Radioiodine (I-131)
What is Hyperthyroidism?
Hyperthyroidism occurs when the thyroid gland, which regulates metabolic rate, over-produces thyroid hormones. It’s the most common endocrine disease in cats older than 8 years of age. In about 99% of cases, hyperthyroidism is caused by a noncancerous tumor of the thyroid gland.
The over-production of thyroid hormone over-stimulates many organ systems, causing such symptoms as:
- Behavioral changes
- Decreased or excessive appetite
- Hyperactivity or lethargy
- Weight loss
- Vomiting
- Rapid heartbeat
- Increased water consumption and litter box output
- Shedding of fur
- Diarrhea
Left untreated, hyperthyroidism can be fatal, so prompt diagnosis and treatment are critical.
How Does Radioiodine (I-131) Work?
Radioactive iodine is given via a single injection under your cat’s skin and is absorbed into your cat’s bloodstream. Because the thyroid gland uses iodine for hormone production, it promptly takes up the radioactive iodine which destroys hormone-producing thyroid tumor cells.
This treatment is safe and effective with cure rates of better than 94% with just one treatment (cats can have a second treatment, if necessary). Most cats treated with I-131 will have normal hormone levels within 4 weeks.
Because it takes a few days for the radioactivity to dissipate, by law your cat must remain hospitalized for three days after treatment. We have a specially designed room where your pet will stay; it has large windows so you can see your pet from outside (and he/she can see you) when you visit.
Are There Other Treatments for Hyperthyroidism in Cats?
Surgery and anti-thyroid drugs are treatment options. However, neither one cures hyperthyroidism, and each has certain risks:
- Surgery: Many hyperthyroid cats have heart problems so are at higher risk of anesthesia complications during surgery. Further, 80% of cats will develop another tumor within 1½ years of surgery because it’s often difficult to surgically remove the entire thyroid tumor. There also is an increased risk of hypocalcemia after surgery and many cats still require post-surgical I-131 therapy.
- Medication: Daily oral medication typically causes side effects such as nausea, vomiting, lethargy, hair loss, or pruritus (itchiness). Regular veterinary visits to monitor thyroid hormone levels are required for the rest of your cat’s life. Plus, the medication can cause lowered white blood cells and blood-clotting abilities as well as liver and kidney damage.
The Advantages of Radioiodine (I-131)
In contrast, I-131 offers significant advantages:
- No daily pills
- More cost-effective
- No anesthesia required
- No long-term monitoring
- Destroys thyroid tumors regardless of location
- Thyroid function returns to normal, usually within 4 weeks
The Disdvantages of Radioiodine (I-131)
- Not reversible
- In some cases, it can unmask underlying renal disease
To Refer a Patient or Schedule a Consultation with a Radioiodine (I-131) Specialist:
Pet Parents
Please contact internalmedicine@northstarvets.com to schedule your appointment and complete pre-consultation paperwork.
Referring Veterinarians
Please call 609-259-8300 or contact internalmedicine@northstarvets.com with questions about in-hospital cases, or to discuss a pending case.
