Objective. Rheumatoid arthritis (RA) is characterized by high levels of cytokines such as tumor necrosis factor (TNF). TNF appears to have an etiologic role in thyroid dysfunction, and thyroid dysfunction is a common comorbidity in RA. Anti-TNF treatment might limit thyroid dysfunction. Thus, changes in thyroid hormones were studied during TNF-blocking therapy in patients with RA. Methods. At baseline and after 6 months' treatment with adalimumab, thyroid function [thyroidstimulating hormone (TSH), free thyroxine (fT4), and antibodies against thyroid peroxidase (TPOabs)] were assessed in 138 consecutive adalimumab-treated patients with RA who were naive for TNF-blocking agents. Patients were categorized as hypothyroid, hyperthyroid, or euthyroid. In these groups, changes in thyroid function were determined. Results. Prevalences of hypothyroidism, hyperthyroidism, and TPOabs were 13%, 5%, and 15%, respectively. After 6 months, TPOabs decreased from 267 to 201 IU/ml (p = 0.048). In hypothyroid patients without concomitant L-thyroxine, a trend for declining levels of TSH was observed. Subgroup analysis revealed that in patients who were hypothyroid and TPOabs-positive and L-thyroxine-naive, TSH levels decreased significantly, from 12.5 (interquartile range 6.7-18.4) to 7.1 (interquartile range 4.9-13.8) mU/l (p = 0.043). Conclusion. Anti-TNF treatment improves thyroid function in hypothyroid patients with RA (especially in those who are L-thyroxine-naive and TPOabs-positive), providing further evidence that inflammatory cytokines such as TNF have a pathogenic role in thyroid dysfunction. The Journal of Rheumatology Copyright © 2011. All rights reserved.