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X-linked Hypophosphatemic rickets (XLHR) is a rare form of rickets that mainly affects children but, in some cases, it can be missed and not diagnosed until later in life. We present a post-menopausal female that was misdiagnosed with osteoporosis for many years until complete work up was done, and she was found to have osteomalacia due to hypophosphatemia. Clinical case: A 59-year-old female was evaluated following admission to the hospital for a worsening femur fracture on imaging and had received ORIF. She was diagnosed with osteoporosis at the age of 45 and endorses a history of multiple femur fractures from low impact trauma. Despite previous bisphosphonate therapy, she continued to have recurrent fractures.[RC1] She reported no family history of early osteoporosis, but her mother was diagnosed with rickets as a child. Secondary workup for osteoporosis revealed normal 25OH vitamin D, SPEP, TSH, PTH and serum calcium, endomysial antibodies, and 24-hour urine calcium levels. However, the patient had persistently elevated alkaline phosphatase levels (150-200) and low phosphate levels (1.8-2.4). This raised the possibility of Paget’s disease, so a bone scan and lumbar X-ray were obtained which were normal. Given low phosphate levels, fibroblast growth factor (FGF)-23 was obtained and was elevated. This left the differential between tumor-induced osteomalacia (TIO) vs hypophosphatemic rickets. Ga-DOTATE scan and PET scan were negative, so the patient subsequently underwent genetic testing. She was found to have a phosphate regulating endopeptidase homologue (PHEX) gene mutation and was finally diagnosed with XLHR Her PHEX mutation was caused by a novel variant, c.1366 T&amp;gt;C or W456R, which has only been documented once in the literature. The patient was treated with 2 gm per day of phosphate supplementation in divided doses and calcitriol 0.25 mcg once daily which normalized her phosphate and 1,25 vitamin D levels. 1 month later after treatment, she reported significant improvements in bone pain, and her DEXA scans were stable for the following 4 years. Discussion: XLHR is a heterogeneous group of inherited disorders characterized by hypophosphatemia and impaired bone mineralization leading to rickets. It results from mutations affecting the PHEX gene of which more than 300 pathogenic variants have been described. The mutation causes excess FGF-23 which leads to osteomalacia and chronic hypophosphatemia. This condition can be difficult to distinguish from TIO as both present with low phosphate and elevated FGF-23 but can be differentiated with genetic testing. Recognition of the correct diagnosis is prudent to providing correct treatment. The current treatment for XLH is calcitriol and phosphorus replacement. Recently, burosumab was FDA approved in 2018 for treatment in adults.

MON-373 Low Bone Mineral Density Does Not Equal Osteoporosis: The Finding of XLHR with a Novel Phex Mutation