PTH and Marchiafava-Bignami disease: In the impossibility to directly assess CYB27B2, circulating levels of 1–25(OH)2D might be considered a proxy of tubular synthesis, and 1,25(OH)2D/PTH ratio as a composite marker of tubular endocrine reserve and setpoint of transition from adaptive to maladaptive response in CKD-MBD.