Calcium and Bone Metabolism

Calcium is one of the most tightly regulated ions in the body — far more tightly than glucose or sodium. Your serum calcium can vary by only a few percent before nerves misfire, muscles cramp or the heart's rhythm falters, yet the skeleton holds roughly a kilogram of calcium in reserve. Understanding how the body defends this narrow set-point — using parathyroid hormone, vitamin D and a supporting cast of hormones acting on gut, kidney and bone — unlocks a huge swath of clinical medicine: osteoporosis, kidney stones, thyroid surgery complications, chronic kidney disease and some of the most common findings on routine blood tests.

This page teaches the control system first, then walks through what happens when it breaks.

Learning Objectives

• Describe the distribution of body calcium and why the ionized fraction matters clinically.
• Explain how parathyroid hormone (PTH) and vitamin D (calcitriol) act on bone, kidney and gut to defend serum calcium.
• Trace the activation pathway of vitamin D from skin/diet to active hormone.
• Distinguish osteoporosis from osteomalacia and interpret a basic DEXA report.
• Work through the diagnostic approach to hypercalcemia and hypocalcemia.
• Recall the historical discovery of the parathyroid glands and vitamin D and why it mattered.

Quick Answer

Serum calcium is defended within a very narrow range (about 8.5–10.5 mg/dL total; ionized ~4.5–5.3 mg/dL) by two main hormones. PTH, secreted by the four parathyroid glands when calcium falls, raises calcium by pulling it from bone, reclaiming it in the kidney, and switching on renal activation of vitamin D. Calcitriol (active vitamin D) raises calcium mainly by boosting gut absorption of calcium and phosphate. The calcium-sensing receptor on parathyroid cells is the thermostat that reads calcium and tunes PTH output. When this system fails, you get predictable disease: primary hyperparathyroidism and malignancy cause most hypercalcemia; vitamin D deficiency causes rickets/osteomalacia; and long-term imbalance between bone resorption and formation causes osteoporosis. Diagnosis almost always starts with measuring calcium, PTH, phosphate and vitamin D together.

Where It Came From

For most of the 19th century the parathyroid glands did not officially exist. They were first described in 1852 by Richard Owen during a dissection of an Indian rhinoceros at the London Zoo, and independently and more thoroughly in humans by the Swedish anatomy student Ivar Sandström in 1880, who named them the "glandulae parathyroideae." His paper was largely ignored — they were tiny, easily mistaken for fat or lymph nodes, and no one knew what they did.

The need to understand them became urgent and tragic. As surgeons in the 1880s began removing goitrous thyroid glands, some patients developed violent muscle spasms (tetany) and died. Eugène Gley and later Jacques-Louis Reverdin and Theodor Kocher realized these deaths followed inadvertent removal of the parathyroids, not the thyroid itself. In 1925 James Collip prepared an active parathyroid extract that could raise blood calcium and reverse tetany — proving these glands secreted a calcium-controlling hormone. This is the classic lesson still drilled into every surgical trainee: damage the parathyroids during neck surgery and you cause life-threatening hypocalcemia.

Vitamin D has a parallel story rooted in a disease of poverty and cities. Rickets — soft, bowed bones in children — exploded during the Industrial Revolution as families crowded into smoky, sunless northern cities. In 1919 Edward Mellanby produced rickets in dogs by diet and cured it with cod-liver oil, isolating a "fat-soluble factor." Elmer McCollum named it vitamin D in 1922. The twist came when Alfred Hess and others showed that sunlight alone could also cure rickets — because ultraviolet light manufactures vitamin D in skin. Adolf Windaus won the 1928 Nobel Prize for working out its chemistry. The realization that vitamin D is really a hormone the body can make itself, not merely a dietary vitamin, reshaped nutrition and public health, leading to milk fortification and the near-eradication of childhood rickets in developed countries.

Calcium in the Body: Distribution and Set-Point

About 99% of the body's calcium sits in bone as hydroxyapatite, a crystalline calcium-phosphate mineral that gives bone its rigidity and doubles as a vast reservoir. Only about 1% is in soft tissue and extracellular fluid — but that tiny circulating pool is what nerves, muscles and clotting depend on.

Serum calcium travels in three forms:

• Ionized (free) calcium (~50%) — the biologically active fraction the body actually senses and regulates.
• Protein-bound (~40%) — mostly to albumin; a reservoir that is not immediately active.
• Complexed (~10%) — bound to citrate, phosphate, bicarbonate.

