When a severe abnormality in mineral metabolism occurs in a relatively short period of time, such as hypophosphatemia in tumor-induced osteomalacia due to an acquired excess of FGF23, skeletal deformities might ensue, resembling a very high-turnover disease, such as historical severe forms of untreated primary hyperparathyroidism, which may constitute a phenocopy of this disease. Here, FGF23 is linked to osteomalacia.