Section of kidney showing thyroidization. Many tubules are dilated with hyaline casts giving the kidney a thyroid-like appearance. Patchy areas of collected chronic inflammatory cells are present. A couple of glomeruli are noted in the mid lower field. Urine reflux due to uretero-vesicle incompetence with or without superimposed infection.
Most cases are due to ascending infections from urine flow abnormalities.
Urinary outflow obstruction results in urine stasis providing an environment for bacterial proliferation. Obstruction also predisposes to vesicoureteral reflux which can enable bacteria to reach the kidney. In general, any abnormality to the flow of urine from the calyces to the urethra increases the risk for pyelonephritis. There is irregular loss of renal parenchyma with scarring. There is distortion of the associated calyces which commonly appear dilated because of the reduction in medullary and cortical tissue.