Calcium Homeostasis

Why the setpoint is defended so tightly

The body holds about a kilogram of calcium, and 99% of it is locked in the hydroxyapatite of bone. The fraction circulating in blood is tiny — a few grams — yet it is defended more aggressively than almost any other ion. The reason is that ionized calcium (the free, unbound form) is a signaling molecule, not just a building block. It stabilizes the resting state of voltage-gated sodium channels on nerve and muscle membranes, it is the trigger for cardiac and skeletal contraction, it is a cofactor in the coagulation cascade (factor IV), and it gates neurotransmitter and hormone release at synapses and secretory cells. Move ionized calcium too far in either direction and excitable tissues misbehave within minutes.

Only the ionized fraction is sensed and regulated. Of total serum calcium, roughly 45% is ionized, about 40% is bound to albumin, and the remaining 10-15% is complexed to anions such as phosphate, bicarbonate, and citrate. This distribution explains two clinical traps. First, a low serum albumin lowers the measured total calcium without touching the active ionized fraction — so a malnourished or cirrhotic patient can have a frighteningly low "calcium" on the chemistry panel yet be physiologically normal. The bedside correction adds 0.8 mg/dL to the measured value for each 1 g/dL that albumin sits below 4 g/dL. Second, blood pH shifts calcium on and off albumin: alkalosis frees up albumin binding sites, so calcium sticks to protein and the ionized level drops. This is exactly why a panic attack with hyperventilation can produce perioral tingling and carpal spasm despite a perfectly normal total calcium.

The three-hormone control loop

Calcium balance is a classic negative-feedback servo, and the sensor is the calcium-sensing receptor (CaSR), a G-protein-coupled receptor sitting on the surface of the parathyroid chief cells. The CaSR is unusual in that its ligand is an ion rather than a peptide, and it is exquisitely sensitive: a drop in ionized calcium of just a few percent relieves its inhibitory signaling and releases parathyroid hormone (PTH) within seconds. PTH is the master regulator. It acts on three target organs to push calcium back up:

• Bone. PTH binds receptors on osteoblasts, which respond by displaying RANKL. RANKL activates osteoclasts, which dissolve mineralized matrix and liberate both calcium and phosphate into the blood. (Note the counterintuitive wiring: PTH never touches osteoclasts directly — it works through the bone-building cell.)
• Kidney. PTH increases active calcium reabsorption in the distal convoluted tubule, reclaiming calcium that would otherwise be lost in urine. At the same time it blocks phosphate reabsorption in the proximal tubule, dumping phosphate — which keeps the calcium-phosphate product from rising and prevents precipitation.
• Vitamin D activation. PTH switches on renal 1-alpha-hydroxylase, the enzyme that converts inactive 25-hydroxyvitamin D into the active hormone calcitriol (1,25-dihydroxyvitamin D).

Calcitriol is the gut arm of the system. Vitamin D begins as cholecalciferol made in skin under ultraviolet light (or absorbed from diet), is hydroxylated by the liver to 25-hydroxyvitamin D — the storage form that labs measure — and is finally activated in the kidney. Active calcitriol binds the vitamin D receptor inside intestinal enterocytes and switches on the calcium-transport machinery (the TRPV6 channel and calbindin), raising fractional intestinal calcium absorption from a passive baseline of 10-15% up to 30-40%. Calcitriol also feeds back to suppress PTH gene transcription, closing the loop. Without it, dietary calcium passes straight through, and the skeleton is robbed to keep blood calcium normal — causing soft, undermineralized bone: rickets in growing children and osteomalacia in adults.

The third hormone, calcitonin, runs the opposite direction. Secreted by the parafollicular C-cells of the thyroid when calcium climbs, it inhibits osteoclasts and increases urinary calcium excretion. In humans its physiologic contribution is surprisingly minor — patients with no thyroid (and therefore no calcitonin) and patients with calcitonin-spewing medullary thyroid cancer both keep normal calcium — but it is clinically useful as a fast-acting drug for severe hypercalcemia and for Paget disease of bone.

