Key Points Question What is the incidence of impaired kidney function in adolescents with anorexia nervosa, and how does it correlate with body mass index and physiologic parameters of anorexia nervosa severity? Findings In this case-control study of 395 adolescent patients with recent diagnosis of anorexia nervosa, impaired kidney function was found in 36.8%. Minimal heart rate and free triiodothyronine levels showed correlation with eGFR but not with admission body mass index. Meaning Results of this case-control study suggest that impaired kidney function may be a better parameter of anorexia nervosa severity than body mass index.