parathyroid hormone (PTH)
Parathyroid hormone is the body's emergency response team for low calcium. The moment blood calcium dips, the parathyroid glands release PTH, which then works on three fronts at once to push calcium back up.
First, PTH acts on bone, telling osteoblasts to recruit and activate osteoclasts, which dissolve a little bone mineral and free calcium into the blood. Second, it acts on the kidney, where it forces the tubules to reabsorb more calcium (so less is lost in urine) while dumping more phosphate. Third, and more slowly, it switches on the kidney enzyme that converts vitamin D into its active form, calcitriol, which then boosts calcium absorption from food in the gut.
PTH is a peptide hormone with a short half-life, so its level can change minute to minute, allowing tight, rapid control. There is an important paradox in how it affects bone: continuous high PTH (as in disease) breaks bone down, but brief daily pulses (as with the drug teriparatide) actually stimulate bone formation, a fact exploited in osteoporosis treatment.
A blood test showing high PTH together with high calcium points toward a parathyroid gland that is overproducing the hormone, a hallmark of primary hyperparathyroidism.
High PTH with high calcium is the classic biochemical signature.
PTH does not act on osteoclasts directly; it signals osteoblasts and stromal cells, which then produce RANKL to activate osteoclasts. This indirect route is central to why PTH dosing schedule changes its effect.