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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# 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.

## In detail

- 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.

**Also called**
Ca²⁺, Phosphorus, Hydroxyapatite, PTH, FGF23

## Clinical notes

- 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.

## Related conditions

- [Rickets and osteomalacia](https://durgical.com/en/conditions/rickets-and-osteomalacia/)
- [Osteoporosis](https://durgical.com/en/conditions/osteoporosis/)
- [Primary hyperparathyroidism](https://durgical.com/en/conditions/primary-hyperparathyroidism/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Kidney stones (urolithiasis)](https://durgical.com/en/conditions/kidney-stones/)

## Related

- [Vitamin D (calcitriol)](https://durgical.com/en/biochemistry/molecules/vitamin-d/)
- [Magnesium](https://durgical.com/en/biochemistry/topics/magnesium/)
- [Fluoride](https://durgical.com/en/biochemistry/topics/fluoride/)
- [Vitamin K (phylloquinone)](https://durgical.com/en/biochemistry/molecules/vitamin-k/)
- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)

## Test yourself

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.

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

Sources: [Calcium in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002412.htm) (MedlinePlus, US National Library of Medicine, 2026); [Phosphorus in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002424.htm) (MedlinePlus, US National Library of Medicine, 2026); [Calcium Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/calcium-blood-test/) (MedlinePlus, US National Library of Medicine, 2026); [Phosphate in Blood: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/phosphate-in-blood/) (MedlinePlus, US National Library of Medicine, 2026); [Hypercalcemia: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000365.htm) (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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