Which hormone gives the skin a darkened or tanned appearance?

Adrenocorticotropic hormone (ACTH) darkens the skin by stimulating melanocytes to produce melanin. In contrast to normal tanning, the darkening is often accentuated in palmar creases and mucous membranes. (A note of caution: dark skin creases and mucous membranes may be a normal variant in more darkly pigmented races.) The most common cause of elevated ACTH levels is Addison’s disease, in which hypofunctioning adrenals and deficient serum cortisol remove the negative feedback inhibition of the pituitary gland that increases ACTH production.

Slominski A, Tobin DJ, Shibahara S, Wortsman J: Melanin pigmentation in mammalian skin and its hormonal regulation, Physiol Rev 84:1155–1228, 2004.