The parathyroid glands secrete parathyroid hormone (PTH) when serum calcium falls below ~8.5 mg/dL, and PTH increases blood calcium by promoting bone resorption (via RANKL signaling), renal calcium reabsorption, and synthesis of active vitamin D (1,25-dihydroxyvitamin D3). PTH simultaneously increases phosphate excretion, maintaining calcium-phosphate balance essential for neuromuscular and cardiac function.
From the endocrine system overview, you know that endocrine glands monitor internal conditions and release hormones to maintain homeostasis. Calcium homeostasis is among the most tightly regulated parameters in the body — serum calcium is maintained within a narrow range of 8.5–10.5 mg/dL because even small deviations cause serious problems. Too little calcium (hypocalcemia) makes neurons hyperexcitable, causing muscle spasms, tingling, and potentially fatal cardiac arrhythmias. Too much calcium (hypercalcemia) depresses neural function, causes kidney stones, and weakens bones. Parathyroid hormone (PTH) is the primary minute-to-minute regulator that prevents calcium from drifting outside this range.
The four parathyroid glands, small lentil-sized structures embedded in the posterior surface of the thyroid, contain chief cells equipped with calcium-sensing receptors (CaSR) on their surface. These receptors continuously monitor blood calcium concentration. When calcium drops below the set point, CaSR signaling decreases, which releases the brake on PTH secretion — the glands release preformed PTH from storage granules within seconds. When calcium rises, CaSR activation suppresses PTH release. This is a direct negative feedback loop that does not require the hypothalamus or pituitary: the parathyroid glands sense and respond autonomously.
PTH raises blood calcium through three coordinated actions at three different organs. In bone, PTH stimulates osteoblasts to express RANKL, which activates osteoclasts — the cells that break down bone matrix and release stored calcium and phosphate into the blood. In the kidney, PTH increases calcium reabsorption in the distal convoluted tubule (so less calcium is lost in urine) and simultaneously increases phosphate excretion in the proximal tubule (so phosphate does not rise alongside calcium, which would risk calcium-phosphate crystal deposition in soft tissues). In the kidney again, PTH stimulates the enzyme 1α-hydroxylase, which converts 25-hydroxyvitamin D (the circulating storage form) into 1,25-dihydroxyvitamin D₃ (calcitriol) — the active hormone. Calcitriol then acts on the small intestine to increase dietary calcium and phosphate absorption, providing a longer-term supply.
The interplay between PTH and vitamin D creates a multi-layered defense of calcium levels. PTH handles rapid corrections by mobilizing calcium from bone and reducing renal losses. Vitamin D handles sustained supply by enhancing intestinal absorption. When PTH is inappropriately elevated — as in primary hyperparathyroidism from a parathyroid adenoma — the result is chronic hypercalcemia, bone loss, kidney stones, and muscle weakness (the classic triad of "bones, stones, and groans"). When PTH is absent — as after accidental surgical removal of the parathyroid glands — severe hypocalcemia develops rapidly, requiring emergency calcium replacement and lifelong vitamin D supplementation.