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Treatment of feline hyperthyroidism

There are currently three alternatives to treating hyperthyroidism: Radioactive iodine (I131) therapy, medication (pills or transdermal ear gel), or surgery.


Symptoms of hyperthyroidism can vary, but may include vomiting, fast heartbeat, excessive fur licking/pulling, ravenous appetite, weight loss, yowling, excessive drinking, or excessive urination. In some cases, there may be no overt symptoms and diagnosis is made as part of a wellness exam.

When hyperthyroidism is present, the cat’s metabolism is in overdrive and burning calories at a fast rate. Left untreated, hyperthyroidism can cause heart failure.


Unfortunately, hyperthyroidism is often not diagnosed until later stages – when the cat is very thin and has a poor coat.

Hyperthyroidism is often diagnosed with a blood test. A “total T4” or simply “T4” can be diagnostic. Care must be taken in evaluating T4 levels, as the reference range used by labs include young healthy cats. For cats over 10 years of age, T4 levels should be in the lower half of the range, and they should decrease over time. T4 levels in the upper half of the range, or increasing over time should be regarded with suspicion. The “free T4” blood test can also be useful in diagnosis.

In some cases, total T4 and free T4 are in normal range or inconclusive. A technetium or scintigraphic scan by a specialist can provide absolute diagnosis.


There are three options for treatment. Radioactive iodine (I131) therapy involves a one-time injection or pill given by a specialist. Quarantine is required for five to seven days. Then the cat returns home and the caregiver follows a few safety protocols for a few weeks. This treatment is generally pricey, but basically a one-time expense. Cost can vary widely by center. A technetium (or scintigraphic) scan ensures the most accuracy in dosing, but is not available at all centers.

Another option, methimazole (brand name Tapazole), is a pill or transdermal gel applied to the ear once or twice daily. It can cause stomach upset. Sometimes starting with a lower dose and gradually increasing it can alleviate the nausea. In rarer cases, more serious side effects can occur. Medication requires frequent bloodwork to check the dose. In the long run, it may be more expensive than radioactive iodine.

The third option is surgery. Surgery can result in over or under-treatment, which necessitates a second surgery or medication to correct the condition.


Cats diagnosed with hyperthyroidism can have great quantity and quality of life if treated. Blood tests should be performed routinely on older cats, and scrutinized not only in relation to reference ranges but also trends from prior bloodwork. Technitium scans should be considered for any cat showing symptoms to ensure proper diagnosis.

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Case Studies

Omaha's T4 level was 3.7. While this was within the lab's reference range of 1 to 4 and Omaha didn't exhibit any overt symptoms of hyperthyroidism, it was suspiciously high in a cat older than ten years. Older cats generally have T4 levels in the lower end of the range, and the level should decrease over time. A technitium scan done at a local specialty center confirmed Omaha was indeed hyperthyroid, and he was successfully treated by radioactive iodine therapy. His T4 level, retested a few months after therapy, was 1.4.

Latifah's T4 level was 2.2, well within the lab's reference range of 1 to 4. However, she exhibited many of the symptoms of hyperthyroidism: excessive hunger, weight loss, and poor coat condition. A technitium scan done at a local specialty center confirmed Latifah was indeed hyperthyroid, and she was successfully treated by radioactive iodine therapy. Her T4 level, retested after therapy, was 1 and later 1.8.

For futher information, visit:

Feline Hyperthyroidism by Dr. Lisa Pierson, DVM

Is Hill's y/d a Nutritious Diet for Hyperthyroid Cats? Insights into Veterinary Endocrinology by Dr. Mark E. Peterson

Hyperthyroidism Information Center by Mar Vista Animal Medical Center: This site is a wealth of information, including diagnosis and treatment

Hyperthyroidism in Cats by Max's House

Hyperthyroidism: Cats can Live a Normal Life DVM news, September 2001

Feline Hyperthyroidism WSAVA 2001, David Bruyette

Hyperthyroidism in Cats by Feline Advisory Bureau

Multivariate analysis of risk factors for feline hyperthyroidism in New Zealand New Zealand Vet Journal, 2005: Several factors analyzed as potential contributors to hyper-thyroidism. Though pop-top cans are often cited as a causal factor, this study shows several other factors had higher correlations, which calls into question any determination that pop-top cans are at fault.

Feline Hyperthyroidism and its Relation with Renal Function WSAVA 2006 by Sylvie Daminet, DVM, PhD, DACVIM, DECVIM-CA

Use and Misuse of FT4ED in Diagnosing Feline Hyperthyroidism by Antech Diagnostics, October 2001

Feline hyperthyroidism: Weigh the pros and cons of medical, surgical management DVM news, February 2006

Hyperthyroidism by All About Cats Health & Wellness Center

Thyroid Basics by Newman Veterinary

Feline Hyperthyroidism Gulf Coast Veterinary Specialists

Feline Hypertension: Risks, Diagnosis and Management Clarke E. Atkins, DVM, DACVIM (Internal Medicine & Cardiology)

Feline Hyperthyroidism

Influence of Systemic Nonthyroidal Illness on Serum Concentration of Thyroxine in Hyperthyroid Cats

Thyroid Hormone Levels in Cats: Colony Average and the Decrease with Age

Subclinical Hyperthyroidism in Cats: A Spontaneous Model of Subclinical Toxic Nodular Goiter in Humans?

Thyrotropin-releasing hormone stimulation test to assess thyroid function in severely sick cats.

Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease

Feline Hyperthyroidism In Cats With Normal T4 Levels: The Importance Of Scintigraphy

Survival times for cats with hyperthyroidism treated with iodine 131, methimazole, or both: 167 cases (1996-2003).

Scintigraphy for Feline Hyperthyroidism - Winn Feline Foundation

Scintigraphic findings in 120 hyperthyroid cats - J Feline Med Surg. 2009 Feb;11(2):96-106. Epub 2008 Sep 10

Date last updated: 2/2/2013

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