Parathyroid hormone

Where it comes from

Made in

Chief cells of the four parathyroid glands behind the thyroid

Sent out by

A fall in blood calcium, sensed by the calcium-sensing receptor; high phosphate and low vitamin D raise it

Acts on

• Bone (osteoblasts, and through them osteoclasts)
• Kidney (calcium reabsorption, phosphate excretion, vitamin D activation)
• Gut, indirectly through vitamin D

What it does

• Raises blood calcium by freeing it from bone, keeping it in the kidney and activating vitamin D
• Lowers blood phosphate by stopping the kidney reabsorbing it
• Builds bone when it comes in brief pulses and dissolves it when it is constantly high ¹

In medicine

• Measured for hyperparathyroidism, the commonest cause of high calcium, and for hypoparathyroidism after thyroid surgery
• Teriparatide (PTH 1-34) and abaloparatide build bone in osteoporosis by daily injection; teriparatide cut new vertebral fractures by about two thirds over 21 months ¹²
• Palopegteriparatide (Europe 2023, US 2024), a long-acting PTH (1-34) prodrug, replaces the hormone in hypoparathyroidism; the earlier PTH (1-84) product was recalled in the US in 2019 and discontinued worldwide at the end of 2024 ³

Beyond the body’s own use

• Teriparatide carried a boxed warning about osteosarcoma, taken from rat studies, and a two-year lifetime limit; both were removed in 2020 and 2021 after fifteen years of surveillance found no excess of the tumour in treated adults ⁴
• Daily injection is the only route that works: PTH is digested if swallowed, and a level held constantly high dissolves bone instead of building it ¹
• It is given off licence for fractures that will not knit, for atypical femoral fracture and for osteonecrosis of the jaw; the trials behind those uses are small and none is an approved indication