Autoimmune Thyroid Disease Risk Associated with Onset of Rheumatoid Arthritis

The risk of autoimmune thyroid disease (AITD) increases during the 5-year period before diagnosis of rheumatoid arthritis (RA), but appears to drop following RA diagnosis, according to a new study.
Although prior research revealed an association between AITD and RA, there has been limited information on the risk of developing these conditions over time and the risk of AITD after RA diagnosis.
The study, published in JAMA Network Open, sought to determine when and how the increased risk of AITD develops in association with the time before or after onset of RA. The researchers used data from the Swedish Rheumatology Quality Register and linkage to other nationwide registers to identify 8090 adults with new-onset RA and 80,782 population-based participants as reference.
According to the findings, the prevalence of thyroxine-treated AITD was 10.3% among patients with RA by the time of RA diagnosis compared with 7.15% among the control group.
The study showed that the increased risk developed during the 5 years before diagnosis of RA and peaked by the time of RA diagnosis. Additionally, the risk of AITD decreased 2 to 5 years after RA diagnosis, the researchers noted.
The study indicated that the co-occurrence of RA and AITD may be due to several factors, including the increasing number of identified, shared susceptibility genes for both diseases. However, the researchers noted that it remains unclear whether there is an increased prevalence of antithyroid antibodies among patients with RA.
Although the pattern of risk before diagnosis of RA was similar for hyperthyroidism and hypothyroidism, the decrease in risk after diagnosis of RA was more pronounced for hyperthyroidism, according to the study. The researchers also indicated that a protective effect of antirheumatic therapies on the development of AITD may contribute to the decreased risk after RA diagnosis.
“This pattern of risk raises the questions whether AITD may influence the pathogenesis of RA (or vice versa) and, conversely, the question whether antirheumatic therapies may prevent AITD,” the researchers wrote in the study.
Regarding the clinical implications, the researchers concluded that patients with AITD who have joint symptoms should be evaluated for potential new-onset RA and those with diagnosed RA may be reassured that the risk of developing AITD has decreased.  
Waldenlind K, Saevarsdottir S, Bengtsson C, et al. Risk of thyroxine-treated autoimmune thyroid disease associated with disease onset in patients with rheumatoid arthritis. JAMA Network Open. 2018. doi:10.1001/jamanetworkopen.2018.3567

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