Up to 80% of filtered salt and water is returned back into the circulation in the proximal tubule. Several solutes, such as phosphate, glucose, low-molecular weight proteins, and amino acids are exclusively reabsorbed in this segment, so their appearance in urine is a sign of proximal tubular dysfunction. An entire orchestra of specialized apical and basolateral transporters, as well as paracellular molecules, mediate this reabsorption. Defects in proximal tubular function can be isolated (e.g. isolated renal glycosuria, aminoacidurias, or hypophosphataemic rickets) or generalized. In the latter case it is called the Fanconi–Debre–de Toni syndrome, based on the initial clinical descriptions. However, in clinical practice it is usually referred to as just the ‘renal Fanconi syndrome’. Severity of proximal tubular dysfunction can vary, and may coexist with some degree of loss of glomerular filtration capacity. Causes include a wide range of insults to proximal tubular cells, including a number of genetic conditions, drugs and poisons.
Renal proximal tubular dysfunction in Crohn's disease.