July 2026 Research Update

This month added a clearer picture that burosumab (an antibody that blocks FGF23) can improve the key XLH biology in children, with higher blood phosphate and better rickets measures over about a year, while also bringing monitoring questions into sharper focus over longer treatment. In a 64-week study in Chinese children, phosphate and kidney phosphate handling increased and X-ray rickets scores improved, alongside a small improvement in a walking test. In a separate 3-year follow-up after switching from conventional therapy, parathyroid hormone (PTH) rose over time and a minority developed secondary hyperparathyroidism, suggesting this is a lab trend worth understanding better during longer use. [1] [2]

In the 64-week pediatric study, burosumab was followed by higher serum phosphorus and higher TmP/GFR (a way to describe how well the kidneys hold onto phosphate), and the rickets severity scores on X-rays improved from baseline. Reported day-to-day function also moved in a positive direction, with the 6-minute walk test percent predicted increasing by week 64 in the subgroup with available data. For families, this connects lab changes to visible bone healing signals on imaging and to measured walking ability, even though the study did not compare against a placebo group. [1]

Safety reporting in the 64-week study described many treatment-emergent events that were mostly mild or moderate, with no treatment interruptions due to adverse events and no anti-burosumab antibodies detected. Kidney ultrasound findings included nephrolithiasis in a few children and some changes in nephrocalcinosis grading over time, which keeps attention on kidney monitoring as part of research follow-up. The study also noted stable PTH on average in that shorter timeframe, which contrasts with the longer-term cohort below and helps explain why different follow-up lengths can surface different issues. [1]

Over 3 years of burosumab in 37 children who switched from phosphate supplements and vitamin D analogues, PTH increased over time and 4 children developed secondary hyperparathyroidism. The report also described a link between higher phosphate levels during burosumab and higher PTH, including a correlation when serum phosphate rose above 1.3 mmol/L. For patients, this is an early signal that the parathyroid response may change over longer periods, even when phosphorus improves, and it highlights an area where researchers are still learning what patterns are most common. [2]

For many families, the most immediately understandable message is that recent pediatric burosumab studies continue to show the expected direction of change: phosphate levels rise, rickets on X-rays improves, and some measures of movement improve over months to a year. At the same time, the longer follow-up study adds a specific monitoring question: PTH may increase over years in some children, and a small group developed secondary hyperparathyroidism, which is a lab-defined change rather than a symptom by itself. Taken together, these results may matter now mainly as context for what researchers are tracking during treatment and may matter later if longer and comparative studies clarify how often PTH changes happen and what they mean for bone and kidney health. [1] [2]

What remains uncertain is how much of the reported improvement and the lab changes are specific to burosumab versus what might have happened over time without a comparison group, because the 64-week trial was single-arm and the 3-year report followed one treated group. It is also not yet clear how to connect PTH changes to long-term outcomes, since the long-term study did not report tertiary hyperparathyroidism and the short-term study reported stable PTH on average. This month helps clarify which benefits are being measured most consistently (phosphate, kidney phosphate handling, rickets scores, walking tests) and which possible longer-term effects are still being mapped (PTH trends and kidney findings on ultrasound). [1] [2]

Two studies this month helped fill in the long view of XLH by describing how the condition can add up over years when treatment is limited or starts late. In adults who were untreated or minimally treated, researchers reported a broad burden including short stature, skeletal and joint problems, pain, dental issues, and reduced quality of life, with men showing greater burden on several measures. In a Brazilian cohort followed under conventional therapy, later diagnosis was tied to worse growth status at diagnosis and was independently linked to shorter final height, keeping early recognition on the radar as a factor that can shape adult height outcomes. [3] [4]

Among 52 adults with XLH who had never been treated or had only minimal phosphate/vitamin D treatment, the report described severe disproportionate short stature, high BMI, widespread skeletal deformities, pseudofractures, enthesopathies (tendon or ligament attachment changes), osteoarthritis, and pain with reduced physical function. Dental and periodontal problems were also common, and patient-reported measures reflected reduced quality of life. For patients and caregivers, this paints a coherent picture of XLH as a whole-body condition that can affect mobility, comfort, and daily functioning over decades. [3]

In that adult natural history report, men had higher disease burden than women in several areas, including more compromised stature and higher markers related to FGF23 and bone turnover. The study also reported no nephrocalcinosis in this adult group and noted that a few had transient secondary hyperparathyroidism during childhood phosphate/vitamin D that resolved after stopping. This helps separate which complications were prominent in that specific cohort versus which were not observed, without assuming the same pattern applies to every adult with XLH. [3]

