Alvarez-Cienfuegos and his colleagues found a negative correlation between plasma klotho levels and disease activity (DAS28-ESR) (correlation coefficient =  − 0.22, p = 0.07) in the included 63 female RA patients, but no significant correlation between FGF23, an important ligand for klotho, and disease activity (correlation coefficient = 0.08, p = 0.52) [15]. Here, FGF23 is linked to rheumatoid arthritis.