What is Tumor-Induced Osteomalacia?

Tumor-induced osteomalacia (TIO) is caused by slow-growing tumors that overexpress a protein called Fibroblast Growth Factor 23 (FGF23). FGF23 is involved in phosphate absorption, which plays an important role in bone health. A variant of TIO called epidermal nevus syndrome (ENS) is associated with skin lesions and results in similar symptoms.  

TIO tumors are often located in the extremities (for example, a hand or a foot). A TIO tumor is unlikely to spread, but can be difficult to locate by X-ray or MRI. People with TIO may experience muscle weakness, fatigue, and bone pain (especially in the ankles, legs, hips and back). TIO tumors may be suspected in people who have a rapid onset of symptoms without a family history of hypophosphatemia.

The diagnosis is confirmed by a blood test for FGF23 and locating the tumor through imaging.


Finding and removing the tumor through surgery is currently the most effective treatment for TIO since these tumors rarely spread. There is a small chance that TIO could come back, or recur, with the growth of a new tumor.

Phosphate supplements and calcitriol (a form of vitamin D) are used in some patients with TIO.

Ultragenyx is collaborating with Kyowa Hakko Kirin Co., Ltd. (KHK) to develop KRN23 for TIO in cases where tumor removal (called resection) is not possible or is incomplete.

Patient Support

Cancer Net

XLH Network

Nevus Outreach

If you are a member of a TIO patient organization and would like to be listed here, please contact us.

Current Activities

Ultragenyx is collaborating with Kyowa Hakko Kirin Co., Ltd. (KHK) to develop burosumab (KRN23), an investigational product,  for TIO. 

Ultragenyx is conducting a Phase 2 study of burosumab in adult TIO patients. For more information on this study, visit the study page.

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