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Hypophosphatemic Rickets

(Vitamin D–Resistant Rickets)

By

Christopher J. LaRosa

, MD, Perelman School of Medicine at The University of Pennsylvania

Last full review/revision Jan 2020| Content last modified Jan 2020
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Hypophosphatemic rickets is a disorder characterized by hypophosphatemia, defective intestinal absorption of calcium, and rickets or osteomalacia unresponsive to vitamin D. It is usually hereditary. Symptoms are bone pain, fractures, and growth abnormalities. Diagnosis is by serum phosphate, alkaline phosphatase, and 1,25-dihydroxyvitamin D3 levels. Treatment is oral phosphate plus calcitriol; burosumab is given for X-linked hypophosphatemia.

Familial hypophosphatemic rickets is usually inherited as an X-linked dominant trait; other familial patterns occur but are rarer (1).

Sporadic acquired cases sometimes are caused by benign mesenchymal tumors that produce a humoral factor that decreases proximal renal tubular resorption of phosphate (tumor-induced osteomalacia).

General reference

  • 1. Bitzan M, Goodyer PR: Hypophosphatemic rickets. Pediatr Clin N Am 66(1):179–207, 2019. doi: 10.1016/j.pcl.2018.09.004.

Pathophysiology

The observed abnormality is decreased proximal renal tubular resorption of phosphate, resulting in renal phosphate wasting and hypophosphatemia. This defect is due to circulating factors called phosphatonins. The principle phosphatonin in hereditary hypophosphatemic rickets is fibroblast growth factor-23 (FGF-23). Decreased intestinal calcium and phosphate absorption also occurs. Deficient bone mineralization is due to low phosphate levels and osteoblast dysfunction rather than to the low calcium and elevated parathyroid hormone (PTH) levels as in calcipenic rickets (see Vitamin D Deficiency and Dependency). Because 1,25-dihydroxyvitamin D3 levels are normal to slightly low, a defect in conversion is presumed; hypophosphatemia would normally cause elevated 1,25-dihydroxyvitamin D3 levels.

There are several forms of hypophosphatemic rickets (see table Forms of Hereditary Hypophosphatemic Rickets). A form of hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is known to occur due to mutations in the proximal tubule type 2c sodium-phosphate cotransporter (NaPi2c). Defective phosphate transport and hypophosphatemia in this case result in appropriately increased 1,25-dihydroxyvitamin D3 levels, thus leading to hypercalciuria.

Table
icon

Forms of Hereditary Hypophosphatemic Rickets*

Form

Pattern of Inheritance

Gene (Location)

Gene Mutation Effect

Clinical Features

X-linked hypophosphatemia (HLH)

PHEX (Xp22.1)

Inactivating mutation results in increased phosphatonin (FGF-23) production

Renal phosphate wasting with normal serum calcium, normal or low urine calcium, high alkaline phosphatase, normal or paradoxically high intact parathyroid hormone

Rickets/osteomalacia, delayed dentition, dental abscess, craniofacial abnormalities, hearing loss, hypertension, nephrocalcinosis (results from management)

Autosomal recessive hypophosphatemic rickets (ARHR)

ARHR1: DMP1 (4q22.1)

ARHR2: ENPP1 (6q23.2)

ARHR3: FAM20C (7p22.3)†

Loss-of-function mutations associated with inappropriate increases in FGF-23

General features of renal phosphate wasting, normal serum calcium, low/normal urine calcium, and high alkaline phosphatase

Rickets/osteomalacia, short stature, long bone deformities, spinal immobility, enthesopathies, dental and facial bony abnormalities, learning disabilities

Autosomal dominant hypophosphatemic rickets

Autosomal dominant (variable penetrance)

FGF23 (12p13.32)

Unable to cleave FGF-23, leading to elevated serum levels

Although features can be similar to XLH, age at presentation and penetrance is variable.

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH)

SLC34A3 (9q34.3)

Normal/low FGF-23

Defect in the proximal tubule NaPi2c

Phosphate wasting and high 1,25-dihydroxyvitamin D3, causing hypercalciuria, stones, and nephrolithiasis and/or nephrocalcinosis

Hypophosphatemia, hypercalcemia, and nephrocalcinosis (HHN)

SLC34A1 (5q35)

Suppressed FGF-23

Abnormal NaPi2a

Elevated levels of 1,25-dihydroxyvitamin D3, hypophosphatemia, increased calcitriol, hypercalcemia, hypercalciuria, and nephrocalcinosis.

* This table provides a brief overview of single-gene forms of hereditary hypophosphatemic rickets associated with either increased FGF-23 or normal to suppressed FGF-23.

† This form can occur in osteosclerotic dysplasia and as part of Raine syndrome (a rare skeletal dysplasia).

FGF-23 = fibroblast growth factor-23; NaPi2a = renal type 2a sodium-phosphate cotransporter; NaPi2c = renal type 2c sodium-phosphate cotransporter.

