Calcium and phosphate !

Calcium and phosphate form bone mineral and control nerves, muscles and energy; three hormones keep their blood levels steady.

Updated 2026-11-01

Key facts

99% of body calcium and 85% of body phosphorus are in bone as hydroxyapatite.
Only ionised calcium (about half of the total) is active; the rest is bound to albumin or anions. Low albumin lowers total calcium without changing the ionised fraction.
PTH: raises calcium and lowers phosphate (bone resorption, renal calcium saving, renal phosphate loss, more calcitriol).
Calcitriol: raises calcium and phosphate (gut absorption). FGF23: lowers phosphate and suppresses calcitriol.

Calcium sources: dairy, sardines with bones, nixtamalised maize tortillas, leafy greens, fortified foods. Phosphate: almost every protein food and the additives in processed foods.
Calcium roles: muscle contraction (troponin C), release of neurotransmitters and hormones, clotting (factor IV), second messenger (calmodulin).
Phosphate roles: ATP and other nucleotides, DNA and RNA, phospholipids, 2,3-BPG, acid-base buffer, protein phosphorylation.
The parathyroid gland senses calcium with a calcium-sensing receptor; when it falls, PTH is released within seconds.
Hypocalcaemia: tetany, perioral tingling, Chvostek and Trousseau signs, long QT, seizures. Causes: hypoparathyroidism, vitamin D deficiency, low magnesium, kidney disease, pancreatitis.
Hypercalcaemia: 'bones, stones, abdominal groans and psychic moans'; polyuria, constipation, confusion, short QT. Main causes: primary hyperparathyroidism and cancer.
Hypophosphataemia: muscle weakness, respiratory failure, haemolysis; typical of refeeding syndrome. Hyperphosphataemia: in chronic kidney disease, with vascular calcification.
Lab: total (albumin-corrected) or ionised calcium, phosphate, PTH, 25-OH-D, magnesium, alkaline phosphatase, urine calcium.

In chronic kidney disease phosphate rises, calcitriol falls and PTH rises (secondary hyperparathyroidism); this is why dietary phosphate is limited.
Hypercalcaemia of cancer is usually caused by PTH-related peptide (PTHrP) or bone metastases.
Low magnesium blocks PTH secretion and action: hypocalcaemia does not correct until magnesium is replaced.

Think of the answer, then tap the question to see it.

What does PTH do to blood calcium and phosphate?

It raises calcium and lowers phosphate: bone resorption, renal calcium saving, renal phosphate loss and more calcitriol.

Why can low albumin show a low total calcium without symptoms?

Protein-bound calcium falls, but ionised calcium, the active part, stays normal.

Which bone hormone makes the kidney waste phosphate?

FGF23, from osteocytes, which also suppresses renal 1α-hydroxylase.

Why does low magnesium cause hypocalcaemia?

Without magnesium the parathyroid cannot release PTH and tissues respond poorly to it.

Why does refeeding syndrome cause hypophosphataemia?

Insulin drives phosphate into cells to make ATP and phosphorylated glucose, and blood phosphate falls sharply.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Calcium in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Phosphorus in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Calcium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Phosphate in Blood: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Hypercalcemia: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.