Diabetes insipidus (DI) in pregnancy is a rare condition with an incidence of 1/30,000 (1). The classic clinical picture is that of increased thirst, polydipsia and polyuria, however presentations of oligohydramnios have also been described (2).
As the physiological changes seen in pregnancy closely resemble the pathological appearance of DI and primary polydipsia, differentiating these states clinically and biochemically can be challenging. While water deprivation tests are routinely completed in the non-pregnant population, it is seldom used in the evaluation of DI in pregnancy due to maternal and fetal risk with dehydration and hypernatraemia (3).
We present the case of LG, a 36 year-old G4P3 female presenting at 22 weeks gestation with new thirst and polyuria >8L/day. Given her complex social and psychiatric history, a high degree of suspicion remained for primary polydipsia, necessitating the completion of multiple investigations including MRI brain and a modified water deprivation test to definitively diagnose DI in pregnancy.
Gestational diabetes insipidus (GDI), secondary to increased vasopressinase production or decreased metabolism, is a transient condition of pregnancy, usually appearing in third trimester and resolving within two weeks post-partum (3) at which time desmopressin can be weaned. Surprisingly, LG’s excessive polyuria and thirst persisted post-partum, with repeat water deprivation testing at four months demonstrating persistent DI.
LB’s early onset of DI in second trimester, absence of symptoms in previous pregnancies, antibody-positive thyroid disease and failure of her DI to resolve post-partum, are suggestive of an acquired decrease in ADH reserve rather than GDI. Our case typifies the difficulties encountered by endocrinologists and obstetricians alike in identifying and formally diagnosing DI in pregnancy, especially when primary polydipsia is suspected. It also demonstrates the importance of considering other causes of DI in pregnancy such as subclinical posterior pituitary pathology, which takes a different course post-partum.