Hyperphosphataemic tumoral calcinosis (HTC) is a rare condition associated with chronic hyperphosphataemia leading to abnormal calcium deposits in soft tissue causing deformity, pain and immobility. It is more commonly associated with conditions such as chronic renal disease, and less commonly related to dysregulation of Fibroblast Growth Factor 23 (FGF23), a hormone responsible for phosphate homeostasis. Autosomal recessive HTC is associated with inactivating mutations in FGF23, GALNT3 (enzyme responsible for full FGF23 bioactivity), or KLOTHO (co-receptor for FGF23). There is one reported case in the literature of an auto-immune aetiology; an 8-year-old boy with HTC who subsequently developed Type 1 diabetes prompting an autoimmune screen1. We report the case of a 54-year-old female who presented with diffuse, painful HTC with normal renal function, low FGF-23 and homozygosity around the FGF-23 gene on chromosomal array. The unusual late age of onset led us to consider an auto-immune aetiology: positive chromatin antibodies were detected, and her symptoms and biochemistry responded to prednisolone. Further investigations are in progress including direct FGF23 sequencing and testing for antibodies directed against FGF23. Our case highlights the importance of considering FGF23 deficiency or resistance in the diagnosis of HTC.