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Primary Polydipsia — RACP Adult Medicine MCQ

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HardEndocrinologyPrimary PolydipsiaRACP Adult Medicine

An adult with confirmed hypotonic polyuria has baseline copeptin 3.8 pmol/L, excluding nephrogenic diabetes insipidus. During supervised hypertonic-saline testing, sodium reaches the protocol target but stimulated copeptin remains low. Which diagnosis is supported?

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Correct answer: D — Central diabetes insipidus with deficient osmoregulated vasopressin secretion

Explanation lettering: D = shown as B · E = shown as D · B = shown as E

E is correct because an inadequate copeptin response to a verified hyperosmotic stimulus indicates deficient vasopressin secretion and supports central diabetes insipidus. A should generate an appropriate stimulated copeptin response once osmotic stimulation is achieved. B typically has a markedly raised baseline copeptin and should not proceed to this test. C must be excluded before classifying hypotonic polyuria. D reverses the physiology. Hypertonic testing requires close sodium monitoring and should follow the Australian protocol rather than unsupervised water deprivation.

Reference: Endocrine Society of Australia, Harmonisation of endocrine dynamic testing: https://www.endocrinesociety.org.au/downloads/20211216%20Harmonisation%20of%20Endocrine%20Dynamic%20Testing%201.9%20Dec%2016%202021.pdf