How PTH defends blood calcium
The four small parathyroid glands behind the thyroid sense blood calcium through a calcium-sensing receptor. When serum calcium falls, they release parathyroid hormone (PTH). PTH raises calcium through three coordinated actions: it stimulates osteoclast-driven release of calcium from bone, it increases calcium reabsorption in the kidney while promoting phosphate excretion, and it activates the enzyme that converts vitamin D to its active form, calcitriol.
Active vitamin D in turn increases calcium absorption from the gut, so PTH and vitamin D work together to restore calcium toward its normal narrow range.
Feedback and what disrupts it
As calcium returns to normal, the parathyroid glands sense the rise and reduce PTH output, completing a negative feedback loop. The hormone calcitonin from the thyroid can lower calcium modestly, but PTH and vitamin D are the dominant regulators in humans.
Persistent imbalance, such as in hyperparathyroidism or vitamin D deficiency, can affect bones, kidneys, and other tissues. This is general educational information, not medical advice; abnormal calcium or PTH levels should be evaluated by a qualified clinician.