We report the case of a rare association of Hashimoto's thyroiditis with Crohn's disease . 17-year-old boy with a 3-year history of fistulizing Crohn's disease treated with biotherapy , admitted to the gastroenterology department for an exacerbation of his disease . He complained of diarrhea, abdominal pain, bradycardia and asthenia. The biological workup was in favor of a high CRP and anemia (Hb at 11g/dl). The etiological workup of the bradycardia showed: a TSH at 8.5IU/ml (0. 4-4.78IU/ml) LT4 at 0.46ng/dl (0.7-1.48ng/dl) . Antithyroperoxidase antibodies were 567IU/ml and antityroglobulin antibodies at 150IU/ml (lower than 0.6) . Cervical ultrasound was in favor of Hashimoto's thyroiditis. Abdominal imaging revealed active inflammatory ileocoecal involvement. On the basis of these data, a diagnosis of autoimmune Hashimoto's thyroiditis associated with Crohn's disease was made, and the patient was put on Levothyroxine . In front of the increasing number of reported cases of their association; several authors suggested the existence of a correlation between autoimmune thyroiditis and IBD, many explanations were presented, among which the immunological theory described by Roura-Mir C and his team, whose strong argument is the presence of an imbalance of type TH2 in the 2 diseases, Now, it became recognized that the balance Th1/Th2 controls the immune system. The second is that which suggests that a common genetic etiology contributes to the coexistence of these diseases

Keywords : Hashimoto, Thyroiditis , Crohn's Disease