This distribution explains two exam favourites. First, corrected calcium: because much calcium is albumin-bound, a low albumin makes total calcium look low even when ionized calcium is normal. A useful bedside correction adds 0.8 mg/dL to measured calcium for every 1 g/dL that albumin falls below 4 g/dL. Second, pH effects: alkalosis increases calcium binding to albumin, dropping ionized calcium. This is why a patient who hyperventilates from anxiety can develop perioral tingling and carpopedal spasm — respiratory alkalosis has transiently lowered their free calcium.

The Two Master Hormones: PTH and Vitamin D

Parathyroid hormone — the fast responder

The four parathyroid glands carry a calcium-sensing receptor (CaSR) on their surface. When ionized calcium falls, less calcium binds the receptor, and the gland releases PTH within minutes. PTH then acts on three fronts:

1. Bone — stimulates osteoclastic resorption (indirectly, via osteoblast signalling using RANKL), releasing both calcium and phosphate.
2. Kidney — increases calcium reabsorption in the distal tubule, and decreases phosphate reabsorption in the proximal tubule (so it is phosphaturic).
3. Kidney (activation of vitamin D) — stimulates 1-alpha-hydroxylase, the enzyme that produces active vitamin D.

The net effect is elegant: calcium goes up, phosphate goes down. That reciprocal pattern is a diagnostic fingerprint — hyperparathyroidism classically gives high calcium with low phosphate.

Vitamin D — the slow amplifier

Vitamin D is activated in two steps. Skin makes vitamin D3 (cholecalciferol) from 7-dehydrocholesterol under UVB light; diet supplies D2/D3 as well. This precursor is inert and must be hydroxylated twice:

1. In the liver → 25-hydroxyvitamin D (calcidiol). This is the stable circulating form and the one we measure to assess vitamin D status.
2. In the kidney → 1,25-dihydroxyvitamin D (calcitriol), the active hormone, under the control of PTH and low phosphate.

Calcitriol's headline job is to increase intestinal absorption of both calcium and phosphate by inducing calcium-transport proteins in enterocytes. It also permits normal bone mineralization. Without it, dietary calcium simply is not absorbed, no matter how much you eat.

A worked example ties it together: a housebound elderly woman with poor sun exposure becomes vitamin D deficient. Gut calcium absorption falls, serum calcium drifts down, the CaSR senses it, and PTH rises. This secondary hyperparathyroidism keeps her serum calcium in the normal range — but at the cost of chronically leaching calcium from bone and dumping phosphate in the urine, producing soft, undermineralized bone (osteomalacia) and eventually bone pain and fractures.

Calcitonin and FGF23 — the supporting cast

Calcitonin, from thyroid C-cells, lowers calcium by inhibiting osteoclasts — but in humans it is a minor player (thyroidectomy does not cause hypercalcemia). FGF23, secreted by bone, lowers phosphate and suppresses calcitriol, and is central to the mineral chaos of chronic kidney disease.

Osteoporosis: When the Reservoir Runs Down

Osteoporosis is a disorder of bone quantity and quality, not blood calcium — serum calcium is typically normal. Throughout life, bone is continuously remodelled: osteoclasts resorb old bone, osteoblasts lay down new. Peak bone mass is reached in the late 20s. After menopause, the loss of estrogen removes a brake on osteoclasts, and resorption outpaces formation. Bone becomes porous and fragile, and fractures — especially of hip, spine and wrist — occur with minimal trauma.

Diagnosis uses DEXA (dual-energy X-ray absorptiometry), reported as a T-score (standard deviations from a healthy young adult):

T-score	Interpretation
−1.0 or higher	Normal
−1.0 to −2.5	Osteopenia (low bone mass)
−2.5 or lower	Osteoporosis
−2.5 or lower plus a fragility fracture	Severe/established osteoporosis

Management combines lifestyle (weight-bearing exercise, adequate calcium and vitamin D, stopping smoking and excess alcohol, fall prevention) with drugs. Bisphosphonates (alendronate, zoledronic acid) are first-line — they poison osteoclasts and reduce resorption. Others include denosumab (a RANKL antibody), teriparatide (recombinant PTH given intermittently, which paradoxically builds bone), and romososumab. A key teaching point: continuous PTH exposure resorbs bone, but pulsatile PTH stimulates formation — the basis of teriparatide therapy.

Do not confuse osteoporosis with osteomalacia: osteoporosis is too little normally mineralized bone; osteomalacia is bone that is present but poorly mineralized due to vitamin D or phosphate deficiency (rickets is the childhood version, affecting growth plates).

