Thiamine Diphosphate (TDP)

The goal of this study is to examine the effectiveness of thiamine in reducing depressive symptoms in children and adolescents. The investigators hypothesize that thiamine administration will alleviate depressive symptoms in children and adolescents and will have a favorable safety profile. Participants will complete a baseline virtual visit with a study clinician and parent/guardian questionnaires to assess depression symptoms, medical history, current medications, and study eligibility. Participants will complete fasting blood draws before and after the 4-week study period and, if eligible, receive study thiamine capsules by mail to take daily during the study. Participants will attend weekly virtual visits for 4 weeks to monitor depression symptoms, safety, medication adherence, side effects, and any changes in medications or treatments.