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Looking for concurrent disease in hyperthyroid cats undergoing treatment

Nussbaum LK, Scavelli TD, et al. Abdominal ultrasound examination findings in 534 hyperthyroid cats referred for radioiodine treatment between 2007-2010. J Vet Intern Med. 2015; 29:1069-1073.

Hyperthyroidism is the most commonly identified endocrine disease in cats, affecting primarily middle-aged to older cats. The prevalence of concurrent disease in these cats is unknown. One prior study of hyperthyroid cats indicated 14% had renal disease and 54% had cardiac disease at the time of their initial diagnosis. Another study reported diabetes mellitus occurring in upwards of 5.8% of hyperthyroid cats and an additional study noted the prevalence of urinary tract infections was 12% in hyperthyroid cats. In untreated hyperthyroid cases, the complications most commonly associated are renal disease, heart disease and hypertension.

The authors’ objective in this study was to determine the prevalence of concurrent abdominal disease using abdominal ultrasound examination (AUS) in hyperthyroid cats referred for radioactive iodine treatment (RIT). They also planned to use this information to determine whether the requirement for an abdominal ultrasound examination prior to RIT is justified. At the authors’ institution and other locations, cats undergoing RIT for hyperthyroidism are routinely screened first by AUS. The purpose of AUS is to identify concurrent diseases with clinical signs that overlap with hyperthyroidism and to rule out if any concurrent disease may limit the lifespan of the treated cat.

There were 534 cats meeting the inclusion criteria for this study. Their median age was 12.0 years. The most common clinical sign was weight loss followed in descending order by increased appetite, vomiting, increased thirst and urination, and lost or decreased appetite. At the time of diagnosis, the median total T4 concentration for all the cats in the study was 8.4 mg/dL. The study was a retrospective one where the following data was collected: age, breed, sex, body weight, clinical signs, total T4, serum blood urea nitrogen (BUN) concentration, serum creatinine concentration, urine specific gravity, AUS results, and biopsy or cytology (or both) results.

Of the cats in the study, 12 (2.2%) of the 534 cats were not treated with RIT due to abnormalities found on AUS and cytology or biopsy results. Ten of these cats were determined to have malignant disease confirmed by fine needle aspirate or surgical biopsy. The remaining two cats had underlying urinary tract disease that the owners decided not to pursue treating the underlying disease or to undergo RIT. The prevalence of concurrent intra-abdominal disease in this population of cats referred for RIT based on the abdominal ultrasound examinations was 36.1%. Most of the abnormalities found on abdominal ultrasound were not considered significant to deter RIT. The most commonly identified ultrasound abnormalities in hyperthyroid cats referred for RIT were changes consistent with chronic kidney disease. Lymphoma was the most commonly identified malignancy of cats in the study.

The authors state the low prevalence of abnormal AUS findings in the referred population with hyperthyroidism supports a decision to not screen all hyperthyroid cats with AUS prior to RIT. The results also highlight the challenge of identifying cats with concurrent disease using their clinical presentation alone. The study’s findings support not requiring AUS screening prior to RIT in hyperthyroid cats. Their recommendation currently is to use AUS for cases where the suspicion of clinically relevant and serious disease is high.  (VT)

See also:
van Hoek I, Lefebvre HP, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domest Anim Endocrinol. 2009 Jan;36(1):45-56.