A new study published in Scientific Reports discussed was based on the hypothesis that elements of a thyroid hormone network exist in synovial tissue. Here, in 12 patients with rheumatoid arthritis (RA) and 32 with osteoarthritis (OA), serum, synovial fluid, synovial tissue and synovial fibroblasts (SF) were used, in order to characterize the local thyroid hormone network. The findings revealed that serum/synovial fluid thyroid hormone levels were similar in RA and OA. The degradation product termed reverse triiodothyronine (reverse T3) was much lower in serum compared to synovial fluid indicating biodegradation of thyroid hormones in the synovial environment. While superfusion experiments with synovial tissue also demonstrated biodegradation, specifically, in RA. Additionally, cellular membrane transporters of thyroid hormones, deiodinases (DIOs), and thyroid hormone receptors were present in tissue and SF. Density of cells positive for degrading DIOs were higher in RA than OA. TNF increased protein expression of degrading DIOs in rheumatoid arthritis synovial fibroblast (RASF) and osteoarthritis synovial fibroblasts (OASF). On the other hand, gene expression studies of RASF revealed insignificant gene regulation by bioactive T3. Hence, it was inferred that RA and OA synovial tissue/SF show a local thyroid hormone network. Thyroid hormones undergo strong biodegradation in synovium. Meanwhile, bioactive T3 does not influence SF gene expression, whereas SF seem to have a relay function for thyroid hormones.