Internalizing disorders (anxiety and depression) are among the most prevalent psychiatric disorders in youth. Cognitive-behavioral treatment (CBT) is well-establed treatment for internalizing disorders. The evidence originates mostly from disorder-specific CBT for youth. However, the validity of specific diagnosis has been questioned, especially because of high symptom overlap between disorders and the high rates of diagnostic comorbidity. In addition, research has suggested that depressive comorbidity is associated with reduced treatment effectiveness for primary anxiety disorder and vice versa. A recent meta-analysis, of the collective evidence of psychotherapy throught 50 years, revealed that there were no effects of disorder-specific psychotherapy for youths with one or more comorbid disorder. Which indicates that disorder-specific treatment is not sufficient for children with comorbidity.

Transdiagnostic treatments have been developed and evaluated to address this problem. These treatments emphasize effective therapeutic principles which may be applied across multiple disorders. Transidagnostic treatment may be more cost effective and practical than disorder-specific treatments as most therapists only need to be trained extensively in one protocol. Transdiagnostic treatments may also be easily learned by clinicians and especially in generalist settings where one transdiagnostic protocol can be offered to patients with various mental health problems instead of many offered to much fewer. Offering transdiagnostic treatment approach may better serve the diagnostically heterogeneous groups which seek help at general clinics instead of many different disorder-specific treatment protocols. Transdiagnostic CBT protocols have been proved to be effective among adults with internalizing disorders and also among youths. It has also been showed that transdiagnostic CBT was equally effective as anxiety disorder-specific protocol for adults with anxiety disorders.

Treatment Our treatment is a brief group-based cognitive-behavioral treatment which was specifically designed for adolescents with impairing internalizing symptoms, or anxiety/depressive disorders seeking help at the Primary Health Care Clinics in Iceland. The treatment comprises eight weekly 110-minutes sessions were the following components are applied: (1) psychoeducation, (2) cognitive restructuring, (3), behavioral activation, (4) exposure, (5) problem solving, (6) social skills, and (7) mindfulness. The participants receive a workbook with outline of each sessions and assignments related to components (e.g., (e.g., ABC workseets, exposure exercises). They are encouraged to practice skills learned in sessions between sessions. All parents attend two sessions. They receive detailed psychoeducation and are instructed in assisting their children with the homework assignments (week 1), and in week 6 they are instructed how to assist their children with exposure exercises. They also receive a workbook with outline of each sessions. The therapist also has at least one telephone call with each family to follow-up on exercises.

Study design and participants The following study is a randomized controlled trial where participants are allocated to either eight weekly sessions of brief transidagnostic CBT or waitlist monitoring for clinical deterioration. The investigators will randomize by using blockwise procedure determined by a computer-generated algorithm. To ensure that the randomization will not be predicted in advance, it will be centralized at the University of Iceland. Participants will be assigned to study group after being deemed eligible and undergoing consent.

Assessment Participants will be assessed at baseline, midtreatment, posttreatment (week 8), 2 months follow-up, 4 months follow-up, and 1-year follow-up.