CYP19A1 and rheumatoid arthritis: Aromatase is the key enzyme for the conversion of androgens into oestrogens and in patients with RA, low levels of androgens and high levels of oestrone are found in the synovial fluid.16 A possible but speculative explanation of our result could also be provided by an experimental study on mouse models of human RA, in which the administration of anastrozole significantly increased the severity of arthritis.