Bone as the calcium bank

Bone is not inert scaffolding; it is a dynamic mineral reservoir under constant remodeling. Osteoblasts lay down new collagen matrix and mineralize it, while osteoclasts resorb old bone. Around 5-10% of the adult skeleton is replaced every year through this coupled cycle. PTH is the dial on this bank: continuous high PTH (as in untreated hyperparathyroidism) tips the balance toward resorption and thins the skeleton, while intermittent pulsed PTH paradoxically stimulates net bone formation — which is exactly why teriparatide, a once-daily PTH analog, is one of the few drugs that build new bone in osteoporosis rather than merely slowing its loss. Phosphate travels with calcium throughout this story: bone resorption releases both, and one of PTH's jobs is to ensure the freed phosphate is spilled into the urine so it does not drag calcium back out of solution.

When the loop breaks: hypo- and hypercalcemia

Hypocalcemia raises neuromuscular excitability because low ionized calcium destabilizes sodium channels, letting nerves fire too easily. Patients describe perioral numbness, tingling fingertips, and cramping; the classic signs are Chvostek (a facial twitch when the cheek over the facial nerve is tapped) and Trousseau (carpal spasm provoked by inflating a blood-pressure cuff for three minutes). Severe cases bring laryngospasm, frank tetany, seizures, and a prolonged QT interval on the ECG. The most common causes are accidental removal or devascularization of the parathyroid glands during thyroid surgery, vitamin D deficiency, chronic kidney disease (which cannot make calcitriol or excrete phosphate), and acute drops in ionized calcium from alkalosis or the citrate load of a massive blood transfusion.

Hypercalcemia does the opposite, sedating excitable tissue: it causes the "stones, bones, groans, and psychiatric moans" of kidney stones, bone pain, constipation and abdominal pain, and confusion or depression, plus polyuria and a shortened QT. Over 90% of cases trace to one of two causes. Primary hyperparathyroidism — usually a single benign parathyroid adenoma — is the leading cause in outpatients, with an inappropriately high or normal PTH driving calcium up. Malignancy is the leading cause in hospitalized patients, either through PTH-related peptide (PTHrP) secreted by solid tumors, which mimics PTH at its receptor, or through direct bony destruction; here PTH is appropriately suppressed. Measuring PTH alongside calcium is therefore the single most useful step in sorting out the cause. Acute severe hypercalcemia is treated with aggressive IV saline to restore volume and promote calcium excretion, followed by bisphosphonates to shut down osteoclasts and calcitonin for a rapid but short-lived drop.

Hypocalcemia vs hypercalcemia at a glance

Feature	Hypocalcemia (low calcium)	Hypercalcemia (high calcium)
Threshold (total Ca)	< 8.5 mg/dL	> 10.5 mg/dL
Neuromuscular effect	Increased excitability — tetany, spasm	Decreased excitability — weakness, lethargy
Hallmark signs	Chvostek & Trousseau, perioral tingling	Stones, bones, groans, psychiatric moans
ECG change	Prolonged QT interval	Shortened QT interval
Typical PTH response	PTH high (unless hypoparathyroid)	PTH high in hyperparathyroidism, low in cancer
Leading causes	Post-surgical hypoparathyroidism, vitamin D deficiency, CKD	Primary hyperparathyroidism, malignancy (PTHrP)
First-line treatment	IV/oral calcium + active vitamin D	IV saline, then bisphosphonate/calcitonin

Special states that stress the system

• Pregnancy and lactation. The fetal skeleton demands roughly 30 g of calcium, met largely by doubling intestinal absorption via increased calcitriol. During breastfeeding, PTH-related peptide from the breast mobilizes maternal bone, transiently lowering bone density that recovers after weaning.
• Chronic kidney disease. The failing kidney cannot make calcitriol or excrete phosphate, so phosphate rises, calcium falls, and the parathyroids enlarge and pump out PTH continuously — secondary, then autonomous tertiary hyperparathyroidism — driving renal osteodystrophy and vascular calcification.
• Magnesium depletion. Severe hypomagnesemia paradoxically blocks PTH secretion and action, producing a hypocalcemia that will not correct with calcium until the magnesium is replaced.
• Familial hypocalciuric hypercalcemia. An inactivating mutation in the calcium-sensing receptor resets the setpoint higher, so the body defends a mildly elevated calcium with low urinary calcium — a benign mimic of hyperparathyroidism that must not be operated on.

This article is educational and is not medical advice. Diagnosis and treatment of calcium disorders require individualized care from a qualified clinician.