Case Report of a 14-Year-Old Girl with Addison's Disease Under Initial Presumptive Diagnosis of Anorexia Nervosa: Confusingly Similar and Yet so Different? Primary adrenal insufficiency (Addison's disease) is a rare differential diagnosis of anorexia nervosa. This case report presents important differential diagnostic aspects. We prepared a case report of a 14-year-old female patient according to the CARE guidelines, taking the patient's and the child's parents' view into consideration. The diagnosis of primary adrenocortical insufficiency was reached using specific laboratory diagnostics approximately 9 months after the onset of symptoms, including sudden body weight loss. Significant differential diagnostic aspects were the absence of a body schema disorder and skin hyperpigmentation prominent in the physical examination. The patient experienced a high psychosocial burden because of the unclear diagnosis over 9 months. The diagnosis and substitution therapy with hydrocortisone led to a rapid improvement of the physical and psychological symptoms. This case report emphasizes the importance of a thorough somatic differential diagnosis in the context of a suspected anorexia nervosa.