RIT Large

Feline hyperthyroidism, the most common endocrine disorder of cats, is commonly diagnosed via blood testing. Additional diagnostics are often performed to identify other diseases that may complicate treatment and long-term outcome. A common pretreatment diagnostic test before radioactive iodine treatment (RIT) is abdominal ultrasound (AUS) to detect concurrent intra-abdominal disease. When the medical records of 534 cats referred for RIT for hyperthyroidism were reviewed, 193/534 (36.1%) had abnormal AUS findings, including hepatic lesions (5.1%), renal disease (22.8%), and confirmed neoplasia (2.4%). Comparing cats with renal changes on AUS with those with normal-appearing kidneys, there were significant differences in the median urine specific gravity and blood urea nitrogen concentrations. Only 2.2% of cats with abnormal AUS findings were subsequently not treated with RIT. The authors conclude that AUS is typically unnecessary before RIT in hyperthyroid cats and should be reserved for cats for which there is a high clinical suspicion of neoplasia or other significant disease that would preclude RIT.

The findings of this study do not support the use of AUS as a routine screening tool for cats undergoing RIT. Of the 12 cats (2.2%) not treated with RIT, 10 had confirmed malignancy, 1 had hydronephrosis from ureterolithiasis, and 1 had focal bladder wall thickening. A limitation of this study is population bias; the cases evaluated were referred to a specialty hospital for RIT, thus eliminating those cats not referred for RIT because of concurrent disease found by the primary clinician. The prevalence of concurrent abdominal disease may be higher in cats considered poor referral cases. Screening with AUS prior to RIT should be reserved for cases in which there is a high suspicion of clinically relevant disease.—Jonathan Bach, DVM, DACVIM, DACVECC