Pregnancy-unmasked latent autoimmune arginine vasopressin deficiency with anti-rabphilin-3a antibody positivity.

Although pregnancy-associated arginine vasopressin (AVP) disorder is rare, it is often attributed to excessive placental vasopressinase activity. However, vasopressinase-independent mechanisms, including gestational unmasking of latent AVP deficiency (AVP-D), may occur. We report a 37-year-old primigravid female who developed severe polydipsia and polyuria in the third trimester. She had hypotonic polyuria with inappropriately low plasma AVP levels and loss of the posterior pituitary bright spot on T1-weighted magnetic resonance imaging (MRI). AVP-D was suspected; desmopressin promptly relieved her symptoms. Desmopressin became unnecessary immediately following delivery; however, at 6 months postpartum, her polydipsia and polyuria recurred. Anterior pituitary dysfunction subsequently developed. Notably, at 6 months postpartum, the posterior pituitary bright spot on MRI remained absent. Herein, the transient postpartum improvement initially mimicked gestational AVP disorder; however, retrospective testing revealed anti-rabphilin-3A antibody positivity, indicating autoimmune hypophysitis involvement. Persistent MRI abnormalities, postdelivery AVP-D recurrence, and subsequent anterior pituitary dysfunction supported an underlying autoimmune etiology. This case raises the possibility that pregnancy may unmask latent autoimmune AVP-D through a vasopressinase-independent mechanism. In patients who develop AVP-D during pregnancy, anti-rabphilin-3A antibody positivity and persistent loss of the posterior pituitary bright spot on MRI may offer crucial diagnostic clues, supporting the significance of long-term endocrine follow-up.