FGF23 and hyperinsulinemic hypoglycemia, familial, 4: To support the hypothesis of ‘PTH-FGF-23 interdependency’, the authors evaluated one patient with Hyperphosphatemic Familial Tumoral Calcinosis, which is a rare genetic condition of functional FGF-23 deficiency, characterized by hyperphosphatemia, elevated 1,25(OH)2 vitamin D, and increased phosphate reabsorption.