Calcium Homeostasis

Serum calcium is held in a very narrow range (about 2.2 to 2.6 mmol/L) because it is essential for nerve and muscle function, clotting, and as a building block of bone. The body achieves this with three hormones acting on three organs: parathyroid hormone, active vitamin D (calcitriol) and calcitonin acting on bone, gut and kidney. Bone is both the largest calcium store and a tissue under constant remodelling, so calcium balance and skeletal health are inseparable.

Metabolic bone diseases are the conditions that arise when this system goes wrong or when bone turnover itself is disordered. For the exam, the high-value distinctions are simple to state but easy to muddle: osteoporosis is too little bone of normal quality, osteomalacia and rickets are bone that is normal in amount but poorly mineralised, hyperparathyroidism is too much PTH, and Paget disease is chaotic, accelerated remodelling. This topic builds the calcium axis first, then uses it to make sense of each disease and its biochemistry.

About 99 percent of body calcium is stored in bone as hydroxyapatite. The small fraction in blood exists in three forms: ionised (the active, regulated fraction, about half), protein-bound (mostly to albumin), and complexed with anions. Because only ionised calcium is biologically active, a low albumin lowers total calcium without making the patient hypocalcaemic - this is why we correct calcium for albumin or measure ionised calcium.

The three hormones:

The feedback loop, step by step:

• A fall in ionised calcium is sensed by the calcium-sensing receptor on parathyroid cells, which release PTH.
• PTH acts on bone to stimulate osteoclastic resorption (indirectly, through RANKL on osteoblasts) and on the kidney to reabsorb more calcium while excreting phosphate.
• PTH also switches on renal 1-alpha-hydroxylase, converting 25-OH-D to active calcitriol, which increases gut absorption of calcium.
• As calcium rises, PTH secretion is suppressed - a classic negative feedback loop.

Phosphate moves with calcium but in the opposite direction under PTH: PTH is phosphaturic, so high PTH tends to give a high calcium and a low phosphate, while calcitriol raises both. Tracking calcium, phosphate, PTH and alkaline phosphatase together is what lets you read any metabolic bone picture.

Vitamin D begins as an inactive precursor: cholecalciferol (D3) made in skin under UVB light or taken in the diet, and ergocalciferol (D2) from plants. It then passes through two hydroxylation steps:

• Liver: adds a hydroxyl at carbon 25 to make 25-hydroxyvitamin D (25-OH-D). This is the stable, abundant storage form and is the one measured to assess vitamin D status.
• Kidney: adds a hydroxyl at carbon 1 (by 1-alpha-hydroxylase, stimulated by PTH and low phosphate) to make active 1,25-dihydroxyvitamin D (calcitriol).

Calcitriol then increases intestinal absorption of calcium and phosphate, supports normal bone mineralisation, and feeds back to suppress PTH. Two clinical consequences follow directly: liver disease impairs the first step, and kidney disease impairs the activating second step, so chronic kidney disease produces low calcitriol, low calcium and secondary hyperparathyroidism (renal osteodystrophy).

Reading the biochemistry

The single most useful exam skill is reading the calcium / phosphate / PTH / alkaline phosphatase pattern. The table below summarises the classic pictures.

Osteoporosis

Osteoporosis is reduced bone mass with deteriorated microarchitecture, so bone is normal in composition but there is simply less of it, raising fragility-fracture risk. Mineralisation and routine biochemistry are normal, which is the key distinction from osteomalacia. Diagnosis is by DXA (a T-score of -2.5 or lower at hip or spine) or by a low-trauma fragility fracture. Management combines lifestyle measures, calcium and vitamin D sufficiency, and bone-active drugs (anti-resorptives such as bisphosphonates and denosumab, or anabolic agents such as teriparatide and romosozumab in high-risk patients).

Osteomalacia and Rickets

Osteomalacia is defective mineralisation of osteoid in the adult skeleton; in the growing child the same defect at the growth plate is called rickets. The usual cause is vitamin D deficiency (low sunlight, poor intake, malabsorption), but it also follows chronic kidney disease, certain anticonvulsants, and rare phosphate-wasting disorders. Patients have bone pain, proximal muscle weakness and fractures; children develop bowing, metaphyseal widening and growth failure. The radiographic hallmark is the Looser zone (pseudofracture) - a transverse lucent band where unmineralised osteoid has accumulated, as shown in the imaging above. Treatment is vitamin D and calcium replacement, correcting the underlying cause.

Hyperparathyroidism

Primary hyperparathyroidism is the classic cause of high calcium in the well outpatient, usually from a single adenoma, and may be silent or cause stones, bone disease and the wider hypercalcaemia symptoms. Severe disease can produce osteitis fibrosa cystica (brown tumours, subperiosteal resorption, salt-and-pepper skull). Parathyroidectomy is the definitive treatment when criteria are met.

Paget Disease of Bone

Paget disease is disordered, accelerated remodelling producing structurally weak, enlarged bone. It often affects the pelvis, spine, skull and long bones, and may be silent or cause bone pain, deformity (bowing), deafness and rarely high-output cardiac failure or sarcomatous change. Biochemistry shows an isolated, often very high alkaline phosphatase with normal calcium and phosphate. Bisphosphonates, especially a single zoledronic acid infusion, suppress the high turnover and relieve pain.

Calcium homeostasis underpins much of orthopaedic and exam practice. In fracture clinic and fracture liaison services, recognising and correcting vitamin D deficiency and osteoporosis is central to secondary prevention. In basic-science vivas, the PTH-vitamin D axis and the biochemical patterns of metabolic bone disease are classic asks. In trauma and elective surgery, poorly mineralised or low-density bone changes fixation strategy and healing. And in perioperative care, both severe hypercalcaemia and symptomatic hypocalcaemia are emergencies. Being able to read a calcium / phosphate / PTH / alkaline phosphatase panel and act on it is the practical core that examiners reward.

• Osteoporosis and bone-health guidance worldwide (for example AAOS/AOA bone-health initiatives, NICE/NOGG in the UK, and endocrine society guidance) recommends ensuring calcium and vitamin D sufficiency as the foundation on which bone-active drug therapy is added.
• Vitamin D supplementation evidence supports targeted replacement of those who are deficient or at high risk rather than blanket high-dose use in healthy, replete older adults.
• Primary hyperparathyroidism consensus guidelines list bone protection (low bone density or fragility fracture), high calcium, young age and renal involvement among the recognised indications for parathyroidectomy.
• Renal bone disease (CKD-MBD) guidance frames management around controlling phosphate, providing active vitamin D analogues, and managing secondary hyperparathyroidism, with the aim of protecting both bone and the cardiovascular system.