Rickets With Normal Vitamin D Levels: The Genetic Connection

Rickets is often associated with low vitamin D, but in some cases, children show classic signs of rickets despite having normal vitamin D levels. This pattern usually points to a genetic cause, rather than a simple nutritional deficiency. Understanding these genetic forms is key to proper diagnosis and treatment.

What Is Rickets?

Rickets is a condition that affects bone development in children. Instead of growing strong, healthy bones, children with rickets experience softening and weakening of the skeletal structure. Typical symptoms include:

• Bowed legs or knock knees
• Delayed growth and shorter stature
• Pain in the spine, pelvis, or legs
• Dental issues, such as delayed tooth formation
• Muscle weakness

Most cases are linked to vitamin D deficiency—needed for calcium absorption—but that’s not the whole story.

Why Normal Vitamin D Levels Matter

When blood tests show normal or even high levels of 25-hydroxyvitamin D, yet classic signs of rickets persist, doctors look beyond diet and sun exposure. Checking vitamin D rules out nutritional deficiency. If levels are sufficient, a genetic cause is likely.

Key laboratory findings in these cases often include:

• Normal 25-hydroxyvitamin D
• Low blood phosphate (hypophosphatemia)
• Elevated alkaline phosphatase (ALP)
• Parathyroid hormone (PTH) may be normal or mildly elevated

Genetic Forms of Rickets

Several inherited conditions disrupt the body’s ability to process phosphate or activate vitamin D, leading to rickets even when vitamin D levels are normal. The main genetic types include:

1. X-Linked Hypophosphatemic Rickets (XLH)

• Cause: Mutations in the PHEX gene on the X chromosome
• Inheritance: X-linked dominant (affected males and females; males often more severely)
• Mechanism: Excessive fibroblast growth factor 23 (FGF23) causes kidneys to excrete too much phosphate
• Lab findings: Very low serum phosphate, normal calcium, normal 25-hydroxyvitamin D

2. Autosomal Dominant Hypophosphatemic Rickets (ADHR)

• Cause: Mutations in the FGF23 gene itself
• Inheritance: Autosomal dominant (one copy of the mutation causes disease)
• Mechanism: Mutated FGF23 is resistant to breakdown, prolonging its action of wasting phosphate

3. Autosomal Recessive Hypophosphatemic Rickets (ARHR)

• Cause: Mutations in genes such as DMP1, ENPP1, or FAM20C
• Inheritance: Autosomal recessive (both parents carry one copy)
• Mechanism: Various pathways lead to elevated FGF23 or impaired bone mineralization

4. Hereditary Vitamin D–Resistant Rickets (HVDRR)

• Cause: Mutations in the vitamin D receptor (VDR) gene
• Inheritance: Autosomal recessive
• Mechanism: Body cannot respond to activated vitamin D (calcitriol), even if levels are normal or high
• Lab findings: High calcitriol, low calcium, low phosphate, elevated PTH

Recognizing the Signs

Because genetic rickets can mimic nutritional rickets, early recognition is crucial. Watch for:

• Persistent leg bowing or knock knees after walking age
• Delayed motor milestones (sitting, standing, walking)
• Bone pain or tenderness on palpation
• Dental problems, like soft enamel or delayed eruption
• Family history of similar bone issues

If you suspect rickets but vitamin D levels are normal, it’s time for a more detailed evaluation.

Diagnostic Workup

A thorough workup includes:

1. Detailed medical and family history
   • Any relatives with bone deformities, short stature, or early tooth problems?
2. Blood tests
   • 25-hydroxyvitamin D (to confirm normal levels)
   • Serum phosphate and calcium
   • Alkaline phosphatase (ALP)
   • Parathyroid hormone (PTH)
   • Fibroblast growth factor 23 (FGF23), if available
3. Urine tests
   • Phosphate excretion rates
4. Genetic testing
   • Targeted gene panels for PHEX, FGF23, VDR, and related genes
5. Imaging
   • X-rays of wrists, knees, and ankles to look for classic rickets changes (widened growth plates, cupping, fraying)

Early diagnosis allows for tailored treatment and helps prevent long-term complications such as bone deformities and growth failure.

Treatment Approaches

Management varies by genetic type but often includes:

Phosphate and Active Vitamin D

• Oral phosphate supplements to restore serum phosphate
• Calcitriol (1,25-dihydroxyvitamin D) for improved bone mineralization

Medication Targeting FGF23

• Burosumab (an anti-FGF23 antibody) for X-linked hypophosphatemic rickets
• Shown to improve phosphate levels, reduce bone pain, and promote growth

Monitoring and Supportive Care

• Regular blood tests to adjust doses of phosphate and calcitriol
• Dental evaluations for enamel and periodontal health
• Orthopedic consultation for severe bone deformities
• Physical therapy to strengthen muscles and improve mobility

Living With Genetic Rickets

A diagnosis of genetic rickets can feel overwhelming, but many children lead active lives with proper management. Key points:

• Stay on a treatment plan with regular follow-up
• Watch for signs of over-supplementation (hypercalcemia, kidney stones)
• Encourage a balanced diet rich in calcium (dairy or fortified alternatives)
• Maintain a safe level of sun exposure for general health

When to Seek Professional Help

If you’re concerned about bone pain, leg deformities, or growth delays in your child—especially when vitamin D levels are normal—it’s important to take action:

• Consider a free, online symptom check, using the doctor approved Ubie Symptom Checker (https://ubiehealth.com/) to get insights on potential next steps.
• Speak to a pediatrician or pediatric endocrinologist about specialized testing for genetic causes.

Never ignore serious or worsening symptoms. If your child experiences severe bone pain, difficulty walking, or unusual muscle weakness, speak to a doctor right away. In emergencies—such as extreme pain or breathing difficulties—seek immediate medical care.

Key Takeaways

• Rickets typically stems from vitamin D deficiency, but normal 25-hydroxyvitamin D levels suggest a genetic form.
• Genetic rickets often involve phosphate wasting (FGF23-mediated) or resistance to vitamin D action (VDR mutations).
• Diagnosis requires blood and urine tests, imaging, family history, and genetic panels.
• Treatment includes phosphate, active vitamin D, and novel therapies like burosumab.
• Early recognition and consistent follow-up improve long-term outcomes.

If you suspect rickets despite normal vitamin D levels, early evaluation can change the course of your child’s health. Always discuss any concerns or treatment changes with a qualified healthcare provider.