Vitamin D is the clearest case of a vitamin that behaves like a hormone. Cholecalciferol, made in skin from 7-dehydrocholesterol under ultraviolet B light or absorbed from food, is biologically inactive. It is first hydroxylated at carbon 25 in the liver to 25-hydroxycholecalciferol, the main circulating form and the standard clinical marker of vitamin D status. A second hydroxylation at carbon 1, catalyzed by 1-alpha-hydroxylase in the proximal tubule of the kidney, produces calcitriol, the active hormone. Calcitriol acts through the vitamin D receptor, a nuclear receptor, and its principal target is the intestine, where it increases transcription of the calcium-binding protein calbindin and of the epithelial calcium channel, raising calcium and phosphate absorption. It also permits parathyroid hormone to mobilize calcium from bone and supports renal calcium reabsorption. The regulatory logic is a negative feedback loop: low plasma calcium raises parathyroid hormone, which stimulates 1-alpha-hydroxylase and raises calcitriol, which then restores calcium and suppresses parathyroid hormone. When calcitriol is deficient, the intestine absorbs too little calcium and phosphate, so the mineral needed to mineralize newly deposited bone matrix is missing. In growing children the result is rickets, with softened, deformed long bones and widened growth plates; in adults, where bone remodeling continues throughout life, the result is osteomalacia, with soft, painful bones.
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Follow the molecule across the diagram. Cholecalciferol arrives from skin or diet and does nothing on its own. The liver adds a hydroxyl at carbon twenty-five; that is the form you measure in blood. The kidney then adds a second hydroxyl at carbon one, and only now do you have calcitriol, the active hormone. Calcitriol travels to the intestine and increases the proteins that pull calcium and phosphate into the body. Notice the feedback arrow: low calcium drives parathyroid hormone, which drives that final kidney step. Cut the hormone and the intestine absorbs too little mineral, so new bone matrix is laid down but never hardened. In a child that is rickets; in an adult it is osteomalacia.
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