From the Guidelines
Low phosphorous levels, or hypophosphatemia, can be caused by several factors, including X-linked hypophosphataemia (XLH), a genetic disorder characterized by renal phosphate wasting, as well as other conditions such as malabsorption disorders, vitamin D deficiency, and certain medications. The most common causes of hypophosphatemia include poor dietary intake of phosphorous-rich foods, malabsorption disorders like celiac disease or inflammatory bowel disease, and excessive use of phosphate-binding antacids containing aluminum, calcium, or magnesium 1. Vitamin D deficiency can also lead to low phosphorous by reducing intestinal absorption. Certain medications contribute to hypophosphatemia, including diuretics, corticosteroids, and some antiviral drugs. Alcoholism frequently causes low phosphorous due to poor nutrition and impaired kidney function. Hyperparathyroidism increases phosphorous excretion through the kidneys. Refeeding syndrome, which occurs when nutrition is reintroduced too quickly after malnutrition, can cause phosphorous to shift into cells, lowering blood levels. Severe burns, diabetic ketoacidosis, and respiratory alkalosis may also deplete phosphorous.
Some of the key risk factors for hypophosphatemia include recurrent or ongoing blood loss, malabsorptive disorders, normal renal function, severe iron deficiency, lower body weight, low baseline serum phosphate, and higher serum parathyroid hormone (PTH) levels 1. Treatment depends on the underlying cause but often includes phosphorous supplementation, typically as potassium phosphate or sodium phosphate, at doses of 2.5-5 mg/kg/day, while addressing the primary condition. However, in cases of treatment-emergent hypophosphatemia, cessation of the offending agent, such as certain intravenous iron formulations, is the most important management step 1. Severe cases may require intravenous phosphate replacement under careful monitoring to avoid complications.
It is essential to note that phosphate repletion should be avoided in cases of hypophosphatemia with secondary hyperparathyroidism, as it can raise parathyroid hormone and worsen the phosphaturia, ultimately worsening hypophosphatemia 1. Instead, treatment should be directed at mitigating secondary hyperparathyroidism, such as with vitamin D supplementation. Regular monitoring of serum phosphate levels is crucial in patients at risk for chronic low serum phosphate, especially those receiving certain intravenous iron formulations 1.
From the Research
Causes of Low Phosphorous
- Inadequate intake, decreased intestinal absorption, excessive urinary excretion, or a shift of phosphate from the extracellular to the intracellular compartments can cause hypophosphatemia 2
- Renal phosphate wasting can result from genetic or acquired renal disorders, such as vitamin D deficiency, hyperparathyroidism, oncogenic osteomalacia, and Fanconi syndrome 2
- Acquired renal phosphate wasting syndromes can also result from conditions like malnutrition, intestinal malabsorption, excess alcohol intake, and certain drugs 3
- Genetic disorders, such as X-linked hypophosphatemic rickets, can cause chronic hypophosphatemia, especially in children, but also in adults with hypophosphatemia of unknown origin 3
- Medical interventions, such as those for alcoholism and diabetic ketoacidosis, can produce a shift of phosphorus within the body, leading to hypophosphatemia 4
- Other causes of hypophosphatemia include primary hyperparathyroidism, secondary hyperparathyroidism, and primary renal phosphate wasting, which can be divided into three types based on serum calcium levels 2
Mechanisms and Regulation
- Phosphate homeostasis involves the regulation of intestinal absorption, redistribution, and renal tubular absorption of the mineral, with plasma phosphorus levels maintained within a narrow range 5
- Major regulators of phosphate homeostasis, such as parathormone, fibroblast growth factor 23, and 1,25-dihydroxyvitamin D, respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption 6
- Any acquired or genetic dysfunction in the major organs or regulators involved in phosphate homeostasis can induce hypo- or hyperphosphatemia 6