Phosphate, the renal tubule, and the musculoskeletal system

A component of ATP, phosphate is at the hub of the energy-related mechanisms operative in muscle cells. Together with calcium, phosphate is involved in bone tissue mineralization: thus, a chronic alteration in the metabolism of phosphate can induce bone and joint disorders. Diagnosis of chronic hypophosphatemia. Serum phosphate, calcium, and creatinine should be assayed simultaneously. Serum calcium is increased in hypophosphatemia caused by hyperparathyroidism and decreased in osteomalacia. Urinary phosphate excretion should be measured in patients with a normal serum calcium level and a serum phosphate level lower than 0.80 mmol/L. A decrease in urinary phosphate excretion to less than 10 mmol/24 h strongly suggests a gastrointestinal disorder, such as malabsorption, antacid use, or chronic alcohol abuse. In patients with a urinary phosphate excretion greater than 20 mmol/24 h, the maximal rate of tubular reabsorption of phosphate (TmPO4) and the ratio of TmPO4 over glomerular filtration rate (GFR) should be determined to look for phosphate diabetes. Manifestations and causes of phosphate diabetes in adults. Moderately severe phosphate diabetes in adults manifests as chronic fatigue, depression, spinal pain, and polyarthralgia, with osteoporosis ascribable to increased bone resorption. Although many cases are idiopathic, investigations should be done to look for X-linked vitamin D-resistant rickets missed during childhood, a mesenchymatous tumor, or Fanconi's syndrome with renal wasting of phosphate, glucose, and amino acids. Management of phosphate diabetes. Phosphate supplementation and, in patients with normal urinary calcium excretion, calcitriol produce some improvement in the symptoms and increase the bone mineral density. Whether dipyramidole is clinically effective remains unclear.