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[Physiology](https://durgical.com/en/physiology/) · [Endocrine](https://durgical.com/en/physiology/#s-endocrine)

# Calcium regulation: PTH, vitamin D and calcitonin

- Regulation
- Exam priority: high

When blood calcium falls, the parathyroid releases PTH, which pulls it from bone, saves it in the kidney and activates vitamin D.

Updated 2026-11-01

Also called: Parathyroid hormone, Calcitriol, Calcium homeostasis

**See it in 3D:** [Parathyroid glands](https://durgical.com/en/anatomy/parathyroid-glands/) · [Thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/) · [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- PTH: ↑ bone resorption (via osteoblasts and RANKL), ↑ distal tubule Ca²⁺ reabsorption, ↓ proximal phosphate reabsorption, ↑ 1α-hydroxylase.
- Calcitriol (1,25-(OH)₂D): ↑ gut absorption of calcium and phosphate; it is activated in the kidney.
- Only ionized calcium (≈45–50%) is active; with low albumin, total calcium misleads.
- Alkalosis (e.g., hyperventilation) increases albumin binding and lowers ionized calcium: tetany.
- Severe hypomagnesemia blocks PTH secretion and action: hypocalcemia that does not correct until magnesium is replaced.
- Hypercalcemia with high PTH = primary hyperparathyroidism; with suppressed PTH = cancer (PTHrP) or other causes.

## What it is

Ionized calcium is held in a narrow range because nerves, muscles and clotting depend on it. Parathyroid chief cells sense it with a calcium-sensing receptor (CaSR) and answer within seconds with PTH. PTH acts on bone and kidney, and through calcitriol on the gut as well. Calcitonin plays a minor role in adults.

## How it works, step by step

- Ionized calcium falls and stops activating the chief cell's CaSR.
- Stored PTH is released within seconds; synthesis rises within hours and the gland grows over weeks.
- In bone PTH acts on osteoblasts: RANKL rises and osteoprotegerin falls, and osteoclasts release calcium and phosphate.
- In the kidney it raises distal tubule calcium reabsorption and blocks proximal phosphate cotransport (phosphaturia).
- It also stimulates proximal tubule 1α-hydroxylase: calcitriol rises.
- Calcitriol increases gut calcium absorption; calcium normalizes and brakes PTH (and calcitriol represses its own gene).

### Hormone axes and feedback

Calcium regulation (PTH, calcitriol, calcitonin) **Lesion** None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓

Illustrative schematic drawn by Durgical: simplified axes and classic lab patterns; not for clinical decisions.

## Equations

**Albumin-corrected calcium**Cacorr = Catotal + 0.8 × (4 − albumin)

**Ca**
mg/dL

**albumin**
g/dL

It is an estimate; when in doubt, ionized calcium is measured.

## Adult reference values (they vary by laboratory)

| Parameter | Value | Note |
| --- | --- | --- |
| Total calcium | ≈8.5–10.5 mg/dL (2.1–2.6 mmol/L) | |
| Ionized calcium | ≈4.5–5.3 mg/dL (1.1–1.3 mmol/L) | |
| Intact PTH | ≈10–65 pg/mL | |
| 25-OH vitamin D | sufficiency ≥20–30 ng/mL | the cut-off varies by guideline |
| Phosphate | ≈2.5–4.5 mg/dL | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Lab patterns

| Disorder | Ca²⁺ | PO₄³⁻ | PTH |
| --- | --- | --- | --- |
| [Primary hyperparathyroidism](https://durgical.com/en/conditions/primary-hyperparathyroidism/) | ↑ | ↓ | ↑ |
| [Hypercalcemia of malignancy (PTHrP)](https://durgical.com/en/conditions/hypercalcaemia/) | ↑ | ↓ | ↓ |
| [Hypoparathyroidism (e.g., after thyroidectomy)](https://durgical.com/en/conditions/hypocalcaemia/) | ↓ | ↑ | ↓ |
| [Vitamin D deficiency](https://durgical.com/en/conditions/rickets-and-osteomalacia/) | ↓ or normal | ↓ | ↑ |
| [Secondary hyperparathyroidism of chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/) | ↓ or normal | ↑ | ↑ |

## Clinical notes

- Hypocalcemia: perioral tingling, carpopedal spasm, Chvostek and Trousseau signs, long QT.
- Hypercalcemia: kidney stones, bone pain, constipation, polyuria, confusion and short QT.
- Chronic kidney disease retains phosphate and loses 1α-hydroxylase: low calcitriol and high PTH.
- Vitamin D deficiency: rickets in children and osteomalacia in adults; less common in the tropics, but it appears with little sun exposure.

## Memory aid

PTH "collects from bone, saves in the kidney and orders from the gut": three offices, one goal, raise calcium.

## Test yourself

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

## Why does primary hyperparathyroidism come with low phosphate?

PTH inhibits proximal tubule sodium–phosphate cotransport; the kidney wastes phosphate and its level falls, even though PTH also releases it from bone.

## Why can anxious hyperventilation cause tingling and carpopedal spasm?

Respiratory alkalosis frees more negative charges on albumin, which binds more calcium; ionized calcium falls although total calcium is normal.

## Why doesn't hypocalcemia in a patient with hypomagnesemia correct with calcium alone?

Magnesium is needed to secrete PTH and for PTH to act on its tissues; until it is replaced, the parathyroid cannot respond.

## Why does chronic kidney disease raise PTH?

The failing kidney retains phosphate and makes little calcitriol; calcium absorption falls, ionized calcium tends to drop, and high phosphate and FGF-23 stimulate the parathyroid.

## Hypercalcemia with suppressed PTH: which group of causes does it suggest?

Non-parathyroid causes: PTHrP-secreting tumors, bone metastases, calcitriol-making granulomas or vitamin D excess; the parathyroid responds normally by shutting off.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/physiology/calcium-regulation-pth-vitamin-d-and-calcitonin/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Parathyroid glands](https://durgical.com/en/anatomy/parathyroid-glands/)
- [Thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics

- [Bone remodeling](https://durgical.com/en/physiology/bone-remodeling/)
- [Phosphate and FGF-23](https://durgical.com/en/physiology/phosphate-and-fgf-23/)
- [Hormones: types, receptors and feedback](https://durgical.com/en/physiology/hormones-types-receptors-and-feedback/)
- [Metabolic alkalosis and its compensation](https://durgical.com/en/physiology/metabolic-alkalosis-and-its-compensation/)

## Related conditions

- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)
- [Hypercalcaemia](https://durgical.com/en/conditions/hypercalcaemia/)
- [Primary hyperparathyroidism](https://durgical.com/en/conditions/primary-hyperparathyroidism/)
- [Rickets and osteomalacia](https://durgical.com/en/conditions/rickets-and-osteomalacia/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)

## In Biochemistry

- [Vitamin D (calcitriol)](https://durgical.com/en/biochemistry/molecules/vitamin-d/)
- [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [17.5 The Parathyroid Glands](https://openstax.org/books/anatomy-and-physiology-2e/pages/17-5-the-parathyroid-glands) (OpenStax (Rice University), 2022); [Parathyroid Hormone (PTH) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/parathyroid-hormone-pth-test/) (MedlinePlus, US National Library of Medicine, 2024); [Primary Hyperparathyroidism](https://www.ncbi.nlm.nih.gov/books/NBK441895/) (StatPearls (NCBI Bookshelf), 2024); [Vitamin D Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/vitamin-d-test/) (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 80). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/calcium-regulation-pth-vitamin-d-and-calcitonin/