The relationship between RA and thyroid dysfunction has been taken into account since the 1960s when an association between RA and Hashimoto’s thyroiditis based on the main role of autoimmunity was found [15, 16]. It was demonstrated that the prevalence of thyroid dysfunction with or without AITD estimated 6–33.8% in RA patients . However, thyroid dysfunction, AITD, and anti-TPO positivity prevalence differ among RA patients based on geographical locations. Among RA patients, AITD range from 0.5% in Morocco  to 27% in Slovakia ; and anti-TPO differ from 5%  in Egypt to 37% in Italy .
Since in early research the control group was absent, some recent studies have investigated the association between thyroid hormone dysfunction and AITD in RA patients . In numerous studies were demonstrated that RA is a substantial risk factor for thyroid dysfunction . Hence, Sara McCoy et al. found no difference in prevalence or development of hypothyroidism between 650 RA and 650 non-RA patients . Since thyroid dysfunction or AITD and RA disease association were not well studied in the Iranian population, this study with enrolling 250 RA patients and 248 non-inflammatory rheumatological patients was designed to investigate this issue. This study was demonstrated a significant relationship between thyroid dysfunction and RA which was twice as common in RA patients as in controls. Prakash Joshi et al., in a prospective cross-sectional survey enrolling 52 RA patients, estimated hypothyroidism was 3.5 times more prevalent compared with the general population . Several studies only consider overt hypothyroidism except for the subclinical stage, yet some studies included both stages. In most of these studies, overt hypothyroidism was the most common thyroid dysfunction [11, 25–27]; however, several of them showed subclinical hypothyroidism as the most common one  and the others found no significant difference between subgroups . Most of the research demonstrated a significant association only between RA and hypothyroidism but not for hyperthyroidism. However, Qian Li et al. conducted a meta-analysis moreover case-control study including 65 RA patients and 550 controls among the Chinese population and found that both hypo and hyperthyroidism were significantly highly prevalent in RA patients than that in controls . Also, a retrospective cross-sectional study in Israel found RA was associated with both hyperthyroidism and especially hypothyroidism . This study was estimated that RA was just associated with overt hypothyroidism. Nevertheless, no association between RA and subclinical hypothyroidism or hyperthyroidism was observed.
Moreover, recent studies pay more attention to the prevalence of anti-thyroid antibodies positivity and AITD in RA patients. Xi-Feng Pan et al., in a meta-analysis, estimated that the presence of anti-TPO is 2.3 times more common in RA patients than that in healthy individuals. Although anti-thyroid autoantibodies were positively associated with RA disease in Asian and African populations, no significant association was detected in most of the studies run in America and Europe. The reason for this contrast might lie in geographical genetic, and environmental differences or preliminary studies to find its real association . This study also demonstrated that the anti-TPO positivity was 2.65 times greater in RA patients compared with controls (32% vs. 15%).
The association between AITD, an organ-specific autoimmune disease, and systematic autoimmune diseases like systemic lupus erythematosus, primary Sjögren’s syndrome, and especially RA was shown in several studies . Yet some studies observed no significant correlation, for instance, a case-control study including 100 RA patients and 55 controls was showed that AITD was more common in RA patients than that in controls, but it was not significantly different between the two groups . It was demonstrated in this study that AITD was 2.5 times more prevalent in RA patients compared with controls (19.7% vs. 8.6%).
Nonetheless, this research is subject to several limitations such as lack of measuring other kinds of anti-thyroid antibodies and detailed factors affecting the prevalence of thyroid abnormality in RA patients, for instance lifestyle, genetic background, and so on; hence, it leaves room for further studies in this field.
In conclusion, this study was demonstrated that RA disease was an independent covariant for thyroid dysfunction (especially for overt hypothyroidism) and the presence of anti-TPO positivity as well as AITD among the Iranian population like several other populations.