Hypophosphatemia

Peter Thorne

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Background

• Required for metabolic pathways (ATP production!)

• Most renal reabsorption occurs in proximal tubule via sodium-phosphate cotransporter

• Common causes

  • Internal redistribution, reduced intestinal absorption

  • Refeeding syndrome

  • Alkalemia

  • Phos binders on purpose or inadvertently (calcium, aluminum, magnesium antacids)

  • Excessive loss (diarrhea, CRRT, increased urinary excretion)

  • Proximal tubular dysfunction such as in Fanconi Syndrome

  • Hyperparathyroidism causes renal phos wasting

  • Post-parathyroidectomy leading to hungry bone syndrome

  • Vitamin D deficiency or resistance

Presentation

• Mild Hypophosphatemia (serum >2.0) rarely symptomatic

• PO4-3\< 2.0: Muscle weakness

• PO4-3\< 1.0: Heart failure, respiratory failure, rhabdomyolysis, seizures

• Failure to wean from ventilator

Evaluation

• Urine PO4-3 level if cause not readily apparent

• Calculate Fe PO4-3 ([U PO4-3 x PCr x 100]/[P PO4-3x UCr])

  • Fe PO4-3 \< 5% = normal renal response to hypophos: redistribution or ↓ absorption

  • Fe PO4-3 > 5% = renal phos wasting

Management

• Caution replacing in pts with impaired renal function: start with half suggested dose

• If K+ > 4 and patient requires IV repletion, may need to use sodium PO4-3 in place of K+ PO4-3 IV; po preferred unless severe or symptomatic, or patient cannot take po

  • K-Phos neutral: oral, each 250mg tablet has 8 mmol of PO4-3 and 1.1mEq of K+

  • K+ PO4-3: IV, each mL has 3mmol PO4-3, 4.4 meq K+

  • Na+ PO4-3: IV, each mL has 3mmol PO4-3

• PO4-3>1.5: PO: 40 – 80 mmol K+Phos neutral (aim for 1 mmol/kg) divided into 3-4 doses/day

• PO4-3 1.25 - 1.5: oral 100 mmol K+ PO4-3neutral in 3-4 divided doses if asymptomatic

  • IV: 30 mmol K+ PO4-3over 6 hours (aim for 0.4mmol/kg) if symptomatic

• PO4-3\<1.25: IV: 80mmol K+Phos over approximately 12 hours (aim for 0.5mmol/kg)

  • Check serum PO4-3 2-12 hrs after last dose of PO4-3 to determine if additional needs