The various forms of MSH are generated from different cleavages of the proopiomelanocortin protein, which also yields other important neuropeptides like adrenocorticotropic hormone.
[1]: 554 [2] Melanocytes in skin make and secrete MSH in response to ultraviolet light, where it increases synthesis of melanin.
[5]: 1210 Acting through melanocortin 1 receptor, α-MSH stimulates the production and release of melanin (a process referred to as melanogenesis) by melanocytes in skin and hair.
Pigmentation increases in humans during pregnancy; though the exact endocrine cause is not known, α- and β-melanocyte-stimulating hormone are thought to be involved.
[7] Cushing's disease due to excess adrenocorticotropic hormone (ACTH) may also result in hyperpigmentation, such as acanthosis nigricans in the axilla.