BDNF and anxiety disorder: Only a few studies linked biological parameters to disease course in anxiety disorders: C-reactive protein (CRP) levels were longitudinally associated with anxiety symptoms (Copeland, Shanahan, Worthman, Angold, & Costello, 2012), increasing cortisol levels were linked to higher 6-month anxiety severity in girls (Schiefelbein & Susman, 2006), and lower Brain-Derived Neurotropic Factor (BDNF) levels were found in patients with a poor response to treatment (Kobayashi et al., 2005).