Higher circulating levels of pro-inflammatory cytokines (IL-6—A, TNF-α—B, and IFN-γ—C) were found in the group of patients with periodontitis and diabetes than in the non-diabetic group both at baseline and 1 month after the end of the treatment (IL-6, p = 0.0098 at T0 and p = 0.0061 at T1; TNF-α, p = 0.012 at T0 and p = 0.022 at T1; and IFN-γ, p = 0.032 at T0 and p = 0.029 at T1). The gene discussed is IFNG; the disease is periodontitis.