Synonyms: Serum phosphate, urinary phosphate
Related tests: Calcium, parathyroid hormone
Clinical significance
Phosphorus is found mainly in bone (80%), muscles and body fluids; in the blood it is present in red blood cells and plasma, in both inorganic and organic form. The test measures the concentration of the inorganic fraction.
Absorbed in the first part of the intestine, phosphorus is eliminated mainly by the kidneys but also in the stools; in the kidneys, after glomerular filtration, phosphorus is reabsorbed in the tubules under the control of parathyroid hormone and calcitonin (which reduce reabsorption) and growth hormone (which increases reabsorption).
The concentration of phosphate must remain in balance with that of calcium (the product of the two ions must remain constant) and is regulated mainly by parathyroid hormone.
A rise in serum phosphorus is caused by hypoparathyroidism, hypervitaminosis D, chronic kidney failure and acromegaly.
A fall in serum phosphorus is caused by childhood hypopituitarism, hyperparathyroidism and vitamin D deficiency.
Increased urinary phosphorus excretion is caused by hyperparathyroidism, excessive dietary intake and osteolysis.
Reduced urinary phosphorus excretion is caused by growth, pregnancy, breastfeeding and reduced dietary intake.
Clinical indications
Suspected disorder of phosphorus metabolism; differential diagnosis in suspected hyper- or hypoparathyroidism.
Sample type
The patient must have a blood sample taken.
The patient must collect a 24-hour urine sample.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.