Melanocyte-stimulating hormone is a naturally occurring hormone produced by the pituitary gland. When released, it encourages the production of melanin in both the skin and hair.
The hormone is produced normally during pregnancy, resulting in a darkening of the skin and nipples.
The hormone is also produced in response to abnormal function of the adrenal glands, which is why patients with Cushing's disease and Addison's disease often exhibit acanthosis nigricans.
The hormone does not account for differences in skin tone by race. However, redheads appear to have fewer receptors for the hormone, resulting in them tanning more slowly.
The hormone is being developed as a pharmaceutical. It is hoped it will assist patients who are photosensitive, such as patients with certain types of porphyria.
Melanocyte-stimulating hormone at Wikipedia