Adapted from Bitzan M, Goodyer PR: Hypophosphatemic rickets. Pediatr Clin N Am 66(1):179–207, 2019. doi: 10.1016/j.pcl.2018.09.004.

Symptoms and Signs

The disease manifests as a spectrum of abnormalities, from hypophosphatemia alone to growth retardation and short stature to severe rickets or osteomalacia. Children usually present after they begin walking, with bowing of the legs and other bone deformities, pseudofractures (ie, x-ray findings in osteomalacia that may represent areas of prior stress fractures that have been replaced by inadequately mineralized osteoid vs areas of bony erosions), bone pain, and short stature. Bony outgrowth at muscle attachments may limit motion.

Rickets of the spine or pelvis, dental enamel defects, and tetany that occur in dietary vitamin D deficiency are rarely present in hypophosphatemic rickets.

Patients with HHRH may present with nephrolithiasis and/or nephrocalcinosis.

Diagnosis

  • Serum levels of calcium, phosphate, alkaline phosphatase, 1,25-dihydroxyvitamin D3, PTH, FGF-23, and creatinine

  • Urinary phosphate and creatinine levels (for calculation of the tubular reabsorption of phosphate)

  • Bone x-rays

Serum phosphate levels are depressed, but urinary phosphate excretion is large. Serum calcium and PTH are normal, and alkaline phosphatase often is elevated. Hypophosphatemia-induced stimulation of calcitriol production does not occur. Typically, calcidiol levels are normal, whereas calcitriol levels are normal to low. In calcipenic rickets, hypocalcemia is present, hypophosphatemia is mild or absent, and urinary phosphate is not elevated.

Treatment

  • Oral phosphate and calcitriol

  • Burosumab for X-linked hypophosphatemia

Treatment of hypophosphatemic rickets consists of neutral phosphate solution or tablets. Starting dose in children is 10 mg/kg (based on elemental phosphorus) 4 times a day. Phosphate supplementation lowers ionized calcium concentrations and further inhibits calcitriol conversion, leading to secondary hyperparathyroidism and exacerbating urinary phosphate wasting. Therefore, oral vitamin D is given as calcitriol, initially 5 to 10 ng/kg 2 times a day. This, however, is not the case with HHRH or HHN (hypophosphatemia, hypercalcemia, and nephrocalcinosis), where 1,25-dihydroxyvitamin D3 levels are elevated and dosing with calcitriol can be detrimental.

Phosphate dose may need to be increased to achieve bone growth or relieve bone pain. Diarrhea may limit oral phosphate dosage. Increase in plasma phosphate and decrease in alkaline phosphatase concentrations, healing of rickets, and improvement of growth rate occur. Hypercalcemia, hypercalciuria, and nephrocalcinosis with reduced renal function may complicate treatment. Patients undergoing treatment need frequent follow-up evaluations.

Burosumab is an anti–FGF-23 monoclonal antibody that has become the treatment of choice for X-linked hypophosphatemia (XLH) and has replaced the conventional therapy described above (1). Dosing in children < 10 kg is started at 1 mg/kg (rounded to nearest 1 mg) subcutaneously every 2 weeks. For children 6 months to < 18 years and > 10 kg, starting dose is 0.8 mg/kg (rounded to the nearest 10 mg) subcutaneously every 2 weeks. For adults ≥ 18 years, starting dose is 1 mg/kg (rounded to the nearest 10 mg) subcutaneously every 4 weeks. The dose may be titrated upwards according to the manufacturer’s instructions to a maximum of 2 mg/kg or 90 mg as needed to normalize serum phosphate.

Iron deficiency upregulates expression of bone FGF-23 and can exacerbate conditions with high FGF-23 levels/impaired FGF cleavage. Therefore, repletion of iron is essential for patients with iron deficiency in the setting of high FGF-23 hypophosphatemic conditions.

Adults with oncogenic rickets may dramatically improve once the mesenchymal tumor that causes the disorder is removed. Otherwise, oncogenic rickets is treated with calcitriol 5 to 10 ng/kg orally 2 times a day and elemental phosphorus 250 mg to 1 g orally 3 or 4 times a day.

Treatment reference

Key Points

  • Decreased renal resorption of phosphate results in renal phosphate wasting and hypophosphatemia.

  • There is deficient bone mineralization due to low phosphate levels and osteoblast dysfunction.

  • Children have growth retardation, bone pain and deformities (eg, leg bowing), and short stature.

  • Patients with hypophosphatemic rickets with hypercalciuria (HHRH) may present with nephrolithiasis and/or nephrocalcinosis.

  • Diagnose by finding low serum phosphate levels, elevated urinary phosphate, and normal serum calcium and parathyroid hormone.

  • Treat with oral phosphate supplements and, except for HHRH, vitamin D (given as calcitriol).

  • Use burosumab for X-linked hypophosphatemia.

Drugs Mentioned In This Article

Drug Name Select Trade
ROCALTROL
Burosumab
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