An inverted sensor
Most hormones rise when their trigger rises. PTH is the opposite: it goes up when calcium goes down. The reason sits on the surface of every parathyroid cell — a calcium-sensing receptor (a member of the G protein-coupled receptor family) that constantly reads ionized calcium. When calcium is high, the receptor is occupied and tells the gland to stay quiet. When calcium dips, the receptor empties, the brake comes off, and stored PTH is released within minutes. So the parathyroid gland is wired to defend against *low* calcium above all.
Three jobs, one direction
Every action of PTH points the same way: raise blood calcium. It does three things.
- Bone — pull calcium out fast. PTH drives bone resorption. Curiously it does not grip the bone-eating osteoclast directly; instead it acts on bone-building cells, which then signal osteoclasts to dissolve mineral and free calcium (and phosphate) into the blood. We unpack that hand-off in the bone guide.
- Kidney — keep the calcium, dump the phosphate. PTH tells the kidney to reabsorb more calcium from the urine (so less is lost) while excreting *more* phosphate. Dumping phosphate matters: bone breakdown released phosphate too, and getting rid of it keeps calcium free in the blood rather than complexed.
- Kidney — switch on vitamin D. PTH activates the enzyme that turns stored vitamin D into its potent form, calcitriol. Calcitriol then opens the gut to absorb dietary calcium. This is the slow arm of the response, building over hours to days.
Notice the elegant division of labor. Bone and kidney give PTH a response in *minutes*. The gut, via calcitriol, gives a response over *days*. Short-term emergencies are met by raiding bone; long-term balance is met by absorbing more from food so the skeleton can be repaid. A useful one-liner: PTH raises calcium and lowers phosphate in the blood.
Closing the loop
As calcium climbs back toward the set point, the calcium-sensing receptor fills again and PTH secretion falls — clean negative feedback. Calcitriol adds a second brake: it directly tells the parathyroid glands to make less PTH. So rising calcium *and* rising active vitamin D both quiet the gland, which prevents overshoot.
Worked trace — calcium drops after an overnight fast t=0 min ionized Ca dips below set point t=2 min Ca-sensing receptor empties -> PTH released t=5 min kidney: reabsorb Ca up, phosphate excretion up t=15 min bone: resorption signaled -> Ca + Pi to blood t=30 min ionized Ca rising; PTH already easing off t=hours PTH-driven calcitriol made -> gut absorbs more Ca t=next day Ca normal, PTH back to baseline (loop reset) Net on blood: Ca UP Phosphate DOWN