A 47-year-old female patient presented progressively worsening pain in the chest wall, back, and bilateral lower extremities as well as muscle weakness.

She did not have a family history of bone disease or fractures.

Physical examination was unremarkable excluding bone and muscular abnormalities.

Bone scintigraphy using technetium-99m methylenediphosphate showed increased uptake in the shoulder, multiple ribs, thoracic and lumbar spines, bilateral sacroiliac joints, left ilium, and left foot (Fig.1).

Plain radiographs revealed osteoporosis change of thoracic and lumbar spine as well as compression change of several spines, consistent with findings of magnetic resonance imaging and computed tomography (CT) (Fig.1).

Decreased bone mass was presented in the lumbar spine (T-score: −3.4), femoral neck (T-score: −3.1), and hip (T-score: −3.5).

Main laboratory data are shown in Table ​1.

She demonstrated hypophosphatemia, hypokalemia, hypouricemia, elevated level of ALP, C-telopeptides and chloride, and decreased carbon dioxide combining power.

The urinalysis revealed a high pH value, increased level of potassium, and decreased level of specific gravity, chlorine, and phosphate.

Persistent glycosuria and proteinuria were repeatedly found, despite normal HbA1c and plasma glucose level.

Other laboratory test results including thyroid function test, serum parathyroid hormone, 25-hydroxyvitamin D, and protein electrophoresis were within normal range.

In addition, all of serum tumor markers were negative.

Due to limited technique, we cannot determine the level of serum fibroblast growth factor 23.

With the clinical diagnosis of HO, fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) and technetium-99m octreotide (99mTc-OCT) scintigraphy were performed to confirm whether the occult causative tumor exist.

However, the results of these 2 tests were negative except that mild uptake in the seventh rib was found on PET/CT, which identified no evidence of a neoplastic lesion potentially responsible for HO (Fig.2).

The immunological examination showed elevated level of serum IgG, IgM, and IgA, as well as positive antinuclear antibody, anti-SSA antibody, and rheumatic factor.

Subsequently, Schirmer test was abnormal and lip biopsy supported the diagnosis of SS (Fig.3).

Eventually, this patient was diagnosed with HO secondary to SS, and she was then treated with alkalinization (citrate 4 g/day and potassium citrate 3 g/day for 2 weeks), steroids (prednisone 20 mg/day for 1 month, 10 mg/day for 4 months), neutral phosphate (1.0 g/day for 5 months), calcium supplements (600 mg/day for 5 months), and together with activated vitamin D (0.5 g/day for 5 months).

So far, she recovered uneventfully with relieved pain and increased serum phosphorus level.

This case report was approved by the ethics committee of West China Hospital of Sichuan University, Chengdu, China, and the written informed consent was obtained.