Adult fibrous dysplasia/McCune–Albright syndrome with hypoparathyroidism: additional skeletal lesions and an unusual bone turnover profile

Fibrous dysplasia (FD) is a rare, mosaic skeletal disorder characterized by replacement of normal bone with fibro-osseous tissue. Disease activity typically peaks in childhood and diminishes by adulthood. FD is associated with accelerated bone turnover, which generally correlates with the extent and severity of skeletal involvement. We report the case of a 42-year-old woman with mild polyostotic FD and precocious puberty. At the age of 31 years, the patient underwent total thyroidectomy for papillary thyroid cancer complicated by permanent hypoparathyroidism. On follow-up, 18 F- Sodium Fluoride PET/CT (NaF) identified rib lesions that were not evident on prior 18 F-fluorodeoxyglucose PET/CT performed 19 years ago when FD was diagnosed. All newly detected skeletal foci had morphological features consistent with FD on the CT component of the NaF PET/CT. The patient had neither clinical evidence of rib trauma, nor biochemical or structural evidence of recurrent thyroid cancer. In contrast to expected biochemical profile in polyostotic FD and possibly related to suppressed skeletal remodeling associated with hypoparathyroidism, serum osteocalcin and alkaline phosphatase were within normal ranges. The only persistently elevated marker was procollagen type 1 N-terminal propeptide, which remained 3–4 times above the upper limit of normal. Next-generation sequencing, following target enrichment, identified the causal variant Arg201Cys in the GNAS gene in the DNA extracted from the dysplastic right iliac bone. Based on the history of precocious menarche at 8 years of age, the diagnosis was revised to FD/McCune–Albright syndrome (FD/MAS). The case demonstrates additional skeletal lesions identified on interval imaging in an adult patient with mild FD/MAS despite the expected quiescence of the disease. It also highlights an unusual pattern of bone turnover markers, possibly influenced by the combined effects of FD/MAS and permanent hypoparathyroidism.