Phosphorus is a trace element found in the body in organic and inorganic compounds. For clinicians, inorganic phosphorus is of most interest because of its function in regulating acid-base balance and its involvement in the metabolism of proteins, fats and carbohydrates. Phosphorus is part of tooth enamel and bones.

The thyroid gland is responsible for phosphorus metabolism, which reduces its concentration by excretion through the kidneys. Insulin transfers phosphorus from the blood into the cells. Phosphorus levels are directly related to calcium, and the calcium/phosphorus ratio, which is normally 1.9 to 2 (for example, rickets has an increase above 3), has an important diagnostic value.

Clinical manifestation of reduced inorganic phosphorus levels in the blood (hypophosphatemia) is seen at values below 0.32 mmol/l. There is muscle weakness, destruction of muscle fibres (rhabdomyolysis), respiratory and cardiac failure. Nervous system disorders: speech impairment, confusion, seizures and even coma. There may also be destruction of blood cells and "acidification" of the blood. Chronic hypophosphatemia causes bone destruction in adults and rickets in children.

Increased phosphorus concentration leads to "ossification" (deposition of calcium) of many tissues, including vascular walls, cornea, skin, kidneys and periarticular tissue.