Various shades and colors of human skin are created by the brown pigment, melanin. Without melanin, the skin would be pale white with varying shades of pink caused by blood flow through the skin. Fair-skinned people produce very little melanin, darker-skinned people produce moderate amounts, and very dark-skinned people produce the most. People with albinism have little or no melanin.
Melanin is produced by specialized cells (melanocytes) that are interspersed among the other cells in the top layer of the skin, the epidermis. After melanin is produced, it spreads into other nearby skin cells.
An abnormally high amount of melanin (hyperpigmentation) may affect large areas of the body or small patches. When exposed to sunlight, melanocytes produce increased amounts of melanin, causing the skin to darken, or tan. In some fair-skinned people, certain melanocytes produce more melanin than others in response to sunlight. This uneven melanin production results in spots of pigmentation known as freckles. A tendency to freckle runs in families. Increased amounts of melanin can be produced in response to hormonal changes, such as those that may take place in Addison disease, in pregnancy, or with hormonal contraceptive use. Some cases of skin darkening, however, are not related to increased melanin at all, but rather to abnormal pigments that make their way into the skin. Diseases such as hemochromatosis or hemosiderosis or some drugs and chemicals that are applied to the skin, swallowed, or injected can cause skin darkening. Drugs and chemicals that can cause skin darkening include amiodarone, hydroquinone, antimalarial drugs, tetracycline antibiotics, phenothiazines, and some cancer chemotherapy drugs, tricyclic antidepressants, and heavy metals. A buildup of bilirubin (the main pigment in bile) causes the skin to turn yellow (jaundice). Hyperpigmentation can also develop after injuries or inflammation caused by disorders such as acne and lupus.
An abnormally low amount of melanin (hypopigmentation) may affect large areas of the body or small patches. Decreased melanin usually results from a previous injury to the skin, such as a blister, ulcer, burn, exposure to a chemical, or skin infection. Sometimes pigment loss results from an inflammatory condition of the skin or, in rare instances, is hereditary.
Last full review/revision January 2013 by Peter C. Schalock, MD