What is feline hyperthyroidism?

Feline hyperthyroidism is a common endocrine disorder in middle to older-aged cats (typically over 10 years). It’s caused by increased production of thyroid hormones (T3 and T4) due to abnormal thyroid glands. Most cases stem from benign tumors (adenomas), often affecting both thyroid lobes, though around 30% are unilateral. Rarely, about 2-5% of cases result from malignant tumors known as thyroid adenocarcinomas.

Why do cats become hyperthyroid?

Hyperthyroidism in most cats occurs when a part of the thyroid gland becomes overactive and secretes excessive thyroid hormone. Occasionally, it’s due to thyroid cancer. Identifying the cause is essential, as it affects treatment options and long-term expectations for your cat’s health. We assess this during your cat’s evaluation before treatment.

What are the signs of feline hyperthyroidism?

Signs include weight loss, increased appetite, higher water intake, and frequent urination. Gastrointestinal symptoms like vomiting and diarrhea are common, as are behavioral changes such as hyperactivity, restlessness, and aggression. Hyperthyroidism can also cause heart problems (thyrotoxic hypertrophic cardiomyopathy) and hypertension, potentially harming organs like the eyes, kidneys, heart, and brain.

How is feline hyperthyroidism diagnosed?

Diagnosis generally includes reviewing your cat’s medical history, clinical signs, physical exam findings (e.g., thyroid enlargement or rapid heartbeat), and elevated thyroxine (T4) levels. If T4 levels are inconclusive, additional tests like free T4 by serum dialysis or thyroid scintigraphy may be recommended. Blood chemistry panels and urinalysis help assess kidney and heart health, which hyperthyroidism can impact.

What are the treatment options for feline hyperthyroidism?

Treatment options include:

• Antithyroid medications like Methimazole

• Surgery to remove abnormal thyroid tissue

• Dietary management to control thyroid hormone levels

• I-131 radioactive iodine injection

What is I-131 for cats?

Iodine-131 (I-131), or radioiodine, is a radioactive form of iodine widely considered the gold-standard treatment for feline hyperthyroidism. I-131 targets and destroys thyroid tumor tissue, almost always curing the condition. It’s non-invasive, painless, permanent, and has very few side effects, making it an ideal choice for many cats.

How does I-131 feline treatment work?

I-131 is absorbed into the bloodstream and taken up by thyroid tumor cells, sparing healthy tissue. The radiation then destroys the tumor cells. Because thyroid tumors act as “magnets” for I-131, this treatment also targets tumors in the chest and other areas that may be hard to detect. Cats undergoing this treatment emit radiation temporarily, so isolation from people and other pets is required during this period.

How is I-131 administered to cats?

I-131 is given as a single subcutaneous injection, similar to a vaccine, and is highly effective, with a 95-98% success rate. In about 2-5% of cases, a second injection may be necessary 3-6 months after the first.

Are there side effects of I-131 treatment?

Side effects are rare. In about 2% of cases, cats may develop mild hypothyroidism, often symptomless and managed with hormone supplements if needed. Although unlikely, some cats may not respond to treatment, usually due to undetected thyroid cancer, which would require additional treatment.

Does hyperthyroidism affect kidney health?

Hyperthyroidism can mask kidney disease, so a methimazole trial may be recommended if your cat has elevated kidney markers (BUN, creatinine, or SOMA) or a low urine specific gravity. This helps assess kidney health before proceeding with I-131.

Which cats are eligible for I-131 treatment?

Cats with hyperthyroidism who are in stable medical condition are good candidates for I-131 treatment. During the isolation period, only essential care is provided to minimize radiation exposure to personnel. Cats that require daily medical support are not eligible for this treatment.

How long does it take for I-131 to work?

The treatment works over time, with most cats returning to normal thyroid levels within a few weeks. In some cases, thyroid function may take up to six months to fully normalize.

Will my cat be radioactive after returning home?

Yes, your cat will emit minimal radiation upon discharge, which is deemed safe by the State of Texas Department of State Health Services. However, to keep exposure as low as possible, limit close contact (like sleeping on your bed or sitting on your lap) for the first two weeks. It’s also recommended to avoid prolonged exposure to children and pregnant women, though your cat can interact with other pets.

What should I expect for my cat's I-131 treatment at your facility?

Your cat will receive the injection on Monday and stay in an isolation ward until Wednesday. We may shave a small area between the shoulders for the injection site. For comfort, please bring your cat’s regular diet and a familiar item with your scent, like a T-shirt or towel. We provide bedding and bowls, but all items brought in will be discarded due to radiation safety protocols.

When should my cat’s thyroid levels be checked after treatment?

Your cat’s thyroid levels should be re-evaluated about two months after treatment to confirm that thyroid function has returned to normal.

Should my cat stop anti-thyroid drugs before radioiodine therapy?

If your cat is stable without Tapazole® (methimazole), discontinue it two weeks before treatment, as these drugs can affect diagnostic scans and lower the radiation dose to the thyroid gland. If your cat requires Tapazole for stability, consult your vet before stopping.

How does the scheduling and appointment process work?

Complete our Contact Form, and our team will contact you within 24-48 hours to review your cat’s history and set up a diagnostic appointment. We’ll image your cat’s thyroid during this visit to confirm suitability for I-131. If your cat qualifies, we’ll schedule the treatment, and your cat will stay in our treatment ward for up to three days, depending on how quickly safe radiation levels are reached.

How long does my cat need to stay at CATT?

Your cat will remain in our treatment ward for up to three days, depending on how quickly radiation levels drop to safe discharge levels. We’ll provide an estimated stay duration and regular progress updates, but a guaranteed discharge time isn’t possible due to variable factors.

Do I need to bring my cat back for follow-up testing?

No, your primary veterinarian can do follow-up blood tests to check thyroid levels.

References:

1. Peterson ME: Hyperthyroidism in Cats, in Rand J (ed): Clinical Endocrinology of Companion Animals, Oxford, England, Wiley-Blackwell, 2013, pp 295-310.

2. Turrel J, Feldman E, Nelson R et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981-1986). J Am Vet Med Assoc. 1988:193(3),359-364.

3. Peterson ME: Radioiodine treatment of hyperthyroidism. Clinical Techniques in Small Animal Practice 21:34-39, 2006.

4. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc . Dec 1995; 207(11):1422-8.

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