Hypophosphatemia is defined as a serum phosphate level of less than 2.5 mg/dL (0.8 mmol/L). Phosphate is critical for a remarkably wide array of cellular processes. It is one of the major components of the skeleton, providing mineral strength to bone. Phosphate is an integral component of the nucleic acids that comprise DNA and RNA. Phosphate bonds of ATP carry the energy required for all cellular functions. It also functions as a buffer in bone, serum, and urine.
The addition and deletion of phosphate groups to enzymes and proteins are common mechanisms for the regulation of their activity. In view of the sheer breadth of influence of this mineral, the fact that phosphate homeostasis is a highly regulated process is not surprising.
Phosphate in the body
The bulk of total body phosphate resides in bone as part of the mineralized extracellular matrix. This phosphate pool is accessible, although in a somewhat limited fashion. Approximately 300 mg of phosphate per day enters and exits bone tissue. Excessive losses or failure to add phosphate to bone leads to osteomalacia.
Phosphate is a predominantly intracellular anion with a concentration of approximately 100 mmol/L, although determination of the precise intracellular concentration has been difficult. Most intracellular phosphate is either complexed or bound to proteins and lipids. In response to kinases and phosphatases, these phosphate ions attach and detach from different molecules, forming a constantly shifting pool. Intracellular phosphate is essential for most, if not all, cellular processes; however, because the intracellular concentration of phosphate is greater than the extracellular concentration, phosphate entry into cells requires a facilitated transport process.
Several sodium-coupled transport proteins have been identified that enable intracellular uptake of phosphate by taking advantage of the steep extracellular-to-intracellular sodium gradient. Type 1 sodium phosphate cotransporters are expressed predominantly in kidney cells on the apical membranes of proximal tubule cells and, possibly, the distal tubule cells. They are capable of transporting organic ions and stimulating chloride conductance in addition to phosphate. Their role in phosphate homeostasis is not clear. Other sites of expression include the liver and brain.
Type 2 sodium phosphate cotransporters are expressed in the kidneys, bones, and intestines. In epithelial cells, these transporters are responsible for transepithelial transport, ie, absorption of phosphate from intestine and reabsorption of phosphate from renal tubular fluid. Type 2a transporters are expressed in the apical membranes of kidney proximal tubules, are very specific for phosphate, and are regulated by several physiologic mediators of phosphate homeostasis, such as parathyroid hormone (PTH), dopamine, and dietary phosphate. Currently, these transporters are believed to be most critical for maintenance of renal phosphate homeostasis. Impaired expression or function of these transporters is associated with nephrolithiasis. [1, 2]
Type 2b transporters are very similar, but not identical, to type 2a transporters. They are expressed in the small intestine and are also up-regulated under conditions of dietary phosphate deprivation. Type 2c transporters, initially described as growth-related phosphate transporters, are a third member of the type 2 sodium phosphate cotransporter family. They are expressed exclusively on the S1 segment of the proximal tubule and together with Type 2a transporters are essential for normal phosphate homeostasis.  Similarly to type 2a transporters, type 2c transporters are also regulated by diet and PTH. Loss of type 2c function results in hereditary hypophosphatemic rickets with hypercalciuria. 
Type 3 transporters were initially identified as viral transport proteins. Almost all cells express type 3 sodium phosphate cotransporters; therefore, these transporters presumably play a housekeeping role in ensuring adequate phosphate for all cells. The factors that regulate the activity of these transporter proteins are not completely understood. Evidence suggests, however, that these transporters also participate in the regulation of renal and intestinal transepithelial transport [5, 6] and in the regulation of bone mineralization. 
Circulating phosphate exists as either the univalent or divalent hydrogenated species. Because the ionization constant of acid (pK) of phosphate is 6.8, at the normal ambient serum pH of 7.4 the univalent species is 4 times as prevalent as the divalent species. Serum phosphate concentration varies with age, time of day, fasting state, and season. Serum phosphate concentration is higher in children than adults; the reference range is 4-7 mg/dL in children compared with 3-4.5 mg/dL in adults. A diurnal variation exists, with the highest phosphate level occurring near noon.
Serum phosphate concentration is regulated by diet, hormones, and physical factors such as pH, as discussed in the next section. Importantly, because phosphate enters and exits cells under several influences, the serum concentration of phosphate may not reflect true phosphate stores. Often, persons with alcoholism who have severely deficient phosphate stores may present for medical treatment with a normal serum phosphate concentration. Only after refeeding will serum phosphate levels decline, often abruptly plummeting to dangerously low levels.
