Hi, Thanks for writing in. The kidney consists of renal parenchyma and renal pelvis. Renal parenchyma can be divided into renal medulla and renal cortex. Bilateral renal parenchymal disease means that the renal impairments are in the medullar and cortical areas affecting both the two kidneys. Thinning of renal parenchyma is a condition in which the cortex is thinned out and this can can be due to many reasons. The possibilities are glomerular diseases and chronic interstitial nephritis. Persistent high blood pressure or bilateral kidney stones can also cause this. Evaluation of kidney functions in detail is required and he must take guidance from a nephrologist.