Melanocyte-stimulating hormone is a name for a group of peptide hormones produced by the skin, pituitary gland, and hypothalamus. In response to ultraviolet (UV) radiation, its production by the skin and pituitary is enhanced, and this plays a key role in producing colored pigmentation found in the skin, hair, and eyes. It does this by making specialized skin cells called melanocytes produce a pigment called melanin; melanin protects cells from DNA damage, which can lead to skin cancer (melanoma).

MSH secretion from the pituitary is increased by exposure to UV light. Unlike most hormones, MSH release is not thought to be controlled by a direct feedback mechanism. Melanocyte-stimulating hormone is produced from the same precursor molecule as adrenocorticotropic hormone (ACTH) It is produced by the pituitary gland, and Its key function is to stimulate the production and release of cortisol from the outer part of the adrenal gland.

A direct consequence of high levels of MSH is the increased production of melanin. Abnormal darkening of the skin is found in patients with primary adrenal insufficiency (Addison's disease). In Addison's disease, the adrenal glands do not produce enough hormones (including cortisol).

Melanocyte-stimulating hormone deficiency can cause increased inflammation, pain, and sleeping problems. A deficiency in MSH results in a lack of skin pigmentation and subsequent loss of natural protection from UV rays of the sun.