Melanocyte stimulating hormone - MSH - is a peptide derived from pro-opiomelanocortin in the anterior pituitary. It seems to be modulated by the release of two hormones within the hypothalamus: melanoncyte stimulating hormone releasing factor (MSHRF) and melanocyte stimulating hormone release-inhibiting factor (MSH-RIF). These pass down haematogenously to the adenohypophyseal capillaries. In phylogenetically lower organisms, MSH is clearly involved with pigmentation. In humans MSH may have an important role in the regulation of body weight.
Excessive ACTH in Addison's disease can give rise to melanotic pigmentation due to the over-production of a peptide precursor with a similar structure to MSH. But, hypophyseal ablation does not cause dark skin to lighten.
External factors e.g. ultraviolet light exposure, seem more powerful determinants of melanin pigmentation.
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