Category: endocrine_axis
Coupled homeostatic loop. Plasma Ca2+ low → parathyroid CaSR senses → PTH released → (1) bone: stimulates osteoclast resorption via RANKL on osteoblasts; (2) kidney: increases Ca2+ reabsorption + PO4 excretion + activates CYP27B1 → calcitriol; (3) gut: indirect via calcitriol → upregulates calbindin / TRPV6 for active Ca absorption. Net: plasma Ca rises. Inverse for high Ca → calcitonin from thyroid C cells (lesser role in humans). Pharmacology: bisphosphonates inhibit osteoclasts (alendronate, zoledronate); denosumab is anti-RANKL mAb; teriparatide is recombinant PTH(1-34) — paradoxically anabolic when given pulsatile (vs continuous which is resorptive); romosozumab is anti-sclerostin (Wnt activator → bone formation); cinacalcet is a CaSR PAM for secondary hyperPTH + parathyroid carcinoma.