Thyroidization (thyroid-like appearance) in renal tissue which is made up of a colloid-like hyaline cast formation of Tamm-Horsfall glycoprotein (THP) is a common finding in chronic pyelonephritis and obstructive nephropathy. This type of pathological change is sometimes observed in renal allograft specimens. We examined allograft specimens for thyroidization and other pathological findings related to thyroidization to characterize the conditions causing such changes. One-hundred three patients who underwent renal transplantation between January 2006 and April 2008 at Gifu University Hospital (251 renal allograft biopsy specimens) were enrolled in this study. Sixteen patients had thyroidization (11 mild, 4 moderate, and 1 severe). In four patients, THP reflux on Bowman's capsule was found, and in three patients interstitial THP deposits were observed. In four patients, tubulointerstitial nephritis was diagnosed. Fifteen of 16 patients were examined for vesicoureteral reflux (VUR) with voiding cystourethrography. Three of 15 patients had VUR. In the past medical histories of the 16 patients with thyroidization, three had low capacity bladders, two had prostate diseases, and six had previous urinary tract infections. In cases of thyroidization with additional findings, including THP reflux into Bowman's space and interstitial THP deposits, we need to examine the patients for the presence of urinary tract diseases. In cases of thyroidization and tubulointerstitial nephritis urinary tract infections were suspected. Such subclinical urological diseases in the grafts might affect the prognosis of renal function. Therefore, appropriate management of urinary tract diseases is required.