Testing a patient’s urine is part of the diagnostic process for confirming X-linked hypophosphatemia (XLH), a genetic condition characterized by high levels of a protein called fibroblast growth factor 23 (FGF23).
High FGF23 leads to low phosphate levels in the blood by stimulating phosphate excretion. Because phosphate is crucial for bone health, phosphate wasting leads to soft and weak bones. Consequently, XLH patients have lower limb deformities, pain, poor mineralization of the teeth, and short stature, among other symptoms.
Urine tests, key to diagnosing XLH, also can be used to determine the right dose of supplements that should be used for treating the condition.
Urinary phosphate levels
People with XLH have high levels of FGF23, which inhibits the reabsorption of phosphate in the kidneys, causing them to excrete more phosphate. This leads to high phosphate levels in the urine. Measurement of urinary phosphate is part of the diagnostic process to detect XLH.
Urinary calcium levels
XLH treatment involves supplementation with phosphate salts and calcitriol, the active form of vitamin D. Both of these can lead to increased secretion of calcium into the urine. This is known as hypercalciuria.
Phosphate supplementation can cause hyperparathyroidism, in which the parathyroid glands become overactive and secrete too much parathyroid hormone (PTH). PTH promotes calcium release from the bones, leading to an increase in blood calcium levels. To normalize the calcium levels, the body excretes calcium via the urine, leading to hypercalciuria.
Calcitriol causes hypercalciuria by stimulating calcium uptake from food in the intestines. The excess calcium is then excreted via the urine.
Because hypercalciuria is associated with the development of kidney stones, calcium levels in the urine need to be checked regularly. Hypercalciuria is determined by measuring the ratio of calcium/creatine in the urine. Hypercalciuria can be reduced by adjusting the phosphate and calcitriol doses.
Further urine tests
Urine tests also can help to distinguish XLH from other conditions. For example, patients with Fanconi syndrome also have high levels of phosphate in the urine. However, while Fanconi syndrome patients also have glycosuria (sugar in the urine), bicarbonaturia (bicarbonate in the urine), and proteinuria (protein in the urine), these measures are typically normal in XLH patients.
Last updated: Jan. 2, 2020
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