Crysvita (burosumab)

What it is

Crysvita (burosumab) is a biologic medicine for X-linked hypophosphatemia (XLH) — an inherited condition that causes low phosphate in the blood, soft and weak bones, bowing of the legs, short stature, dental problems, and fractures — and for tumor-induced osteomalacia, bone softening caused by a small tumor. In both conditions, a hormone called FGF23 is too active and makes the kidneys waste phosphate. Crysvita blocks FGF23, so phosphate levels can return to normal and bones can strengthen. It treats the root cause, which older therapy (phosphate supplements and active vitamin D several times a day) could only partly manage.

Before starting

Before starting, we check your blood phosphate, calcium, parathyroid hormone, and vitamin D levels, plus kidney function. These baselines matter because the dose is set — and adjusted — according to your phosphate level: too little phosphate leaves bones weak; overshooting is avoided for safety. If you take oral phosphate supplements or active vitamin D (such as calcitriol), they must be stopped about a week before starting, because combining them with Crysvita can push phosphate and calcium too high. We also review kidney health with an ultrasound history if relevant, since phosphate balance and the kidneys are closely linked.

How it’s given

It is an injection under the skin given in our office — into the upper arm, belly, buttock, or thigh, rotating sites. Adults are usually treated every 4 weeks; children with XLH are treated every 2 weeks. The dose is calculated from body weight and then fine-tuned using your phosphate results, so doses can change over time. Visits are short, and normal activities resume the same day.

What to expect

Phosphate levels typically rise into the normal range within the first weeks. The improvements patients care about — less bone pain, better stamina, healing of stress fractures, and in children, straighter leg growth — build over months, and we track them with labs, X-rays when needed, and how you are functioning. The most common side effects are soreness or redness where the injection was given, headache, dizziness, restless legs, back or tooth pain, and a drop in vitamin D level, which we monitor and supplement if needed. Call us for unusual muscle weakness, or new severe pain, so we can check your levels.

Everyday guidance

Keep every injection and lab appointment, even when you feel well — the phosphate checks before doses are how we set each dose safely. Do not restart phosphate supplements or active vitamin D on your own while on Crysvita. Dental care matters in XLH: keep regular dentist visits and tell your dentist about your condition, since tooth abscesses are part of the disease, not the medicine. Tell other clinicians you receive Crysvita, especially before any procedure involving the kidneys or bones. Questions: call (361) 400-7700.