BDNF and anxiety disorder: However, most research into biological parameters for anxiety disorders was done cross-sectionally, showing that anxiety disorder status is linked to higher CRP-levels (Copeland et al., 2012; Pitsavos et al., 2006; Vogelzangs, Beekman, De Jonge, & Penninx, 2013), higher metabolic syndrome markers (Carroll et al., 2009; Kahl et al., 2015; Perez-Cornago, Ramírez, Zulet, & Martinez, 2014), higher tumour necrosis factor-α (TNF-α) levels (Hoge et al., 2009; Pitsavos et al., 2006), and lower BDNF levels (Molendijk et al., 2012).