Phosphate is plentiful in the diet. A normal diet provides approximately 1000 mg of phosphate, two thirds of which is absorbed, predominantly in the proximal small intestine. The absorption of phosphate can be increased by increasing vitamin D intake and by ingesting a very low–phosphate diet. Under these conditions, the intestine expresses sodium-coupled phosphate transporters to enhance phosphate uptake.
Regulation of intestinal phosphate transport overall is poorly understood. Although studies had suggested that the majority of small intestine phosphate uptake is accomplished through sodium-independent, unregulated pathways, subsequent investigations have suggested that regulated, sodium-dependent mechanisms may play a greater role in overall intestinal phosphate handling than was previously appreciated. Furthermore, intestinal cells may have a role in renal phosphate handling through elaboration of circulating phosphaturic substances in response to sensing a phosphate load. 
Absorption of phosphate can be blocked by commonly used over-the-counter aluminum-, calcium-, and magnesium-containing antacids. Mild-to-moderate use of such phosphate binders generally poses no threat to phosphate homeostasis because dietary ingestion greatly exceeds body needs. However, very heavy use of these antacids can cause significant phosphate deficits. Stool losses of phosphate are minor, ie, 100-300 mg/d from sloughed intestinal cells and gastrointestinal secretions. However, these losses can be increased dramatically in persons with diseases that cause severe diarrhea or intestinal malabsorption.
Bone loses approximately 300 mg of phosphate per day, but that is generally balanced by an uptake of 300 mg. Bone metabolism of phosphate is influenced by factors that determine bone formation and destruction, ie, PTH, vitamin D, sex hormones, acid-base balance, and generalized inflammation.
The excess ingested phosphate is excreted by the kidneys to maintain phosphate balance. Major sites of regulation of phosphate excretion are the early proximal renal tubule and the distal convoluted tubule. In the proximal tubule, phosphate reabsorption by type 2 sodium phosphate cotransporters is regulated by dietary phosphate, PTH, and vitamin D. High dietary phosphate intake and elevated PTH levels decrease proximal renal tubule phosphate absorption, thus enhancing renal excretion.
Conversely, low dietary phosphate intake, low PTH levels, and high vitamin D levels enhance renal proximal tubule phosphate absorption. To some extent, phosphate regulates its own regulators. High phosphate concentrations in the blood down-regulate the expression of some phosphate transporters, decrease vitamin D production, and increase PTH secretion by the parathyroid gland. Distal tubule phosphate handling is less well understood. PTH increases phosphate absorption in the distal tubule, but the mechanisms by which this occurs are unknown. Renal phosphate excretion can also be increased by the administration of loop diuretics.
PTH and vitamin D were previously the only recognized regulators of phosphate metabolism. However, several novel regulators of mineral homeostasis have been identified through studies of serum factors associated with phosphate wasting syndromes such as oncogenic osteomalacia and the hereditary forms of hypophosphatemic rickets, have been discovered.
The first to be discovered was a phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), a neutral endopeptidase mutated in the syndrome of X-linked hypophosphatemic rickets. The characteristics of this syndrome (ie, hypophosphatemia, renal phosphate wasting, low 1,25-dihydroxyvitamin D levels) and the fact that PHEX was identified as an endopeptidase suggested the possibility that PHEX might be responsible for the catabolism of a non-PTH circulating factor that regulated proximal tubule phosphate transport and vitamin D metabolism. A potential substrate for PHEX was subsequently identified as fibroblast growth factor 23 (FGF23).
Several lines of evidence support a phosphaturic role for FGF23.  Another syndrome of hereditary hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, is characterized by a mutation in the FGF23 gene that renders the protein resistant to proteolytic cleavage and, thus, presumably more available for inhibition of renal phosphate transport. Administration of recombinant FGF23 produces phosphaturia, and FGF23 knockout mice exhibit hyperphosphatemia.
The syndrome of oncogenic osteomalacia, characterized by acquired hypophosphatemic rickets and renal phosphate wasting in association with specific tumors, is associated with overexpression of FGF23. Interestingly, in this syndrome, overexpression of FGF23 is accompanied by 2 other phosphaturic agents, matrix extracellular phosphoglycoprotein (MEPE) and frizzled related protein-4. The roles of these 2 latter proteins and their relationship with FGF23 and PHEX are unknown.
