The body's largest endocrine surface. Skin makes vitamin D, activates local steroids, and runs its own hormone conversions — a gland spread over two square meters.
The vitamin D factory runs in the epidermal layers: UVB photons convert 7-dehydrocholesterol in keratinocytes to cholecalciferol, which the liver and then the kidney activate. Skin also carries 11-beta-HSD enzymes that regenerate or degrade cortisol locally, and aromatase in adipose-cutaneous tissue converting androgens to estrogens — an organ of local hormone economy more than broadcast signaling. The barrier story lives at the integumentary system.
Hormones & Regulation
UV-driven synthesis: latitude, season, and melanin set the factory's output; sunblock trades cancer risk against vitamin D.
Local cortisol activation by 11-beta-HSD1 mirrors the cortisol economy in liver and fat.
Sebocyte and hair-follicle steroid metabolism handle androgen effects at their targets.
Physiological Role
Vitamin D supply: most of the body's input originates in skin — the calcium axis starts here.
Pheromone-adjacent chemistry and vitamin D beyond bone — immune modulation included — are active research.
Melanin's UV shield is endocrine-adjacent: it throttles the same photons that make the hormone.
Interactions
Downstream activation: kidney calcitriol, bone, and parathyroid complete the vitamin D axis.
Sweat glands' endocrine-adjacent signaling — see the eccrine gland.
Skin carcinoid and amyloid disease hint at wider hormone capacity.
Clinical Relevance
Vitamin D deficiency: rickets to osteomalacia — the skin-factory disease of low sun.
Topical steroid atrophy shows local cortisol economy in reverse.
Androgenic skin disease — acne, hirsutism — is cutaneous steroid metabolism made visible.
Photosynthesis of vitamin D at high latitude in winter falls to near zero: the factory runs on photons the sky withholds.
Skin as endocrine target completes the loop: the same organ makes, activates, and receives steroid signals.