Radioactive Iodine Therapy (I-131)

Middle-aged to older felines often experience hyperthyroidism, a disease involving excessive thyroid hormone in the body, usually caused by a non-cancerous tumor called a thyroid adenoma. 

Hyperthyroidism is diagnosed by combining history, physical examination, and laboratory testing. Several treatments may be used for this disease, including daily medication (for the rest of the cat’s life), surgery, and our preferred treatment, radioiodine (I-131) therapy. 

The I-131 treatment center is staffed by Avets clinicians and caregivers, allowing us to offer local follow-up. Because we are a 24-hour emergency and specialty veterinary hospital, our veterinarians and staff are always in attendance to your pets, including monitoring the video feed from the I-131 ward. 

  • Radioiodine (I-131) is considered the treatment of choice—it’s cost-effective when weighed against the long-term cost of medical therapy and is typically successful. 
  • A complete blood count, chemistry panel, total T4, and urinalysis must be performed within a month prior to treatment. 
  • A thorough pre-treatment consultation is performed to confirm the cat is a good candidate prior to every treatment. Methimazole is usually discontinued 1-2 weeks prior to the treatment to optimize the results of radioactive iodine treatment. 
  • An injection of radioactive iodine is given under the cat’s skin, absorbed into the body, and concentrated in the thyroid gland. The cat remains in a specially designed ward in the hospital until the radiation level is low enough to go home (usually 1 week). All treatments are administered by Avets’ board-certified internal medicine specialist. The pet is discharged to the owner with instructions on post-treatment care in the home to avoid excessive exposure and ensure adequate monitoring for the patient. Additional precautions are then taken for a few weeks to limit radiation exposure from the cat. 
  • Most (>95%) cats are cured of the disease with one treatment, a few need a second treatment and a very small number have a malignant tumor that will require surgery or referral for higher doses of I-131. Additional complications are usually mild and self-limiting, such as signs attributable to a sore throat. 

Post-treatment information for pet owners: 

  • For two weeks after discharge, the pet should be confined to the home. During this time, interactions with the pet should be minimized. The pet should not be held, allowed to sit on or next to any individual, or sleep on a bed with anyone. Ideally, that the pet should be confined to an unoccupied room during this time. 
  • Anyone who is pregnant or under the age of 18 should not directly interact with the pet and exposure to other animals in the household should be limited. The pet should not be permitted to walk on countertops or eat off plates. All bedding, toys, or potentially contaminated materials should be isolated for three months or washed thoroughly. 
  • A litter box containing dust-free litter should be provided in an area where children and other pets cannot be exposed. Soiled litter will need to be discarded in a sanitary sewage system (toilet) daily or dirty litter should be double-bagged and stored in a low traffic area for a minimum of 3 months before being discarded. Plastic liners should be used to prevent the accumulation of radioactivity in the litter box. 
  • If the pet needs veterinary care during the restriction period, it should be provided by Avets to regulate the potential radiation exposure to caregivers. Repeat lab work will be needed at 1, 3, and 6 months after treatment. This can be performed by the primary care veterinarian.

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