Content continues after advertisement

Failure of Radioiodine Therapy in Cats with Hyperthyroidism

Alex Gallagher, DVM, MS, DACVIM (SAIM), University of Florida

Sign in to Print/View PDF

In the literature

Mullowney D, Chang Y, Glanemann B, Syme HM. Treatment failure in hyperthyroid cats after radioiodine (I-131) injection. J Vet Intern Med. 2021;35(4):1688-1696.


Hyperthyroidism is common in cats, and various treatment options are available, including medication, diet, surgery, and radioactive iodine (RAI; ie, I131) therapy. RAI therapy is considered the treatment of choice because it is minimally invasive and typically achieves a permanent euthyroid state. RAI therapy has resolved hyperthyroidism in ≈95% of cats after a single treatment.1

This study evaluated the outcomes of cats with persistent hyperthyroidism 2 to 4 weeks after receiving RAI therapy and assessed for predictors of ultimate treatment failure. Out of 959 treated cats, 121 continued to have hyperthyroidism (total thyroxine [tT4] concentration, >3.1 µg/dL) 2 to 4 weeks after treatment; initial failure rate was 12.6%. Of these 121 cats, 87 had sufficient follow-up information, with 35 subsequently having tT4 <3.1 µg/dL without further treatment. Most of the 35 cats achieved tT4 concentration <3.1 µg/dL within 6 months (median, 8 months; range, 3-13 months).

Fifty-two cats continued to have hyperthyroidism after RAI therapy, and treatment was considered a failure; this failure rate (ie, 5.6%) is similar to previously reported rates.2-5 Twelve of the 52 cats subsequently underwent thyroidectomy, of which only 4 showed resolution of hyperthyroidism. Of these 12 cats, 5 had thyroid carcinoma, 5 had adenoma, and 2 did not have histopathology.

Repeat RAI therapy was performed in 14 of the 52 cats in which treatment had failed, including one cat with failed thyroidectomy. Ten of the 14 cats had tT4 concentration <3.1 µg/dL after repeat RAI therapy, and 2 cats later became euthyroid without further treatment, suggesting repeat RAI therapy can be successful in cats after initial treatment failure.

Higher tT4 concentration at time of discharge and a higher weight-normalized RAI dose (ie, RAI dose divided by body weight) were independently predictive of treatment failure. tT4 concentration >11.6 µg/dL had a 100% specificity for predicting failure.


Key pearls to put into practice:


RAI therapy successfully treats feline hyperthyroidism in ≈95% of cases and is generally considered the treatment of choice.



Approximately 40% of cats that remain hyperthyroid after an initial dose of RAI may have tT4 concentration <3.1 µg/dL in a median of 8 months and not require further therapy. Cats may need to be monitored for ≥6 months after RAI therapy before repeat therapy is considered.


Treatment failure is likely in cats with posttreatment tT4 concentration >11.6 µg/dL, and repeat RAI therapy should be considered earlier than 6 months.


For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

All Clinician's Brief content is reviewed for accuracy at the time of publication. Previously published content may not reflect recent developments in research and practice.

Material from Clinician's Brief may not be reproduced, distributed, or used in whole or in part without prior permission of Educational Concepts, LLC. For questions or inquiries please contact us.


Clinician's Brief:
The Podcast
Listen as host Alyssa Watson, DVM, talks with the authors of your favorite Clinician’s Brief articles. Dig deeper and explore the conversations behind the content here.
Clinician's Brief provides relevant diagnostic and treatment information for small animal practitioners. It has been ranked the #1 most essential publication by small animal veterinarians for 9 years.*

*2007-2017 PERQ and Essential Media Studies

© 2023 Educational Concepts, L.L.C. dba Brief Media ™ All Rights Reserved. Terms & Conditions | DMCA Copyright | Privacy Policy | Acceptable Use Policy