Small Animal Internal Medicine

Scientific Session

RRS03 - Individualized Radioiodine Treatment of Cats with Hyperthyroidism: Dosing Algorithm to Reduce Hypothyroidism without Reducing Success

Friday, June 15
9:00 AM - 9:25 AM
Location: WSCC 4C-3

Radioiodine (131I) is the treatment of choice for feline hyperthyroidism, but the optimal method for determining the radioiodine dose is controversial.  Standardized, fixed-dose (e.g., 4 mCi) or variable-dose (e.g., 3-5 mCi) protocols are used most often by clinicians, because these result in resolution of hyperthyroidism in most cats.  However, these “higher-dose” protocols have little regard for development of iatrogenic hypothyroidism.  With these protocols, up to 70% of cats that present with mild to moderate hyperthyroidism subsequently develop overt or subclinical iatrogenic hypothyroidism (J Vet Intern Med 2017;31:326-334).  The consequences of iatrogenic hypothyroidism manifest as worsening (or unmasked) kidney disease, more rapid progression of kidney disease, and shortened survival time in cats with CKD.  Therefore, individualized protocols that determine the “lowest possible radioiodine dose” to resolve the hyperthyroidism without inducing iatrogenic hypothyroidism should provide better outcomes for client and cat.

We sought to develop an objective method of determining the 131I dose for each cat individually, that would result in resolution of hyperthyroidism, while minimizing the risk of iatrogenic hypothyroidism, based on results of serum thyroid hormone concentrations, quantitative thyroid scintigraphy, and thyroid uptake of the administered radioiodine.

This prospective study included 450 cats (median age, 12 years) referred to the Animal Endocrine Clinic for treatment with 131I between September 2015 and September 2017. In cats treated with methimazole, the methimazole was discontinued ≥ 1 week prior to evaluation and treatment. Each cat had baseline serum concentrations of T4, T3, and TSH (J Vet Intern Med 2017;31:326-34) determined. Quantitative thyroid scintigraphy was performed with sodium 99m-Tc-pertechnetate (99mTcO-4), with subsequent calculation of each cat’s thyroid volume and percent thyroidal uptake of the 99mTcO-4 (TcTU), as previously described (Vet Radiol Ultrasound 2016;57:427-40). Each cat then received an initial low radioiodine dose (1.0-1.5 mCi, subcutaneously); 24 hours later, we measured the radioactivity over the thyroid tumor (external counting with a Ludlum survey meter) to determine the percent uptake of the administered radioiodine dose by the thyroid tumor(s).

We developed a novel algorithm that based the final 131I dose on the following parameters: 1) serum T4 and T3 concentrations, 2) TcTU; and 3) calculated thyroid volume; and 4) the 24-hour radioiodine uptake. With this algorithm, the final dose was also adjusted to deliver a radiation dose of 200 µCi of 131I /gm of thyroid tumor tissue. If needed, an additional radioiodine dose was administered on day 2 in order to provide enough radioiodine to meet the final calculated dose.

After 131I treatment, recheck examinations were performed at 1, 3, and 6 months, with follow-up serum concentrations of T4, TSH, and creatinine determined at each visit. For analysis, cats were classified into 1 of 4 thyroid categories: euthyroid (T4, 0.9–3.9 µg/dL; TSH ≤ 0.30 ng/mL), overt hypothyroid (T4≤ 0.8 µg/dL; TSH > 0.30 ng/mL), subclinically hypothyroid (T4, 0.9–3.9 µg/dL; TSH > 0.30 ng/mL), and persistently hyperthyroid (T4 ≥ 4.0 µg/dL; TSH < 0.03 ng/mL).

The median calculated 131I  dose administered was 1.9 mCi (interquartile range [IQR], 1.7-2.15 mCi; range, 1.3 to 10.0 mCi). Only 40 of the 450 cats (8.9%) received a radioiodine dose ≥ 3 mCi. Of the 450 treated cats, 339 (75.3%) cats became euthyroid, 14 (3.1%) cats developed overt hypothyroidism, 76 (16.9%) cats became subclinically hypothyroid, and 21 (4.7%) cats remained hyperthyroid. The prevalence of post-treatment azotemia was significantly higher (P < 0.0001) in the cats with overt (50%) and subclinical (36.5%) hypothyroidism than in the cats that remained euthyroid (11.5%) after radioiodine treatment.

In conclusion, our novel algorithm for calculating individual 131I doses based on serum thyroid hormone concentrations, thyroid scintigraphy (thyroid tumor volume and percent TcTU), and radioiodine uptake by the thyroid tumor resulted in “cure rates” similar to historical rates, but dramatically reduced the incidence of overt and subclinical hypothyroidism. Importantly, the lower calculated radioiodine doses (compared to those currently administered by most other protocols), limit radiation exposure to veterinary staff and owners. Finally, by lowering the incidence of iatrogenic hypothyroidism, this individualized approach also lowers the rate of azotemia that develops after radioiodine treatment.

Learning Objectives:

Mark E. Peterson, DVM, Dip ACVIM

Director
Animal Endocrine Clinic

BIOGRAPHY
Mark E. Peterson, DVM, Dipl ACVIM

Dr. Mark E. Peterson is well known for his clinical research and teaching in small animal endocrinology. Mark is the owner and director of the Animal Endocrine Clinic, a specialty referral hospital now devoted primarily to the diagnosis and treatment of cats with thyroid disease, where he conducts most of his clinic research. He also serves as adjunct Professor of Medicine at the New York State College of Veterinary Medicine, Cornell University, where he teaches and collaborates with investigators.
Over the last 40 years, most of Dr. Peterson's clinical and research efforts has been directed toward advancing our understanding of naturally-occurring endocrine disorders of the cat, especially hyperthyroidism, hypothyroidism, diabetes mellitus, calcium disorders, and adrenal disease. He has published more than 500 journal articles, book chapters, and research abstracts. With more than 500 lecture presentations to his credit, Dr. Peterson is a frequent speaker at veterinary colleges and scientific seminars around the world.

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