Oxytocin may play a role in mood regulation. Research has shown the plasma oxytocin level of patients with bipolar I disorder (BD I) during a manic episode was significantly higher than that of BD I patients of other statuses, and also that of healthy subjects. However, whether or not a difference in the level of oxytocin exists between patients with major depressive disorder (MDD) and those with BD II is unclear. This study aimed to investigate the plasma oxytocin levels in MDD and BD II patients in a depressive episode. 119 healthy controls, 135 BD II patients, and 97 MDD patients were enrolled. All of the BD II and MDD patients were drug-naïve, with baseline depressive status 17-item Hamilton Depression Rating Scale scores >15. The plasma oxytocin level of the BD II patients was significantly higher than that of the MDD patients and controls at baseline. After treatment, the plasma oxytocin level of the BD II patients increased significantly; however, in the MDD group, the oxytocin level decreased slightly after treatment. Our findings suggested more significant plasma oxytocin dysregulation in the patients in the BD II group than in the MDD patients and controls, both before and after treatment.
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