Disorders of Calcium

Hypercalcemia

The two causes that account for roughly 90% are primary hyperparathyroidism (usually a single benign parathyroid adenoma, most common in outpatients) and malignancy (most common in inpatients — via PTH-related peptide, bony metastases, or lymphoma-derived calcitriol). The single most useful discriminator is the PTH level:

• Calcium high and PTH high (or inappropriately normal) → primary hyperparathyroidism.
• Calcium high and PTH low/suppressed → look for malignancy, granulomatous disease (sarcoidosis makes calcitriol), vitamin D toxicity, thyrotoxicosis, immobilization.

Symptoms are remembered as "stones, bones, groans, and psychiatric moans": kidney stones, bone pain, abdominal pain/constipation/nausea, and fatigue/confusion/depression. Severe hypercalcemia is a medical emergency treated with aggressive IV normal saline, then bisphosphonates or calcitonin.

Hypocalcemia

The commonest causes are post-surgical hypoparathyroidism (after thyroid or parathyroid surgery — the historical lesson made flesh) and vitamin D deficiency. Symptoms reflect neuromuscular irritability: perioral numbness, tingling, muscle cramps, and in severe cases tetany, laryngospasm and seizures. Two classic bedside signs:

• Chvostek's sign — tapping over the facial nerve causes facial twitching.
• Trousseau's sign — inflating a blood-pressure cuff above systolic for a few minutes provokes carpopedal spasm.

Acute severe hypocalcemia (especially with ECG QT prolongation or seizures) is treated with IV calcium gluconate; chronic cases with oral calcium and active vitamin D analogues.

Real-World Applications

• Surgery wards: Every patient after thyroidectomy has calcium monitored — parathyroid injury is the feared complication, and perioral tingling is an early warning.
• Primary care: An incidental high calcium on a routine panel should trigger a PTH test, not be ignored — it often uncovers asymptomatic primary hyperparathyroidism.
• Women's health: Post-menopausal women are screened with DEXA; ensuring adequate calcium (~1000–1200 mg/day) and vitamin D underpins all osteoporosis care.
• Nephrology: Chronic kidney disease patients lose 1-alpha-hydroxylase activity and retain phosphate, driving secondary hyperparathyroidism and renal bone disease — a whole subspecialty built on this pathway.
• Public health: Vitamin D fortification of milk and infant supplementation reflect the hard-won lesson of the rickets epidemic.

Common Mistakes

• Reading total calcium without checking albumin. A patient with low albumin (nephrotic syndrome, liver disease, sepsis) may have a low total calcium but perfectly normal ionized calcium. Always correct for albumin or measure ionized calcium before treating.
• Thinking vitamin D directly raises blood calcium by acting on bone. Its main job is boosting gut absorption of calcium and phosphate. Its bone role is chiefly to enable normal mineralization, and in deficiency the failure is under-mineralized bone (osteomalacia), not brittle bone.
• Confusing osteoporosis and osteomalacia. Both weaken bone, but osteoporosis is normal bone in reduced quantity (calcium usually normal), while osteomalacia is defective mineralization from vitamin D/phosphate deficiency (calcium and phosphate often low, ALP high). The treatments differ.
• Assuming calcitonin is important in humans. It exists and is used pharmacologically, but endogenously it is minor — losing it (total thyroidectomy) does not raise calcium.
• Interpreting PTH in isolation. PTH is only meaningful alongside the calcium level. A "normal" PTH is actually abnormal if calcium is high — the gland should have shut off.

Comparison and Connections

Feature	PTH	Calcitriol (active vitamin D)
Source	Parathyroid glands	Kidney (final activation)
Trigger	Low ionized calcium (via CaSR)	Low calcium, high PTH, low phosphate
Speed	Fast (minutes)	Slow (hours–days)
Effect on calcium	Raises	Raises
Effect on phosphate	Lowers (phosphaturia)	Raises (gut absorption)
Main site	Bone + kidney	Intestine

Disorder	Calcium	Phosphate	PTH
Primary hyperparathyroidism	High	Low	High
Malignancy (PTHrP)	High	Low	Low
Hypoparathyroidism	Low	High	Low
Vitamin D deficiency	Low/normal	Low	High (secondary)
Chronic kidney disease	Low/normal	High	High (secondary)

For the hormonal control system that surrounds this, see the Endocrinology branch overview. Calcium signalling also connects to muscle and nerve physiology (see Physiology) and to the skeleton itself (see Anatomy).

