production is a byproduct of ACTH synthesis from proopiomelanocortin (POMC). Acanthosis nigricans—hyperpigmentation of intertriginous areas associated with insulin resistance. Melasma, also known as 'chloasma' or the “mask of pregnancy,” when it occurs in pregnant women.— It is a common skin problem that causes dark discolored patchy hyperpigmentation. It typically occurs on the face and is symmetrical, with matching marks on both sides of the face. The condition is much more common in women than men, though men can get it too. According to the American Academy of Dermatology, 90 percent of people who develop melasma are women. Post-Acne marks from post-inflammatory hyperpigmentation. 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. Exposure to certain chemicals such as salicylic acid, bleomycin, and cisplatin. Smoker's melanosis Coeliac disease Cronkhite–Canada syndrome Porphyria Tinea fungal infections such as ringworm. Haemochromatosis—a common