BACKGROUND: The association between chronic idiopathic urticaria (CIU) and autoimmune thyroiditis (AT) is known, as well as major prevalence of antithyroid antibodies in the allergical subjects and other autoimmune diseases. We have evaluated the effects of l-thyroxine on clinical symptoms of CIU in AT patients suggesting the hypothesis of a new thyroid-stimulating hormone (TSH) role in immune system. METHODS: In 20 female patients with CIU + AT, both hypothyroid and euthyroid, we have investigated the therapeutic effects of l-thyroxine dosed to suppress the TSH. Free-T3, Free-T4, TSH, antithyroperoxidase and antithyroglobulin antibodies, total immunoglobulin (Ig)E, Rheuma test and eritro-sedimentation rate were monitored during treatment. Results: In 16 patients a strong decrease of urticaria symptoms has happened after 12 weeks. The TPO Ab and HTG Ab clearly decreased in 14 patients. Furthermore, in two patients with rheumatoid arthritis and in two patients with pollen allergy a strong decrease of rheuma test titer and total IgE has happened. Conclusion: The reason of AT is associated to CIU and others allergical and autoimmune diseases is poorly known. The exclusive hormonal therapy reduces the symptoms of CIU and inflammatory response in many chronic diseases associated to AT. We suggest a stimulatory effect of TSH able to produce considerable changes of the immune response and immune tolerance in patients with AT causing target organs damage. The causal mechanism involves immune, nervous and endocrine system, sharing a common set of hormones, cytokines and receptors, in a unique totally integrated loop (the neuro-immuno-endocrine axis).