Pseudo-(tumor-induced) rickets

An athletic 8-year-old boy developed severe muscle weakness over 2 years. A t the age of 10 years, investigation for possible neuromuscular disease dis closed hypophosphatemia (1.8 mg/dl) and rickets. There was selective renal tubular wasting of inorganic phosphate (Pi) but no history of toxin exposur e, familial bone or kidney disease, or biochemical evidence of vitamin D de ficiency. Urine amino acid quantitation was unremarkable. Serum 1,25-dihydr oxyvitamin D [1,25(OH)(2)D] concentration was in the lower half of the refe rence range. Our presumptive diagnosis was tumor-induced rickets; however, physical examination and bone scanning in search of a neoplasm were unrevea ling. Soon after 1,25(OH)(2)D-3 and Pi treatment began, muscle strength imp roved considerably. After 6 months of therapy, radiographic abnormalities w ere substantially better. During the next 6 years, physical examinations, a second bone scan, whole-body and nasal sinus magnetic resonance imaging, a nd octreotide scintigraphy were unremarkable. When his physes fused at the age of 16 years, assessment of his course showed excellent control of his r ickets requiring decreasing doses of medication. Furthermore, fasting serum Pi levels and tubular maximum phosphorus/glomerular filtration (TmP/GFR) v alues had increased steadily and normalized after 3 years of treatment. Acc ordingly, therapy was stopped. Seven months after stopping medication, he c ontinues to feel completely well. Fasting serum Pi levels, TmP/GFR, other b iochemical parameters of bone and mineral homeostasis, creatinine clearance , and renal sonography are normal. Neither spontaneous or pharmacologic cur e of tumor-induced rickets or osteomalacia nor a patient matching ours has been reported. His disorder, which we call pseudo-(tumor-induced) rickets, should be considered when investigation for oncogenic rickets or osteomalac ia discloses no causal lesion. Consequently, prolonged medical therapy and futile searches for a neoplasm may be avoided.