In the Brazilian cohort study focused on final height, height-for-age at diagnosis, age at diagnosis, and target height (based on family height) were independently associated with final adult height among those analyzed under conventional therapy. Age at diagnosis was also linked to worse growth status at diagnosis, meaning that delays were tied to larger growth gaps by the time XLH was recognized. For families, this adds a practical framing: childhood growth at the time XLH is identified can carry forward into adult height outcomes, even though this study did not test newer treatments for height. [4]

For patients, these studies mainly add context for expectations and for understanding why XLH follow-up often involves more than one body area. The adult natural history report may resonate with adults who experience pain, mobility limits, dental problems, or joint issues, because it describes these as common parts of the disease burden in a largely untreated group. The growth study may matter most to families of children, because it connects later diagnosis and poorer growth at diagnosis with shorter adult height in that setting, although it does not show what would happen with different treatment approaches. [3] [4]

Important uncertainties remain because both reports were observational and reflect the people who were seen at specialized centers, which may not represent everyone with XLH. The adult study also included people across many decades of life and did not compare quality-of-life scores with people without XLH. Even with those limits, this month makes the long-term picture easier to describe in simple terms: untreated or late-recognized XLH can affect height, bones and joints, pain, and oral health, and early growth status is linked to adult height in at least one cohort. [3] [4]

This month included several pieces that are not meant to be generalized, but they broaden what researchers are paying attention to in XLH. One case showed that standard genetic testing can miss a deep intronic PHEX change that whole-genome and RNA testing can uncover, and the person improved after starting burosumab. Another case described a newborn without the family PHEX variant who still showed prolonged changes in phosphate and calcium handling after exposure to burosumab during pregnancy, which later normalized. A separate exploratory study in one treated child suggested that immune cell patterns in blood may shift alongside phosphate and alkaline phosphatase changes, pointing to new biology questions rather than clinical conclusions. [5] [6] [7]

In a 20-year-old with an XLH-like clinical picture but negative standard genetic testing, whole-genome sequencing found a deep intronic PHEX variant that altered RNA splicing, creating abnormal transcripts. After burosumab, the report described higher phosphate measures and reduced bone pain over several months. For families, this helps explain why genetic answers sometimes require more specialized testing, and it shows how confirming the genetic cause can align with targeted treatment in an individual. [5]

In a pregnancy exposure case, a newborn who did not carry the family PHEX variant still had prolonged phosphate retention and transient high calcium soon after birth, consistent with ongoing drug effect, and labs normalized by 6 months without nephrocalcinosis or developmental problems reported. This case matters because it documents what monitoring found in one real-life situation, not because it establishes what will happen in other pregnancies. It also underlines that burosumab can have lasting effects on kidney phosphate handling when exposure occurs before birth. [6]

A single-patient blood study used single-cell RNA sequencing to track immune cells before and during burosumab, comparing results with external healthy pediatric data. The report described shifts in T cell and NK cell subtypes over time, alongside rising phosphate and falling alkaline phosphatase in the same patient, and it proposed possible links to pathways involved in bone resorption. For patients, this is best seen as early-stage research that may eventually connect XLH metabolism and the immune system, but it does not yet translate into a test or a treatment change. [7]

One scoping review collected decades of publications about XLH in dental practice and emphasized that dental problems can be part of XLH across multiple dental specialties. It presented multidisciplinary recommendations focused on diagnosis, prevention, and planning dental care to reduce complications and improve long-term oral health. For patients, the main message is that dental issues are recognized as an important part of XLH in the research literature, even though this review did not measure outcomes in a new patient group. [8]

For patients and caregivers, these reports are most useful for understanding edge cases and the direction of research, rather than for predicting personal outcomes. The genetic case may matter now for families who have a strong XLH clinical picture but no answer on standard tests, because it shows that rarer PHEX changes can be found with broader DNA and RNA approaches. The pregnancy exposure case may matter now mainly as documentation that newborn mineral labs can stay altered for weeks after in-utero exposure, even when the baby does not have XLH, but it is still only one baby. [5] [6]

The immune-cell study and the dental scoping review are better framed as research and care-mapping rather than direct clinical evidence. The single-patient immune findings remain uncertain because they come from one child and from blood rather than bone tissue, but they help clarify which biological pathways researchers are exploring during burosumab treatment. The dental review helps clarify that dental monitoring and planning are consistently discussed across the literature, even though this month did not add new comparative data on how different dental strategies change outcomes. [7] [8]