The physiologic role of FGF23 in the regulation of phosphate homeostasis is still under investigation. FGF23 is produced in several types of tissue, including heart, liver, thyroid/parathyroid, small intestine, and bone tissue. The source of circulating FGF23 has not been conclusively established; however, the highest mRNA expression for FGF23 in mice is in bone. [10, 11] FGF23 production by osteoblasts is stimulated by 1,25 vitamin D.  Conversely, individuals with X-linked hypophosphatemic rickets show inappropriately depressed levels of 1,25 vitamin D due to FGF23-mediated suppression of 1-alpha hydroxylase activity.
Studies in patients with end-stage renal disease found that FGF23 levels rose with decreasing creatinine clearance rates and increasing plasma phosphorus levels. Klotho, a transmembrane protein, is an essential cofactor for the effects of FGF23 on renal proximal tubule cells.  Inactivation or deletion of Klotho expression results in hyperphosphatemia and accelerated aging. The relationship between these 2 functions of Klotho remains unknown.
A study also demonstrated that FGF23 levels rapidly decreased after kidney transplantation suggesting that FGF23 is cleared by the kidney.  Thus, residual FGF23 could contribute to the hypophosphatemia frequently seen in posttransplant patients. In healthy young men without renal disease, phosphate intake did not significantly increase FGF23 levels, suggesting that FGF23 may not play a role in acute phosphate homeostasis. 
One other family of phosphate-regulating factors is the stanniocalcins (STC1 and STC2). In fish, where it was first described, STC1 inhibits calcium entry into the organism through the gills and intestines. However, in mammals, STC1 stimulates phosphate reabsorption in the small intestine and renal proximal tubules and STC2 inhibits the promoter activity of the type 2 sodium phosphate cotransporter, while the effects on calcium homeostasis are of lesser magnitude. Very little is known about the clinical significance of these newly described mineral-regulating agents or about potential interactions with either the PTH-vitamin D axis or with the phosphatonin-PHEX system.
Any of the following three pathogenic mechanisms can cause hypophosphatemia:
Shift from extracellular to intracellular space
Inadequate phosphate intake alone is an uncommon cause of hypophosphatemia. The ease of intestinal absorption of phosphate coupled with the ubiquitous presence of phosphate in almost all ingested food substances ensures that daily phosphate requirements are more than met by even a less-than-ideal diet.
Hypophosphatemia is most often caused by long-term, relatively low phosphate intake in the setting of a sudden increase in intracellular phosphate requirements such as occurs with refeeding. Intestinal malabsorption can contribute to inadequate phosphate intake, especially if coupled with a poor diet. Although generally not essential for adequate phosphate absorption, vitamin D deficiency can contribute to hypophosphatemia by failing to stimulate phosphate absorption in cases of poor dietary ingestion. Case reports also document patients developing hypophosphatemia due to excessive use of antacids, particularly calcium-, magnesium-, or aluminum-containing antacids.
Increased excretion of phosphate is a more common mechanism for the development of hypophosphatemia. The most common cause of increased renal phosphate excretion is hyperparathyroidism due to the ability of PTH to inhibit proximal renal tubule phosphate transport. However, frank hypophosphatemia is not universal and is most often mild.
Increased excretion of phosphate can also be induced by forced saline diuresis due to the inhibitory effect of saline diuresis on all proximal renal tubule transport processes. Again, the degree of hypophosphatemia is generally minimal. Vitamin D deficiency not only impairs intestinal absorption, but also decreases renal absorption of phosphate.
Several genetic and acquired syndromes of phosphate wasting and associated skeletal abnormalities have been described. These include syndromes characterized by isolated proximal tubule phosphate wasting, such as the congenital or acquired rickets syndromes described previously, and Fanconi syndrome, in which phosphate wasting represents one component of a generalized proximal tubule dysfunction. Congenital Fanconi syndromes include Wilson disease and cystinosis, while acquired Fanconi syndrome can be seen with several medications, paraproteinemias, connective tissue disorders, and heavy metals. [15, 16, 17]
Shift from extracellular to intracellular space
This pathogenetic mechanism alone is an uncommon cause of hypophosphatemia, but it can exacerbate hypophosphatemia produced by other mechanisms. Clinical situations in which this mechanism is the major cause of hypophosphatemia are the treatment of diabetic ketoacidosis, refeeding, short-term increases in cellular demand (eg, hungry bones syndrome), and acute respiratory alkalosis.
Exact figures are difficult to determine, mainly because phosphate measurements are often not obtained with routine laboratory studies and are determined only when the care provider has a high index of suspicion for hypophosphatemia. In the general population of hospitalized patients, hypophosphatemia is observed in 1-5% of individuals and is usually mild and asymptomatic. The percentage rises steeply in patients with alcoholism, diabetic ketoacidosis, or sepsis, in whom studies have reported frequency rates of up to 40-80%.
