Phosphate is important for our body, for instance for energy management and as part of our bones. Protein-rich products like milk, cheese, meat and fish have a high content of phosphate. The phosphate balance is maintained by the uptake of phosphate in the gut, storage into the bones and excretion by the kidneys. In the kidney, phosphate is reabsorbed into the blood from the urine via transport proteins in the tubulues. These processes are under the influence of various hormones. The most important hormones are the parathyroid hormone (PTH), vitamin D, and the hormone FGF23 that is mainly secrered by bone cells. Disorders of the transport proteins in the renal tubules are often caused by rare mutations in the genetic material. Disruption of hormones that determine the phosphate metabolism can have various causes.
Most patients with a mild hypophosphataemia, low levels of phosphate in the blood, have few complaints. More severe hypophosphatemia may cause symptoms such as fatigue, muscle weakness, fractures, skeletal abnormalities, short length and calcium deposits. Sometimes mild hypophosphatemia is accidentally found and it is unclear whether the symptoms are actually caused by the low levels of phosphate.
With renal hypophosphataemia, there is a disorder in the re-uptake of phosphate from the renal tubules back to the blood. Virtually all of the phosphate in the blood is filtered through the kidney and ends up in the pro-urine, of which our kidneys produce about 180 liters on daily basis. The vast majority of the water and the substances dissolved therein, including phosphate, is then reabsorbed by the renal tubules into the blood. There are different processes which, if disturbed, can lead to an increased loss of phosphate in the urine, resulting in a low phosphate in the blood.
Determining the cause / diagnosis
The diagnosis of renal hypophosphatemia is made on the basis of an increased phosphate excretion in the urine. This is done by calculating how effectively the renal tubules can re-uptake phosphate in a (2-hour) portion of urine. Furthermore, it is usual to collect a 24-hour urine sample and to determine the height of a number of hormones in the blood that are involved in phosphate metabolism. Depending on these results, additional diagnostic testing could be performed. If a genetic / inherited cause of renal hypophosphatemia is suspected, genetic testing could be done.
The treatment consists of, where possible, elimination of the cause. If the cause is genetic / hereditary or the result of chronic damage to the renal tubules, this is often not possible. A vitamin D deficiency is treated with vitamin D preparations and then phosphate supplementation is frequently started. If there is an excess PTH release, this is engaged with drugs or one or more parathyroid glands will be removed.
Depending on the specific cause, the renal hypophosphatemia will be transient, disappears after a specific therapy or remains chronically present. The prognosis is generally good. Phosphate supplementation is often needed for a longer period and sometimes the complaints of the renal hypophosphatemia remain present.