The four initially classified non-FGF23 related hypophosphatemia patients with an increased FGF23 concentration (leading to the 82% specificity) included 2 patients who in retrospect might have had FGF23-related hypophosphatemia due to ectopic overproduction of FGF23 by a neuroendocrine tumor. Here, FGF23 is linked to neuroendocrine neoplasm.