11. Hypophosphatemia

Definition: serum [PO4] <2.4mg/dL.

Etiology/Risk Factors

Most commonly seen in patients with chronic alcohol use, DKA, or hyperalimentation.

  • Decreased intestinal absorption.
    • Malnutrition: rare by itself due to normal adaptation by renal reabsorption.
    • Malabsorption from vitamin D deficiency.
      • Leads to secondary hyperparathyroidism with increased urinary phosphate excretion.
    • Chronic alcohol use.
    • Phosphate binders.
  • Shifts from serum into cells: stimulation of glycolysis leads to phosphorylated carbohydrate production in liver/muscle, causing decreased serum phosphate.
    • Insulin/glucagon/androgens.
      • Treatment of DKA. 
    • Refeeding syndrome occurs 24-72 hours after initiating nutrition in high-risk malnourished patients (alcohol use, anorexia, chronic diarrhea/vomiting). Requires close electrolyte monitoring.
    • Hyperalimentation.
    • Respiratory alkalosis.
    • Hungry bone syndrome.
    • Sepsis.
  • Increased urinary excretion.
    • Hyperparathyroidism.
    • Proximal renal tubular defect.
    • Vitamin D deficiency. 
  • Continuous renal replacement therapy.

Evaluation

  • The etiology of hypophosphatemia is often apparent from the clinical history and medication review.
  • If not, workup for rare causes includes:
    • Ca, PTH, vitamin D.
    • Fractional excretion of phosphate (serum and urine creatinine and phosphate): >5% suggests renal wasting as the cause.
    • 24-hour urine: urine phosphate >100 mg in 24 hours suggests renal wasting.

Management

  • Evaluate for symptoms: generally seen only with total body depletion and serum PO4 <1 mg/dL.
    • Generalized: weakness, rhabdomyolysis, hematologic dysfunction.
    • CNS: paresthesias, confusion, stupor, seizures, coma.
    • Cardiac: impaired cardiac contractility.
    • Pulmonary: hypercarbic/hypoxic respiratory compromise/failure due to impaired diaphragmatic contractility.
    • GI: ileus.
  • Treatment
    • Treat if PO4 <2 mg/dL. Treat orally if PO4 1-1.9 and with IV if <1 mg/dL and then switch to oral when PO4 >1.5.
    • Oral: 1-2 tabs or packets 3-4x daily.

 Note: phosphate formulations are variable and have similar names. Call pharmacy for the most up-to-date information on available options for oral repletion.

Phosphate Prep Brand

Phosphate content/pkg

Potassium content/pkg

Sodium content/pkg

Phos NaK

250mg (8mmol)

7.1 mEq

6.9 mEq

K-Phos Neutral

250mg (8mmol)

1.1 mEq

13 mEq
  • IV: 15 mmol K-Phos (contains 22 mEq potassium) or Na-Phos (22 mEq sodium) over 2-6 hours.

Serum P04

Serum K+

KPhos (mmol) IV (over 6 hours)

NaPhos (mmol) IV (over 6 hours)

1.5-2mg/dL

<3.5

15

-

1-1.5mg/dL

<3.5

30

-

1.5-2mg/dL

≥3.5

-

15

1-1.5mg/dL

≥3.5

-

30

Key Points

  • Hypophosphatemia is commonly from malabsorption, insulin treatment for DKA, refeeding syndrome, or hungry bone syndrome. 
  • Critically low phosphate (<1.0) should be repleted IV. Otherwise, oral repletion is preferable.

 

Brunelli SM, Goldfarb S. Hypophosphatemia: Clinical consequences and management. J Am Soc Nephrol 2007: 1999-2003, 18.

Gasbeek A, Meinders AE. Hypophosphatemia: An update on its etiology and treatment. Am J Med 2005: 1094-1101, 118.

Weisinger JR, Bellorin-Font E. Magnesium and phosphorus. Lancet 1998;352:391-396.

Marinella MA.  Refeeding syndrome and hypophosphatemia. J Intensive Care Med 2005;20:155-159.