<h3>Background</h3> The association between rheumatoid arthritis and autoimmune diseases have been described since the 1960s ^1,2. Earlier studies with methodological limitations, have not found a clear relationship between these two diseases, while other studies show their correlation in an insignificant percentage³. In South America, there is not population data available to establish this relationship; its importance lies in the need to perform a screening of the thyroid function and antiperoxidase antibodies in patients suffering rheumatoid arthritis, with subclinical and established hypothyroidism that could be early detected, in order to carry out a closer monitoring and initiate a promptly substitution therapy. <h3>Objectives</h3> To Determine the association between autoimmune thyroid disease and rheumatoid arthritis (RA) through antibody anti thyroid frequency expression and abnormal thyroid function tests in patients suffering rheumatoid arthritis compared with those undergoing arthrosis and fibromyalgia. <h3>Methods</h3> Case – control study. The case group were patients older than 18 years of age diagnosed with RA according to ACR/EULAR 2010 criteria. The control group had similar demographic characteristics diagnosed with arthrosis and fibromyalgia, and paired under age and gender. Anti – thyroid antibodies peroxidise (TPO), TSH and free T4 tests were performed. Patients with previous diagnosis of established subclinical hypothyroidism, other autoimmune diseases, pregnant women and patients treated with levothyroxine, amiodarone and lithium carbonate or α – interferon were excluded from this study. <h3>Results</h3> A sample of 86 cases and 86 controls were studied (77 women in each group). The sample showed an average age of 56.4 years (IC 95% 54.2 – 58.6) for the case group, and 58.6 years (IC 95% 56.1 – 61) for the control group. 14 patients (16.3%) had fibromyalgia and 72 patients (83.7%) had arthrosis, in the control group. The TSH average value was higher in the case group. The TPO positive percentage value was 37.2% in patients with RA, and 2.3% for patients in the control group (p&lt;0.001). The probability of finding positive TPO among the cases was 25 times greater compared with the control group. Subclinical hypothyroidism was 26% in patients with RA, and 10% in the control group. Only 6.9% of established hypothyroidism was found in the cases (p&lt;0.001). 51% of patients with RA showed subclinical and established positive TPO; this was not present in the control group <h3>Conclusions</h3> There was an association between autoimmune thyroid disease and RA. A consideration must be made regarding the study of both the thyroid function and TPO in patients with RA. <h3>References</h3> Buchanan WW, Crooks J, Alexander WD, Koutras DA, Wayne EJ, Gray KG, Association of Hashimoto’s thyroiditis and rheumatoid arthritis. Lancet 1961, 1:245-248. Al-Awadhi, Olusi SO, Al-Zaid NS, Parva K, Al-Ali N, Al-Jarallah KH. Prevalence of hypothyroidism among Arabs with rheumatoid arthritis. British Society for rheumatology. 1999, 472-473 Al-Awadhi, Olusi SO, Hasan E.A y Abdullah A: Frequency of Abnormal Thyroid Function Tests in Kuwaiti Arabs with Autoimmune Diseases. Medical Principles and Practice. 2008; 17:61-65 <h3>Disclosure of Interest</h3> None Declared