2021 Aug 14. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–.
ABSTRACT
Phosphate is one of the most important molecular elements to normal cellular functions within the body. It acts as an integral component of nucleic acids and is used to replicate DNA and RNA. It is an energy source for molecular functions through its role in adenosine triphosphate (ATP). It adds and deletes phosphate groups to or from proteins functions as an on/off switch to regulate molecular activity. Given its widespread role in nearly every molecular, cellular function, aberrations in serum phosphate levels can be highly impactful.
Hypophosphatemia is defined as an adult serum phosphate level of less than 2.5 mg/dL. The normal level of serum phosphate in children is considerably higher and 7 mg/dL for infants. Hypophosphatemia is a relatively common laboratory abnormality and is often an incidental finding.
Cellular
In general, phosphate is upregulated through absorption in the intestines and decreased through renal excretion. Excesses are stored in the bones, which act as a buffer to maintain a relatively stable total body content. A typical, nutritious diet provides 1000 to 2000 mg of phosphate daily. Of this, 600 mg to 1200 mg is absorbed via the intestines. Normal serum phosphate levels should be 4 to 7 mg/dL in children and 3 to 4.5 mg/dL in adults. Phosphate exists primarily in the crystallized extracellular matrix of bones within the body, where it is relatively stable and inert. In the absence of pathology, the homeostasis of bone phosphate is neutral with resorption and deposition of approximately 300 mg per day. Bone homeostasis of phosphate is regulated primarily by parathyroid hormone, vitamin D, and sex hormones. Free phosphate within the body is predominantly intracellular at a concentration of approximately 100 mmol/L. This intracellular concentration is maintained using sodium-coupled transport proteins, where extracellular high sodium gradients are used to cotransport phosphate against its concentration gradient into the cellular space. Type 1 sodium phosphate cotransporters are expressed primarily in kidney cells but also are seen in brain and liver tissues. Type 2 sodium phosphate cotransporters function under parathyroid hormone, dopamine, vitamin D, and phosphate concentration regulation. The three types of type 2 transporters are type 2a, type 2b, and type 2c. Type 2a transporters primarily function to modulate renal phosphate homeostasis. Type 2b transporters are expressed in the small intestine and control the dietary uptake of phosphate. Type 2c are thought to be growth-related phosphate transporters and primarily function in the kidneys. Type 3 sodium phosphate cotransporters are present in nearly all cells and function to regulate intracellular phosphate levels.