a metabolic disorder affecting kidney transport, characterized by the failure of the kidney tubules to reabsorb water, phosphate, potassium, glucose, amino acids, and other substances. When the disorder is accompanied by cystinosis (q.v.), a deposition of cystine crystals, it is called Fanconi's syndrome; there is some variation, however, in the designation of these symptoms.

The de Toni–Fanconi syndrome results from injury to kidney tubular function, the primary injury being either hereditary or acquired (e.g., by repeated exposure to toxins). Treatment attempts to eliminate the primary cause; phosphate and potassium supplements and adequate fluid intake also can help offset the disturbance in kidney transport, which may otherwise give rise to a softening of the bones, muscle weakness, and dehydration.