Tumor-induced osteomalacia (TIO) is an acquired disorder characterized by low levels of phosphorus in the body that can result in weak bones

How TIO affects the body

TIO is caused by tumors that produce too much of a protein called fibroblast growth factor 23 (FGF23), which controls the balance of phosphorus in the body.

  • Excess FGF23 causes an abnormal amount of phosphorus to be eliminated through urination
  • Loss of phosphorus leads to hypophosphatemia, a condition in which there are low levels of phosphorus circulating in the bloodstream
  • Hypophosphatemia then leads to osteomalacia, which weakens bones and makes them prone to break easily
Overproduction of fibroblast growth factor 23 (FGF23) leading to TIO

TIO is an acquired disorder, meaning the tumors that cause TIO are not present at birth. TIO may also be referred to as oncogenic osteomalacia.

TIO is caused by slow-growing tumors that can occur anywhere in the body

These tumors are frequently very small and may be difficult or impossible to find.


TIO tumors can be as small as 1 cm (the size of a pea)

The cause of tumors that lead to TIO is unknown, and there is no evidence that TIO is passed down through family members.


Hypophosphatemia from TIO can lead to symptoms such as:

  • Weakening of bones (osteomalacia) resulting in bone pain
  • Bone damage

Symptoms can be so severe that some people with TIO may require a wheelchair.

Slow-growing tumors and TIO

Tumor removal can be curative, but the tumors may be difficult or even impossible to remove under certain circumstances

Tumor-induced osteomalacia: sympton onset to diagnosis

TIO diagnosis can be a long process

A correct diagnosis can take 2.5 to 28 years after first experiencing symptoms.

Misdiagnosis is common. Approximately 95% of patients are misdiagnosed—typically with osteoporosis, inflammatory arthritis, or fibromyalgia because these conditions can have similar symptoms.

TIO can be confirmed through blood and/or urine tests. Body scans may also be performed to locate tumors.