# Hypophosphataemic Rickets

> Hypophosphataemic rickets for NEET-PG Orthopaedics: X-linked vitamin D resistant rickets, phosphate wasting, FGF23 biochemistry and treatment with phosphate.

- Canonical URL: https://prepelephant.com/topics/neet-pg/orthopaedics/hypophosphataemic-rickets
- Exam / course: NEET-PG · Subject: Orthopaedics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Maheshwari's Essential Orthopaedics; Apley's System of Orthopaedics and Fractures
- How to cite: "Hypophosphataemic Rickets", PrepElephant, https://prepelephant.com/topics/neet-pg/orthopaedics/hypophosphataemic-rickets

## Direct answer

Hypophosphataemic rickets is an inherited form of rickets in which renal phosphate wasting keeps serum phosphate low even when vitamin D intake is adequate. The commonest form is X-linked dominant, caused by PHEX mutations that raise fibroblast growth factor 23 (FGF23). Treatment is oral phosphate with active vitamin D, which is why the disease is called vitamin D resistant rickets.

## What you must remember

- **Inheritance:** X-linked dominant in the commonest form, due to PHEX mutations; autosomal dominant and autosomal recessive forms also exist.
- **Pathway:** excess FGF23 suppresses renal tubular phosphate reabsorption and reduces 1-alpha-hydroxylase activity.
- **Biochemistry:** low serum phosphate, normal calcium, raised alkaline phosphatase, normal parathyroid hormone and an inappropriately low or normal 1,25-dihydroxyvitamin D.
- **Clinical picture:** short stature, bowing of the legs with genu varum, and spontaneous dental abscesses from enamel defects.
- **Vitamin D resistance:** the bone disease persists on standard vitamin D doses because the defect lies in phosphate handling.
- **Treatment:** oral phosphate in divided doses with active vitamin D such as calcitriol, with monitoring for nephrocalcinosis and secondary hyperparathyroidism; burosumab, an anti-FGF23 antibody, is a newer option where available.
- **Deformity:** osteotomy with intramedullary rodding corrects established bowing but does not treat the underlying phosphate defect.

## Why the usual rickets treatment fails here

A 4-year-old boy with bowed legs and a dental abscess has had six months of vitamin D without improvement. The question is what the blood and urine reveal about the cause.

| Feature | Nutritional rickets | X-linked hypophosphataemic rickets |
|---|---|---|
| Cause | Deficiency of vitamin D or calcium from diet or sunlight | Renal phosphate wasting from excess FGF23, usually from a PHEX mutation |
| Serum phosphate | Low, partly through secondary hyperparathyroidism | Low, from a primary renal phosphate leak |
| Serum calcium | Low or normal | Normal |
| Parathyroid hormone | Raised, secondary to low calcium and vitamin D | Normal |
| Active vitamin D (1,25) | Variable (25-OH vitamin D is low) | Low or inappropriately normal |
| Response to vitamin D | Corrects the disease | Does not correct the phosphate defect |
| Treatment | Vitamin D with oral calcium | Oral phosphate with active vitamin D |

The discriminator examiners test is the parathyroid level, which is raised in nutritional rickets and normal in the X-linked form. A child who fails to respond to standard vitamin D therapy should prompt a search for renal phosphate wasting.

## Where the exam tries to catch you

Examiners present a child on vitamin D who still has bowing, and expect the phosphate pathway to be recognised.

- **Trap:** treating persistent rickets with more vitamin D. **Correct:** in X-linked hypophosphataemia the defect is renal phosphate wasting, so more vitamin D alone does not correct the bone disease.
- **Trap:** expecting low calcium in the typical case. **Correct:** calcium is usually normal, so the abnormal finding is a low phosphate with a normal parathyroid hormone.
- **Trap:** giving calcitriol alone. **Correct:** active vitamin D must be given with oral phosphate, since calcitriol without phosphate leaves the renal phosphate loss untreated.

## Frequently asked questions

### What distinguishes hypophosphataemic rickets from nutritional rickets?

Hypophosphataemic rickets has low serum phosphate with normal calcium and parathyroid hormone, whereas nutritional rickets has low vitamin D with raised parathyroid hormone. The hypophosphataemic form does not respond to vitamin D alone.

### Why does vitamin D alone fail in X-linked hypophosphataemic rickets?

Excess FGF23 causes renal phosphate wasting and reduces the production of active vitamin D. Vitamin D alone therefore cannot restore phosphate balance, so oral phosphate is needed alongside active vitamin D.

### What is the commonest inheritance pattern in hypophosphataemic rickets?

The commonest form is X-linked dominant, caused by PHEX mutations. Affected females may show milder disease than males, and the pattern is recognised on family history and biochemistry.

### Which complications need monitoring during treatment of hypophosphataemic rickets?

Nephrocalcinosis, secondary hyperparathyroidism and dental problems are the chief complications to monitor during treatment. Regular measurement of phosphate, calcium, alkaline phosphatase and parathyroid hormone guides dose adjustment.

### Does surgery treat hypophosphataemic rickets?

Osteotomy with intramedullary rodding is used for bowing or deformity that persists despite medical therapy. Surgery corrects limb alignment but does not treat the underlying phosphate defect, so medical treatment continues.