Manifestation of Subclinical Diabetes Insipidus due to Pituitary Tumor during Pregnancy

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We describe a case of diabetes insipidus (DI) due to a pituitary tumor in a 33-year-old pregnant woman who developed a sudden onset of polyuria (over 8l/day) and polydipsia at 30 weeks of gestation. Her plasma concentration of vasopressin (AVP) was low compared with high serum osmolality (298mOsm/kg), and her urine output was well controlled by treatment with desmopressin acetate (DDAVP). Cranial magnetic resonance imaging (MRI) demonstrated a 1.8×1.2-cm pituitary tumor, but she did not have any disturbance in the release of anterior pituitary hormones. The serum concentration of cystine aminopeptidase (CAP) was within the normal range for a woman at 34 weeks of gestation. After an uncomplicated delivery of a healthy girl, her polyuria gradually resolved. The size of the pituitary tumor gradually decreased in parallel to a reduction in her urine output, but a silent hemorrhage was detected in her pituitary gland 4 weeks after the delivery. Although pregnancy is sometimes associated with central DI, the occurrence of DI due to pituitary tumor under pregnancy is rare. The basal AVP recovered to within the normal range, but the low response of AVP secretion to high osmolality persisted. In this case, pregnancy may affect the manifestation of subclinical DI. This case may therefore enhance our understanding of the mechanisms of DI during pregnancy.