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Approach to polyuria & polydipsia

The same eight steps, every presentation: threats first, then the history and exam that discriminate, one honest sentence, a mechanism-grouped differential, and the tests read in order.

  1. Immediate threats

    • New diabetes tipping into DKA or hyperosmolar state
    • Severe hypernatremia in the patient who cannot access water
    • Hypercalcemia driving both symptoms

  2. History that discriminates

    • True polyuria (more than 3 L per day) versus frequency of small volumes: measure, not guess
    • Nocturia that wakes; thirst that never settles; craving ice water (classic for DI)
    • Lithium on the chart; recent pituitary surgery or head injury
    • Weight loss, blurred vision, candidal infections: the diabetes cluster

  3. Examination

    • Volume status and orthostatics
    • Visual fields when the pituitary is in question
    • Signs of the causes: dehydration, cachexia, band keratopathy of hypercalcemia

  4. Problem representation

    Three weeks of thirst, 5 L urine daily, weight loss and glucose of 19 (diabetes) versus abrupt polyuria with dilute urine after pituitary surgery (central DI, urgent).

  5. Differential, by mechanism

    Solute diuresis

    • Diabetes mellitus
    • Post-obstructive diuresis
    • Saline loading
    • Mannitol

    Water diuresis

    • Central diabetes insipidus
    • Nephrogenic DI (lithium, hypercalcemia, hypokalemia)
    • Primary polydipsia

  6. Investigations

    • Glucose first: it ends half the workups
    • Paired serum and urine osmolality; sodium; calcium; potassium
    • 24-hour urine volume when the history is vague
    • Water deprivation testing and desmopressin response, specialist-supervised

  7. Interpretation

    • Dilute urine with concentrated serum is the kidney failing to conserve: DI physiology
    • Dilute urine with dilute serum is intake: primary polydipsia
    • High urine osmolality means solute is dragging the water: find the solute

  8. Next steps

    • Treat the found cause; in DI, protect water access above all
    • Lithium-associated DI is managed with the prescriber, never by abrupt discontinuation

Connected

LabsThe DKA panelBlood gasThe DKA gasLabsWater balance, the other direction

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