Practice Questions

Recall

Q: Which enzyme, and in which organ, produces the active form of vitamin D, and what stimulates it? A: 1-alpha-hydroxylase, in the kidney, converts 25-OH vitamin D to 1,25-(OH)2 vitamin D (calcitriol). It is stimulated by PTH and by low phosphate.

Understanding

Q: Why does primary hyperparathyroidism produce high calcium but low phosphate? A: PTH raises calcium (bone resorption, renal reabsorption, vitamin D activation) but simultaneously blocks phosphate reabsorption in the proximal tubule, causing phosphate to be lost in urine. Hence the reciprocal high-calcium/low-phosphate pattern.

Application

Q: A 68-year-old woman has a routine calcium of 11.2 mg/dL (high). What single test best narrows the diagnosis, and how do you interpret it? A: Measure PTH. If PTH is high or inappropriately normal, the diagnosis is primary hyperparathyroidism. If PTH is suppressed, pursue malignancy and other PTH-independent causes.

Analysis

Q: A housebound man has normal serum calcium, low-normal phosphate, high alkaline phosphatase, high PTH and low 25-OH vitamin D, plus bone pain. Explain the whole picture. A: Vitamin D deficiency reduces gut calcium absorption; calcium tends to fall, so PTH rises (secondary hyperparathyroidism), which restores calcium toward normal but drives phosphaturia (low-normal phosphate) and increased bone turnover (high ALP). The underlying bone defect is impaired mineralization — osteomalacia — explaining the pain. Treatment is vitamin D (and calcium) repletion, not bisphosphonates.

FAQ

Is calcium the same as vitamin D? Why do supplements combine them? No. Calcium is the mineral; vitamin D is the hormone that lets your gut absorb it. Combining them makes sense because taking calcium without adequate vitamin D means much of it is not absorbed.

Can you get enough vitamin D from sunlight alone? Often yes in sunny climates with regular exposure, because UVB makes vitamin D in skin. But sunscreen, dark skin, high latitude, winter, aging and indoor lifestyles all reduce synthesis, which is why deficiency is common and supplementation is often advised.

Does drinking lots of milk prevent osteoporosis? Adequate calcium and vitamin D are necessary but not sufficient. Estrogen status, exercise, genetics, smoking and medications matter greatly. Calcium alone does not reverse established osteoporosis — it supports the bone-active drugs that do.

Why does hyperventilating make my hands cramp? Blowing off CO2 causes respiratory alkalosis, which increases calcium binding to albumin and lowers the free (ionized) calcium. Less free calcium makes nerves hyperexcitable, causing tingling and carpopedal spasm — even though total calcium is unchanged.

Why are the parathyroid glands such a big deal in thyroid surgery? They sit right behind the thyroid, are tiny, and are easily bruised or removed by accident. Without them, PTH crashes and calcium falls dangerously, causing tetany. This is exactly the complication that led to their discovery in the 1880s.

Is more vitamin D always better? No. Excess vitamin D causes hypercalcemia, kidney stones and even kidney damage. It is a hormone with a therapeutic window, not a harmless vitamin.

Quick Revision

• 99% of calcium is in bone; the ionized fraction is what's regulated. Correct for albumin; alkalosis lowers ionized calcium.
• PTH: released when calcium falls (via CaSR); raises calcium, lowers phosphate; acts on bone, kidney, and activates vitamin D.
• Vitamin D: skin/diet → liver (25-OH, the one measured) → kidney (1,25-OH, active). Main job: gut absorption of calcium and phosphate.
• Osteoporosis: low bone mass, normal calcium; DEXA T-score ≤ −2.5. First-line drug: bisphosphonates. Pulsatile PTH (teriparatide) builds bone.
• Osteomalacia/rickets: defective mineralization from vitamin D/phosphate deficiency — different from osteoporosis.
• Hypercalcemia: mostly primary hyperparathyroidism (PTH high) or malignancy (PTH low). "Stones, bones, groans, psychiatric moans."
• Hypocalcemia: post-surgical hypoparathyroidism and vitamin D deficiency; Chvostek and Trousseau signs; treat severe cases with IV calcium gluconate.
• Always interpret calcium, phosphate, PTH and vitamin D together.

Related Topics

Prerequisites

• Endocrinology overview
• Physiology — membrane excitability and hormone signalling

Related Topics

• Anatomy — thyroid, parathyroid and skeletal structure
• Nephrology — CKD-mineral bone disease and renal vitamin D activation
• Thyroid and Parathyroid Disorders (see the Endocrinology branch)

Next Topics

• Osteoporosis pharmacology and fracture prevention
• Chronic kidney disease–mineral bone disorder (CKD-MBD)