Adult X-Linked Hypophosphatemia: Switching From Phosphate/Calcitriol to Burosumab
A single patient who has managed a lifelong condition with the same regimen since childhood, still in real pain. The disagreement is whether a newer, targeted therapy is worth disrupting a routine she's followed for decades.
Simone K., a 38-year-old woman, has lived with X-linked hypophosphatemia since infancy, diagnosed after her legs bowed noticeably in early childhood, and has taken oral phosphate and calcitriol on a strict multiple-times-daily schedule for essentially her entire life — a routine she manages around her job teaching high school chemistry with what she calls practiced efficiency. She has avoided major fracture and functions well by most measures. What she has also had, for years, is a chronic dull ache in her legs and lower back that she stopped mentioning to doctors because she had come to assume it was simply part of the condition.
A referral to an endocrinologist specializing in adult metabolic bone disease reopened that assumption. Her serum phosphate sits at 1.8 mg/dL despite an adherent, well-managed regimen — which is the point rather than a footnote: three decades of correctly taken oral phosphate have not brought her into range, because the wasting is renal and driven by FGF23, and the supplement is chasing it downstream rather than addressing it. Her imaging shows the accumulated cost, changes consistent with osteomalacia from years of undercorrected phosphate loss. Her renal ultrasound, at least, is clean of nephrocalcinosis, which is the complication that would have argued against continuing the current regimen on its own terms rather than on the strength of her symptoms. Burosumab, a monoclonal antibody inhibiting FGF23 directly, was not available through most of her adult life. Insogna and colleagues' randomized, placebo-controlled trial in adults with XLH found greater improvement in serum phosphate, stiffness and physical function than continued conventional therapy — a population match rather than an extrapolation, since it enrolled adults with exactly her diagnosis rather than children in whom most XLH evidence was generated. She is candid that part of her hesitation is simply fatigue with change itself after thirty-plus years of the same routine, not doubt that something different might genuinely help.
Correcting the mechanism versus managing around it
I'd switch her to burosumab. Conventional phosphate and calcitriol work around chronically low phosphate without correcting what's actually causing it — burosumab directly inhibits FGF23, the hormone driving her renal phosphate wasting. Insogna and colleagues' randomized, placebo-controlled trial in adults with XLH found burosumab produced meaningfully greater improvement in serum phosphate, stiffness, and physical function than continued conventional therapy. Her persistent bone pain despite decades of conventional therapy is real evidence that working around the mechanism hasn't been enough.
I'd want to be careful about disrupting a routine she's managed successfully for over thirty years without a fracture. Burosumab means new injection training, new monitoring, and real access barriers — that transition shouldn't be underestimated for a patient who's built her adult life around a different regimen.
I'm not saying don't switch — I'm saying "it's worked well enough by the usual measures" and "she's been in chronic pain for years" are both true at once, and I want to make sure the transition burden is actually weighed against a symptom she'd stopped even mentioning.
Worth naming directly: conventional therapy isn't risk-free either, especially over decades of use. Chronic high-dose phosphate and calcitriol carry real, documented risk of nephrocalcinosis and secondary hyperparathyroidism, accumulating the longer it's used the way hers has been.
Her persistent pain despite an adherent regimen, alongside that accumulating risk, tips this toward switching — not because the current regimen has failed by every measure, but because "tolerated for thirty years" isn't the same claim as "without ongoing cost," and her pain is real, current evidence that cost hasn't bought full symptom control.
Agreed: transition to burosumab, with baseline renal imaging already reassuring for nephrocalcinosis, and phosphate levels monitored closely during the switch to confirm the conventional regimen is safely discontinued rather than tapered alongside the new therapy.
Not fully settled: how long to wait before reassessing whether her bone pain has genuinely improved on burosumab versus reflecting years of established chronic pain that may not fully resolve regardless of phosphate correction — the endocrinologist and primary care physician set different informal timelines for that reassessment.