Abstract Disclosure: J. Beck: None. C. Atila: None. J. Refardt: None. Z. Erlic: None. J.B. Drummond: None. B.S. Soares: None. F. Beuschlein: None. B.F. Winzeler: None. M. Christ-Crain: None. Introduction The differential diagnosis between arginine vasopressin deficiency (AVP-D), formally known as central diabetes insipidus, and primary polydipsia (PP) is challenging. In clinical routine, psychopathologic findings are often used as a hallmark for diagnosing PP; thus, it is often referred to as psychogenic polydipsia. Yet, psychopathologic characteristics are barely assessed in patients with AVP-D, and to date, no data exist comparing AVP-D and PP with regard to these features. Therefore, in this study, we aimed to compare levels of anxiety, depression, alexithymia, and overall mental health in patients with AVP-D and PP. Methods This study combined data from two diagnostic studies conducted at three tertiary medical centers. In total, 82 participants (n=39 with AVP-D, n=28 with PP, and n=15 healthy controls [HC]) underwent a psychological evaluation with standardized questionnaires at study inclusion. Anxiety levels were assessed using the State-Trait Anxiety Inventory, mood using the Beck’s Depression Inventory, alexithymia using the Toronto Alexithymia Scale, and overall physical and mental health using the Short Form 36 Health Survey (SF-36). Higher STAI, BDI, and TAS scores indicate higher anxiety, depression, and alexithymia levels. Higher SF-36 scores indicate better health and less disability. Results Compared with HC, patients with AVP-D and PP showed increased levels of anxiety (HC 28 points [24, 31] vs. AVP-D 36 points [31, 45], p<0.01; vs. PP 38 points [33, 46], p<0.01), depression (HC one point [0, 2] vs. AVP-D 7 points [4, 14], p<0.01; vs. PP 7 points [3, 13], p<0.01), and alexithymia (HC 30 points [29, 37] vs. AVP-D 43 points [35, 54], p<0.01; vs. PP 46 points [37, 55], p<0.01). Levels of anxiety, depression, and alexithymia showed no difference between patients with AVP-D and PP (p=0.58, p=0.90, p=0.50). Compared with HC, patients with AVP-D and PP reported comparable reduced self-reported mental health scores (HC 84 [68, 88] vs. AVP-D 60 [52, 80], p=0.05; vs PP 60 [47, 74], p<0.01). Conclusion This is the first study demonstrating comparable increased levels of anxiety, depression, alexithymia, and overall reduced mental health in patients with AVP-D and PP. Based on these data, psychopathological findings should not be used as a hallmark to differentiate between both conditions. Presentation: Thursday, June 15, 2023