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Key Features

Essentials of Diagnosis

  • Serum phosphate < 2.5 mg/dL (< 0.8 mmol/L)

  • Severe hypophosphatemia may cause tissue hypoxia and rhabdomyolysis

  • Renal loss of phosphate can be diagnosed by calculating the fractional excretion of phosphate (FEPO4)

  • Parathyroid hormone (PTH) and fibroblast growth factor (FGF23) are the major factors that increase urine phosphate

General Considerations

  • May occur in the presence of normal phosphate stores

  • Serious depletion of body phosphate stores may exist with low, normal, or high serum phosphate concentrations

  • Moderate hypophosphatemia (1.0–2.4 mg/dL [or 0.32–0.79 mmol/L])

    • Occurs commonly in hospitalized patients

    • May not reflect decreased phosphate stores

  • Severe hypophosphatemia (< 1 mg/dL [or 0.32 mmol/L])

    • Tissue oxygenation and thus cell metabolism are impaired

    • Commonly seen in alcoholic patients

  • In chronic obstructive pulmonary disease and asthma, hypophosphatemia can occur from

    • Xanthine derivatives causing shifts of phosphate intracellularly

    • Phosphaturic effects of β-adrenergic agonists, loop diuretics, xanthine derivatives, and corticosteroids

Etiology

  • Diminished supply or absorption (Table 21–9)

    • Starvation

    • Parenteral alimentation with inadequate phosphate content

    • Malabsorption

    • Vitamin D–resistant osteomalacia

  • Increased loss

    • Phosphaturic drugs (diuretics, theophylline, bronchodilators, corticosteroids)

    • Hyperparathyroidism, hyperthyroidism

    • Renal tubular acidosis (eg, monoclonal gammopathy)

    • Alcoholism

    • Hypokalemic nephropathy

  • Intracellular shift of phosphorus

    • Glucose administration

    • Drugs (anabolic steroids, estrogen, oral contraceptives)

    • Respiratory alkalosis

    • Salicylate poisoning

  • Electrolyte abnormalities

    • Hypercalcemia

    • Hypomagnesemia

    • Metabolic alkalosis

  • Abnormal losses followed by inadequate repletion

    • Diabetes mellitus with acidosis, especially during aggressive therapy

    • Recovery from starvation

    • Chronic alcoholism

    • Severe burns

  • Inhibition of bone remodeling due to treatment with imatinib mesylate

Table 21–9.Causes of hypophosphatemia.

Clinical Findings

Symptoms and Signs

  • Acute, severe hypophosphatemia (< 1.0 mg/dL [or < 0.32 mmol/L])

    • Can lead to rhabdomyolysis, paresthesias, and encephalopathy (irritability, confusion, dysarthria, seizures, and coma)

    • Respiratory failure or failure to wean from mechanical ventilation may occur as a result of diaphragmatic weakness

    • Arrhythmias and heart failure are uncommon but serious manifestations

    • Hematologic manifestations include acute hemolytic anemia from erythrocyte fragility, platelet dysfunction with petechial hemorrhages, and ...

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