A rare genetic disease called XLH makes children very short because their bodies can’t use phosphate properly. Researchers studied a new medicine called burosumab given to toddlers ages 1-4 with XLH. Compared to children who received older treatments, those on burosumab stopped losing height and didn’t gain excessive weight. While the medicine didn’t make them catch up to normal height, it prevented their height from getting worse during the critical early years when kids normally grow quickly. This is important because it suggests starting treatment very early with this new drug may help children with XLH have better outcomes.

The Quick Take

  • What they studied: Whether a new medicine called burosumab helps very young children with a rare genetic disease (XLH) grow taller and healthier compared to older treatments.
  • Who participated: 15 toddlers ages 1-3 years old taking burosumab, compared to 31 similar children from the past who received older treatments (vitamin D and phosphate supplements). All children had XLH, a rare inherited condition affecting bone health.
  • Key finding: Children on burosumab stopped losing height during the first two years of treatment, while children on older treatments continued to get shorter. Burosumab also prevented excessive weight gain that typically happens with XLH.
  • What it means for you: If your young child has XLH, starting burosumab early (before age 4) may help prevent them from becoming even shorter and may help control weight gain. However, this medicine doesn’t fully correct the height loss that already happened—it mainly stops further decline. Talk to your doctor about whether this treatment is right for your child.

The Research Details

Researchers followed 15 very young children with XLH who started taking burosumab between ages 1-3 years old. They measured how tall these children grew over two years and compared them to 31 children from previous years who had received older treatments (vitamin D and phosphate supplements). Both groups started treatment at similar young ages, making them good for comparison. The researchers carefully tracked height, weight, and other health measures to see which treatment worked better.

This type of study is called a prospective follow-up study, which means researchers watched children over time going forward rather than just looking at past records. They used a comparison group (historical cohort) from previous patients to see if the new medicine was better than the old approach. This design helps show whether burosumab truly makes a difference.

XLH is a rare disease that affects how children’s bodies handle phosphate, a mineral needed for strong bones and normal growth. Children with XLH start losing height very early in life, sometimes within the first few months. By studying very young children (ages 1-4), researchers can see if early treatment with burosumab helps during the critical years when kids normally grow the fastest. This timing is important because early intervention might prevent some of the height loss that happens later.

This study has several strengths: it’s the largest group of very young XLH children treated with burosumab studied so far, it followed children for two full years, and it compared them fairly to children who received older treatments. However, the study is relatively small (46 children total), which means results should be confirmed with larger studies. The study was done at specialized medical centers, so results might not apply to all children everywhere. This is real-world data from actual patient care, which is valuable but not as controlled as some other study types.

What the Results Show

Before starting burosumab, children in the study were already getting shorter compared to normal children their age. Their height measurements showed they were about 1.4 units below average by age 2-3 years old. Once they started burosumab, something important happened: their height stopped declining. After one year on burosumab, their height measurements stayed stable (didn’t get worse). After two years, their height remained stable, showing the medicine prevented further height loss.

In contrast, children who received the older treatments (vitamin D and phosphate supplements) continued to get shorter over the same two-year period. They lost an additional 0.7 units of height in the first two years, and by age four, they were about 1.9 units below average—much worse than the burosumab group.

The difference between the two groups is striking: burosumab stopped the decline, while older treatments couldn’t prevent continued height loss. This suggests that starting burosumab very early (before age 4) may be better than waiting or using older medicines.

Weight gain patterns also differed between the groups. Children on older treatments gained excessive weight during the first two years (their weight measurements increased by 0.5 units above normal). This excessive weight gain is a known problem with XLH and can lead to other health issues. Children on burosumab did not show this excessive weight gain—their weight stayed more normal. This is important because it means burosumab may help prevent not just height problems but also weight problems that come with XLH. The study also noted that burosumab was well-tolerated in these very young children, with no major safety concerns reported during the two-year follow-up.

Previous research showed that burosumab worked better in children older than four years, but those studies showed only small improvements in height. This new study is important because it’s the first to look at what happens when children start burosumab much earlier (ages 1-3). The results suggest that earlier treatment may be more effective at preventing height loss. The comparison to older treatments (vitamin D and phosphate supplements) shows that these traditional approaches, while helpful, cannot stop the progressive height decline that happens naturally with XLH. Burosumab appears to work differently and more effectively by addressing the underlying cause of the disease rather than just treating symptoms.

The study is relatively small with only 15 children on burosumab, so results need to be confirmed with larger groups. The comparison group came from the past (historical cohort), so there might be differences in how they were treated or measured compared to today’s standards. The study only followed children for two years, so we don’t know what happens with longer-term treatment. The study was done at specialized medical centers, so results might be different in other hospitals or countries. Finally, while burosumab prevented further height loss, it did not correct the height deficit that had already developed before treatment started—children remained shorter than average even after two years of treatment.

The Bottom Line

For children with XLH diagnosed before age 4: Starting burosumab early appears to be beneficial and should be discussed with your child’s endocrinologist (bone and hormone specialist). The evidence suggests burosumab prevents further height decline and excessive weight gain better than older treatments. However, burosumab does not fully correct height that was already lost, so early diagnosis and treatment are important. Confidence level: Moderate—this is the best evidence we have for very young children, but larger studies would strengthen these findings.

This research is most relevant for: families with children diagnosed with XLH before age 4, pediatric endocrinologists treating XLH, and genetic counselors advising families about treatment options. Children diagnosed after age 4 may benefit differently based on previous research. This does not apply to children with other types of short stature or bone diseases. If your child has been diagnosed with XLH, discuss these findings with your medical team to determine if burosumab is appropriate for your situation.

The study showed that burosumab’s benefits appear within the first year of treatment—height stopped declining by year one. However, the medicine does not quickly make children taller; it prevents them from getting shorter. Most noticeable improvements in preventing height loss and weight gain appear within the first two years. Long-term benefits beyond two years are not yet known from this study, so continued follow-up is important.

Want to Apply This Research?

  • Track your child’s height monthly and weight every 2-3 months using a growth chart app. Record measurements at the same time of day (morning is best) and same scale for consistency. Compare to age-appropriate growth curves to see if height is stable or declining.
  • If your child is starting burosumab, set monthly reminders for growth measurements and medication adherence checks. Use the app to log any side effects or concerns to discuss with your doctor. Create a growth goal tracker showing the target of ‘maintaining current height percentile’ rather than expecting rapid catch-up growth.
  • Establish a long-term tracking system that records height, weight, and BMI every 3 months. Create alerts if height starts declining again (which would indicate a problem) or if weight gain exceeds expected ranges. Share monthly reports with your child’s endocrinologist to monitor treatment effectiveness and adjust if needed.

This research summary is for educational purposes only and should not replace professional medical advice. XLH is a serious rare genetic condition that requires specialized medical care. If your child has been diagnosed with XLH or shows signs of abnormal growth, consult with a pediatric endocrinologist or geneticist before making any treatment decisions. The findings presented here are based on a relatively small study and should be discussed with your healthcare provider to determine if burosumab is appropriate for your child. Individual results may vary, and treatment decisions should be based on your child’s specific medical situation, not on this summary alone.