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Diabetes insipidus is a rare disease, clinically characterized by polyuria and polydipsia. The disease is caused by an insufficient secretion of arginine vasopressin (AVP) from the posterior pituitary in central diabetes insipidus (according to new nomenclature: AVP-deficiency) or by an insufficient AVP action in the kidney in renal diabetes insipidus (according to new nomenclature: AVP-resistance). The most important differential diagnosis is primary polydipsia, characterized by increased thirst and water intake, but without any underlying hormonal disease.

For decades, differential diagnosis of polyuria-polydipsia syndrome was based on the indirect water deprivation test, in which a substantial increase of urine osmolality after prolonged dehydration rules out an endocrine disease. However, diagnostic accuracy of this test is limited, especially in milder partial forms, given the fact that polyuria affects the renal concentration capacity.

Recent studies show promising results regarding a copeptin-based approach. Copeptin is co-secreted with AVP from the posterior pituitary and closely correlates with plasma osmolarity under physiological conditions. Adequately stimulated copeptin concentrations therefore help to distinguish between diabetes insipidus / AVP-deficiency and primary polydipsia, which will be discussed in this narrative review.

Besides AVP, oxytocin is the second hormone secreted from the posterior pituitary. The potential role in social cognition and behavior of oxytocin is well known. Recent studies show an impaired increase in oxytocin following stimulation tests in patients with central diabetes insipidus / AVP deficiency, which might be associated with psychological problems and an impaired quality of life.