Hypophosphatemia has been reported in a significant number of patients following partial hepatectomy for transplantation (up to 55%) and in acute hepatic failure, attributed to an increase in cell utilization due to regeneration of liver tissue. Hypophosphatemia in this setting is associated with a favorable prognosis. Hypophosphatemia is also seen in approximately one third of hematopoietic cell transplantation, but, in this setting, it correlates highly with mortality.
Hypophosphatemia occurs in a significant percentage of kidney transplant recipients (50-80%), in particular immediately after transplantation. In many patients it can persist for the life of the transplant. Hypophosphatemia has also been reported in association with the metabolic syndrome.
The morbidity of hypophosphatemia is highly dependent on cause, duration, and severity.
Mild and transient hypophosphatemia is generally asymptomatic and is not accompanied by long-term complications.
Chronic hypophosphatemia that accompanies chronic phosphate deficiency can result in significant bone disease. This is seen most commonly in osteomalacia due to vitamin D deficiency, long-term antacid abuse, hereditary phosphate wasting syndromes, malnutrition, and tumor-induced osteomalacia. Frequently in these conditions, the hypophosphatemia is accompanied by significant bone pain, fracture rate, nephrocalcinosis, and renal insufficiency. In childhood phosphate wasting syndromes, long-term treatment with phosphate replacement frequently results in renal insufficiency and hyperparathyroidism.
Acute severe hypophosphatemia can manifest as widespread organ dysfunction. Hypophosphatemia in the ICU setting is associated with respiratory insufficiency due to impaired diaphragmatic contractility and depressed cardiac output due to decreased myocardial contractility that reverse with correction of the electrolyte abnormality.
Severe hypophosphatemia is also associated with rhabdomyolysis, cardiac arrhythmias, altered mental status, seizures, hemolysis, impaired hepatic function, and depressed white cell function. The newest recommendation for the use of aggressive insulin therapy in the ICU setting has the potential for increasing the frequency and severity of and the morbidity of hypophosphatemia. Another factor increasing the frequency and severity of hypophosphatemia is the widespread use of continuous therapies for the treatment of acute renal failure.
Because it has been theorized that hypophosphatemia in the early stages of sepsis may contribute to the development of new arrhythmias, Schwartz et al hypothesized that intravenous phosphorus replacement may reduce the incidence of arrhythmias in critically ill patients. In a study of 34 adult septic patients with hypophosphatemia, IV phosphorus replacement was associated with a significantly reduced incidence of arrhythmias when compared with 16 patients from previously published data (38% vs. 63%, P = 0 .04). 
Saito et al noted that hypophosphatemia is a common complication in severely disabled individuals, related to frequent bacterial infections, refeeding following malnutrition, and valproate treatment for epilepsy. Because severe hypophosphatemia is life-threatening, serum phosphate levels should be closely monitored, according to the authors. In a study of 19 severely disabled patients, there were 25 episodes of hypophosphatemia. The causes included febrile illnesses (N = 17), refeeding syndrome (N = 4), and Fanconi syndrome (N = 3); one episode was not identifiable. Significantly increased C-reactive protein levels and reduced sodium levels were present during hypophosphatemia episodes. 
Hypophosphatemia has no race predilection except for the syndrome of X-linked hypophosphatemic rickets, which predominates in Caucasian populations.
Hypophosphatemia has no sex predilection except for the syndrome of X-linked hypophosphatemic rickets, which is seen in male children.
Hypophosphatemia can occur in persons of any age. Acquired hypophosphatemia tends to occur in late adolescence to adulthood. Cases occurring in late adolescence are often related to eating disorders. With aging, hypophosphatemia is often related to alcoholism, tumors, malabsorption, or vitamin D deficiency.
The genetic syndromes of phosphate wasting manifest in infancy or childhood. These syndromes include X-linked hypophosphatemic rickets, vitamin D resistant rickets, autosomal dominant hypophosphatemic rickets, hereditary hypophosphatemia with hypercalciuria, and congenital Fanconi syndrome.
Acquired hypophosphatemia tends to occur in late adolescence to adulthood. Cases occurring in late adolescence are often related to eating disorders. With aging, hypophosphatemia is often related to alcoholism, tumors, malabsorption, malnutrition, or vitamin D deficiency. Hypophosphatemia has been reported in up to 15% of geriatric patients undergoing refeeding. Hypophosphatemia has also been reported in up to 35% of adult patients undergoing open heart surgery and is associated with prolonged mechanical ventilation, increased use of cardiovascular drugs, and prolonged hospitalization.
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