Small Animal Internal Medicine

Research Abstract

EN06 - Diagnosis of Hypothyroidism following Thyroid Stimulating Hormone Stimulation Testing of Radio-Iodine-Treated Hyperthyroid Cats

Thursday, June 14
11:30 AM - 11:45 AM
Location: WSCC 4C-3

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Hyperthyroid cats are commonly treated with radio-iodine (I131) which may result in iatrogenic hypothyroidism. Although the thyroid stimulating hormone (TSH) stimulation test has been used to assess thyroid reserve in several studies of hypothyroid dogs, limited data are available for cats. This study tests the hypothesis that I131 treated cats diagnosed with iatrogenic hypothyroidism using the TSH stimulation test, will have low total-thyroxine (tT4), low free-thyroxine (fT4) and high TSH.


This was a prospective study of 118 client-owned I131 treated cats (> 12 weeks post-treatment). Total thyroxine, fT4, and TSH were measured, and a TSH stimulation test was performed. Previously published criteria from canine hypothyroidism studies and TSH stimulation test responses of 32 mature adult control cats were used to define group cut-off criteria. I131 treated cats were divided into hypothyroid (post-stimulation tT4 ≤ 20nmol/l), euthyroid (post stimulation tT4 ≥ 30nmol/l OR post stimulation tT4 20.0-29.9nmol and pre:post tT4 ratio > 1.5) or intermediate (post stimulation tT4 20.1-29.9nmol/l and tT4 ratio < 1.5 ) groups<./p>

Twenty-five I131-treated cats were diagnosed with hypothyroidism of which 22/25 had high TSH ( ≥ 0.3ng/ml) and 23/25 had low fT4 (< 9pmol/l). Only 6/25 hypothyroid cats had low tT4 (< 10nmol/l) based on the laboratory reference interval (RI); therefore, tT4 below RI is an insensitive test for feline hypothyroidism. Twenty-two cats had intermediate thyroid function. In the euthyroid group 10/71 cats had high TSH, 5/71 had low fT4 and one cat had low tT4. Only 1 euthyroid cat with low fT4 also had high TSH. Measurement of both fT4 and TSH is recommended for diagnosis of iatrogenic feline hypothyroidism.

Jennifer Wakeling, MA, VetMB, CertSAM, PhD, MRCVS

Faculty Instructor
Douglas College, British Columbia, Canada

Dr. Jennifer Wakeling graduated from Cambridge University (U.K.) in 1992 with a 1st class honours degree in Zoology, and from Cambridge University Veterinary School, top of her class for Veterinary Medicine, in 1995. In 1999 she was awarded a Certificate in Small Animal Medicine by the Royal College of Veterinary Surgeons (U.K.). After 8 years in primary care clinical practice Dr. Wakeling returned to academia at the Royal Veterinary College (London) to study for her Ph.D. in feline hyperthyroidism under the tutelage of Dr. Harriet Syme and Prof. Jonathan Elliott which resulted in peer-reviewed publications on subclinical hyperthyroidism, diagnosis of hyperthyroidism, a questionnaire based study of risk factors for the development of hyperthyoidism and a study of urinary iodine excretion in hyperthyroid cats. The Ph.D. entitled "The Aetiopathogenesis of Feline Hyperthyroidism" was successfully defended and awarded in 2008.
Since moving to Vancouver, B.C. (Canada) in 2007 Dr Wakeling has worked as Medical Director of a large small animal hospital (2008-2012) and more recently as a faculty instructor in the Douglas College Veterinary Technology program (2012- present). In 2014 she was awarded an Applied Research and Development grant by the Canadian Natural Sciences and Engineering Research Council (NSERC) for a 3 year study of iatrogenic hypothyroidism in radio-iodine treated cats.

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