Allergen-specific immunotherapy (SIT) is a treatment modality that has been shown to be effective in the treatment of asthma, allergic rhinitis or anaphylaxis due to pollen, mite or venom hypersensitivity. Clinical and immunologic advantages of a successful immunotherapy continue for years after the treatment is finished. Local and systemic side effects have been reported to be associated with SIT, but long-term side effects have not yet been clarified. The term autoimmune thyroiditis includes many different diseases. Impairment of cellular immunity due to genetic defect of suppressor T-cells lies beneath Hashimoto thyroiditis. A 14-year-old girl diagnosed with asthma who developed findings of autoimmune thyroiditis and goiter four months after SIT with grass pollen antigens is presented to emphasize the possibility of autoimmune disease development during the course of SIT and the need for further research about this coexistence.