A 50‑year‑old woman with major depression and metastatic breast cancer presents with a two‑month history of polyuria and polydipsia; labs reveal hypernatremia, low urine osmolality, and otherwise normal renal function and vital signs. Which is the most likely diagnosis: diabetes insipidus, adipsic hypernatremia, or psychogenic polydipsia?

Diabetes Insipidus

The most likely diagnosis is diabetes insipidus (DI), specifically central diabetes insipidus secondary to metastatic disease to the pituitary/hypothalamus. 1

Diagnostic Reasoning

The clinical presentation is pathognomonic for diabetes insipidus based on the classic triad:

• Polyuria and polydipsia for 2 months 1
• Hypernatremia (sodium 150 mmol/L, above normal range of 134-146) 1
• Inappropriately dilute urine (osmolality 110 mOsm/kg, well below the diagnostic threshold of <200 mOsm/kg) in the setting of elevated serum sodium 1, 2

This combination of urine osmolality <200 mOsm/kg with elevated serum sodium definitively confirms diabetes insipidus. 1

Why Not the Other Options?

Psychogenic polydipsia is ruled out because:

• Patients with primary polydipsia typically present with hyponatremia or low-normal sodium, not hypernatremia, due to excessive water intake diluting serum sodium 2
• The hypernatremia (150 mmol/L) is incompatible with psychogenic polydipsia 1

Adipsic hypernatremia is ruled out because:

• This patient complains of excessive thirst (polydipsia), which is the opposite of adipsic hypernatremia where patients lack normal thirst sensation 1
• Adipsic patients cannot maintain adequate hydration because they don't feel thirsty, whereas this patient is actively drinking in response to thirst 1

Critical Clinical Context

The history of metastatic breast cancer with lung metastases is highly relevant. 1, 3

• The European Society for Pediatric Nephrology recommends that all patients with newly developed hormonal deficiencies should undergo MRI with pituitary sequences, as diabetes insipidus is most commonly caused by metastatic diseases 1
• A case report documents a 40-year-old woman with stage IV breast cancer who developed central diabetes insipidus from pituitary metastasis, presenting with polyuria, polydipsia, hypernatremia, and low urine osmolality—nearly identical to this case 3

Immediate Next Steps

MRI of the sella with dedicated pituitary sequences should be obtained urgently, as approximately 50% of central DI cases have identifiable structural causes including tumors and metastatic disease. 1

Plasma copeptin measurement can distinguish between central and nephrogenic DI:

• Copeptin >21.4 pmol/L indicates nephrogenic DI 1, 4
• Copeptin <21.4 pmol/L indicates central DI or primary polydipsia 1, 4

Given the metastatic cancer history, central diabetes insipidus from pituitary/hypothalamic metastases is the most likely etiology. 1, 3

Treatment Considerations

Desmopressin is the treatment of choice for central diabetes insipidus and should be initiated promptly. 1, 2

Critical management principles:

• Ensure free access to water at all times—never restrict fluids in DI patients as this causes life-threatening hypernatremia 1, 2
• For IV rehydration, use 5% dextrose in water (hypotonic fluid), NOT normal saline 1, 3
• Monitor serum sodium within 7 days and at 1 month after starting desmopressin, as hyponatremia is the main treatment complication 1

The patient in the case report with metastatic breast cancer and central DI responded well to a single dose of desmopressin (DDAVP), with urine output decreasing from 350-400 mL/hour to normal levels. 3