Vitamin D is really a hormone
Despite the name, vitamin D behaves like a steroid hormone, not a vitamin. Your skin makes it from a cholesterol relative when sunlight hits it, and food supplies more. But what comes from skin or diet is *inactive*. It must be activated in two steps — one in the liver, one in the kidney — before it can do anything. Think of it as a hormone you can partly stock up from the sun.
- Liver — first hydroxylation. Inactive vitamin D becomes 25-hydroxyvitamin D. This is the storage form and the one labs measure to judge whether your body's vitamin D *supply* is adequate. It is abundant but still not very active.
- Kidney — second hydroxylation. PTH switches on the enzyme that converts 25-hydroxyvitamin D into calcitriol, the fully active hormone (also called 1,25-dihydroxyvitamin D). This step is *regulated* — it is the throttle the body actually controls.
The main job of calcitriol is to open the gut so it absorbs dietary calcium and phosphate efficiently. Without it, you absorb only a small fraction of the calcium you eat, no matter how much you swallow. That single fact explains why severe vitamin D deficiency starves bone of mineral and leads to soft, weak bone — rickets in children, osteomalacia in adults — which we return to later.
Calcitonin: the quiet brake
If PTH is the accelerator that raises calcium, calcitonin is, in theory, the brake. Made by special cells in the thyroid, it is released when calcium runs *high* and it gently opposes osteoclasts to slow calcium release from bone. Tidy symmetry — except in humans it barely matters. People with no thyroid (and therefore almost no calcitonin) keep normal calcium, and people with calcitonin-secreting tumors do not become hypocalcemic.
FGF23: phosphate's own hormone
Calcium gets all the attention, but phosphate has its own dedicated controller, made by bone cells: FGF23. When phosphate runs high, FGF23 rises and does two things — it tells the kidney to dump more phosphate in urine, and it *turns down* the kidney's vitamin-D-activating enzyme so less calcitriol is made and less phosphate is absorbed from the gut. Together that lowers blood phosphate.
Who moves phosphate which way?
Hormone Blood Ca Blood Phosphate
----------- ----------- -------------------
PTH UP DOWN (renal dump)
Calcitriol UP UP (gut absorb)
FGF23 (n/a) DOWN (renal dump +
less calcitriol)
Calcitonin slight DOWN slight DOWN (minor)
Reading it: PTH and FGF23 both throw phosphate out;
calcitriol pulls it in. The kidney is the
shared dial they all turn.Why bother with phosphate at all? Because calcium and phosphate together form the mineral that hardens bone, and if their product in blood gets too high they precipitate in soft tissues — blood vessels, joints — where you do not want stone. So phosphate balance is not a footnote; it is the other half of keeping bone strong and the bloodstream clear.