PTH and rheumatoid arthritis: It will also be important to determine whether there is dysequilibrium in the PTH-FGF23-vitamin D axis in other autoimmune diseases such as type 1 diabetes, systemic lupus erythematosus and rheumatoid arthritis that exhibit an incidence or disease activity which correlates with vitamin D nutrition (Munger et al. 2013; Watad et al. 2017).