Tubulointerstitial nephritis-induced hypophosphatemic osteomalacia in Sjögren's syndrome: a case report and review of the literature.

Sjögren's syndrome (SS) is a chronic autoimmune inflammatory disease that typically affects the salivary and lacrimal glands. Renal involvement is relatively uncommon and may precede other complaints. Tubulointestitial nephritis (TIN) is the most common renal involvement in SS. Osteomalacia occurring as the first manifestation of renal tubular disorder due to SS is very rare. We report a 39-year-old male who presented with polydipsia, polyuria, and multiple bone pain. Bone density test showed severe osteoporosis, and laboratory findings suggested hypokalemia, hypophosphatemia, and vitamin D deficiency, which supported the diagnosis of hypophosphatemic osteomalacia. He had nephrogenic loss of phosphate and potassium, tubular acidification, and concentration dysfunction. And, the diagnosis of chronic TIN was subsequently confirmed by renal biopsy. The patient reported dry mouth and physical examination showed multiple dental caries. Xerophthalmia, abnormal morphology, and function of the salivary glands by sonography and scintigraphy, together with positive anti-SSA and anti-SSB, confirmed the diagnosis of SS. The TIN indicated SS as the underlying cause of osteomalacia. After taking supplements of potassium, phosphate, vitamin D, and sodium bicarbonate for 1 month, bone pain was alleviated and serological potassium and phosphorus were also back to normal. In conclusion, renal involvement in SS may be latent and precede the typical sicca symptoms. Osteomalacia can be the first manifestation of renal disorder due to SS. Therefore, autoantibody investigations as well as the lacrimal and salivary gland examinations for SS should be considered and performed for suspected patients.