TNF and periodontitis: Interleukin‐6 (IL‐6) and tumor necrosis factor‐alpha (TNF‐α) are pro‐inflammatory cytokines that regulate immune response and bone metabolism and have been suggested as one of the most potent cytokines that were associated with periodontitis and RA (McInnes & Schett, 2007; Irwin & Myrillas, 1988; Nibali et al., 2012; Graves & Cochran, 2003).