Diagnostic Tests for Diabetes Insipidus

Diabetes insipidus (DI) is a disorder characterized by excretion of large volumes of hypotonic urine. The underlying cause is either a deficiency of the hormone arginine vasopressin (AVP) in the pituitary gland/hypothalamus [central DI )], or resistance to the actions of AVP in the kidneys [nephrogenic DI )]. In most circumstances, DI is also characterized by excessive consumption of water (polydipsia). A third condition called primary polydipsia can clinically show overlapping features with DI. Both DI and primary polydipsia are collectively referred to as ‘polyuria-polydipsia syndromes. Like other endocrine disorders, an accurate diagnosis of DI can be challenging. This is mainly because the results obtained from diagnostic testing can show significant overlap among the different forms of DI and primary polydipsia. When a case of DI is suspected, the initial step involves the confirmation of the presence of hypotonic polyuria, which is the hallmark of DI. Once hypotonic polyuria is established, the next step is to identify the type of polyuria-polydipsia disorder (central DI vs. nephrogenic DI vs. primary polydipsia). This can be determined either through the water deprivation test or through the copeptin stimulation tests using osmotic and non-osmotic AVP stimulants. Lastly, a detailed history and physical examination must be performed and appropriate laboratory and imaging studies must be undertaken to identify the underlying etiology of DI. This chapter describes the diagnostic steps to be pursued to identify the presence of DI, distinguish the various forms of polyuria-polydipsia disorders, identify the underlying disorders responsible for the DI, the challenges faced with diagnostic testing for DI in clinical practice, and future prospects in the field of DI diagnosis. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.