FGF23 and hyperparathyroidism: Systemic measurements of Ca, P, PTH, and FGF23 revealed that like those of the Cyp27b1‐null mouse, both M1‐IKO and M1/M21‐DIKO mice exhibited hypocalcemia, hypophosphatemia, hyperparathyroidism, and very low levels of FGF23, all indicative of a potential reduction or the absence of circulating 1,25(OH)2D3.