Calcium & bone
A fast hormonal loop that keeps blood calcium within a narrow band.
PTH · calcitonin · PTHrP
Overview
Blood calcium is held within a narrow range by a fast-acting hormonal loop. Parathyroid hormone raises it; calcitonin opposes it; vitamin D (a steroid, not a peptide) supplies the slower arm.
These signals act on bone, kidney, and gut to move calcium in and out of the blood. The same PTH receptor is also engaged by PTH-related protein in development — and pathologically in some cancers.
Key signals
Raises blood calcium via bone, kidney, and (indirectly) gut.
Lowers blood calcium; opposes PTH.
Shares the PTH1 receptor; key in development and in hypercalcemia of malignancy.
Reference notes
- PTH and calcitonin are a classic opposing pair — raise versus lower calcium.
- Intermittent PTH exposure builds bone while continuous exposure breaks it down — a pulsatility effect echoing other axes.
- PTHrP shares the PTH1 receptor, which is why it can drive hypercalcemia in malignancy.