Hypophosphatemia (Low Phosphate)

What phosphate does

Phosphate, the form of phosphorus in the body, is part of ATP (the energy molecule of cells), DNA, cell membranes and bone. It also helps red blood cells release oxygen. About 85% is stored in bone. Blood levels are controlled by the kidneys, parathyroid hormone (PTH), vitamin D and a hormone called FGF23.

Causes

Phosphate shifting into cells

• Refeeding syndrome: when someone who has been starved or severely malnourished starts eating or tube feeding, insulin drives phosphate into cells. This is the most dangerous cause.
• Treatment of diabetic ketoacidosis with insulin.
• Respiratory alkalosis from fast breathing, such as with sepsis, anxiety or alcohol withdrawal.
• Recovery from burns and some leukemias.

Too little absorbed

• poor diet, starvation and eating disorders
• alcohol use disorder, which combines poor intake, vomiting and kidney losses
• phosphate-binding antacids containing aluminum, calcium or magnesium in large amounts
• chronic diarrhea and malabsorption
• vitamin D deficiency

Too much lost in urine

• primary hyperparathyroidism
• IV iron, especially ferric carboxymaltose, which raises FGF23
• kidney tubule disorders (Fanconi syndrome), and some medicines such as tenofovir and ifosfamide
• inherited conditions such as X-linked hypophosphatemia, which causes rickets in children
• tumor-induced osteomalacia, a rare condition caused by small tumors that make FGF23
• after kidney transplant

Symptoms

Mild hypophosphatemia usually causes no symptoms. Symptoms appear mainly with severe or long-lasting deficiency:

• muscles: weakness, including of the breathing muscles and heart, and muscle breakdown (rhabdomyolysis)
• nervous system: confusion, irritability, numbness, seizures and coma
• heart: reduced pumping and heart failure, abnormal rhythms
• blood: red blood cell breakdown, reduced white cell and platelet function
• bones (chronic): bone pain, osteomalacia in adults, rickets in children, fractures

Diagnosis

• blood phosphate level, ideally fasting, with calcium, magnesium, potassium and kidney function
• parathyroid hormone and vitamin D levels
• urine phosphate to see whether the kidneys are losing phosphate (fractional excretion or TmP/GFR)
• FGF23 in suspected inherited or tumor-related causes
• review of diet, alcohol, medicines and recent IV iron

Low magnesium and potassium often occur alongside low phosphate and also need correcting. See electrolyte imbalance.

Treatment

Mild to moderate

• phosphate-rich foods: milk, yogurt, cheese, meat, fish, eggs, beans, nuts and whole grains
• oral phosphate supplements, often causing diarrhea, so doses are divided through the day
• stopping phosphate-binding antacids when possible

Severe

• IV phosphate in hospital, with careful monitoring of phosphate, calcium and kidney function, because too much IV phosphate can drop calcium and damage kidneys
• heart and breathing monitoring

Treating the cause

• slow, monitored refeeding in malnourished patients, with phosphate supplements started early
• vitamin D replacement
• parathyroid surgery for primary hyperparathyroidism
• for X-linked hypophosphatemia, burosumab, an antibody that blocks FGF23, or phosphate with active vitamin D
• removal of a tumor causing tumor-induced osteomalacia

Outlook

Most hypophosphatemia corrects quickly once the cause is addressed. Severe cases, particularly refeeding syndrome, can be life-threatening if missed but are preventable with careful monitoring. Long-term phosphate wasting conditions need specialist care to protect bone health.

Frequently asked questions

What is hypophosphatemia?

Hypophosphatemia is a low level of phosphate in the blood, usually below 2.5 mg/dL. Phosphate is needed for energy, muscles and bones. Mild cases often cause no symptoms, while severe cases can cause muscle weakness, breathing failure and confusion.

What causes low phosphate levels?

Common causes are refeeding after starvation, alcohol use disorder, insulin treatment for diabetic ketoacidosis, phosphate-binding antacids, vitamin D deficiency, overactive parathyroid glands, IV iron, and kidney disorders that waste phosphate. Inherited forms are rare.

What are the symptoms of low phosphate?

Most people with mildly low phosphate have no symptoms. Severe deficiency causes muscle weakness, fatigue, breathing difficulty, confusion, irritability, numbness, seizures and heart problems. Chronic deficiency causes bone pain and soft bones.

What is refeeding syndrome?

Refeeding syndrome is a dangerous shift of fluids and minerals, especially phosphate, when a severely malnourished person starts eating or receives nutrition. It can cause heart failure, breathing problems and confusion. It is prevented by slow refeeding and close blood test monitoring.

How do you raise phosphate levels?

Mild low phosphate can be corrected with phosphate-rich foods such as dairy, meat, fish, eggs, beans and nuts, or oral phosphate supplements. Severe cases need IV phosphate in hospital. Treating the underlying cause is essential to stop it recurring.