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Radioactive Iodine Therapy


Radioactive Iodine, I-131, is a safe and effective, minimally invasive option to treat cats with hyperthyroidism.  Following one subcutaneous injection of I-131, over 98% of affected cats are cured of the disease. If needed, a second injection can be given. There are no harmful side effects.

How Does It Work?

Iodine is normally taken up by the thyroid gland. One form of iodine, I-131, is radioactive. When I-131 enters the thyroid gland it destroys the abnormally functioning cells. This reduces the size of the gland and its ability to produce thyroid hormone.

What Is Hyperthyroidism?
Hyperthyroidism is a clinical condition resulting from the excessive production of thyroid hormones by the thyroid gland. It is more commonly found in middle-age to older cats. In fact, more than 95% of cases occur in cats over 8 years of age. This is usually due to benign changes (hyperplasia or adenomas) in the thyroid gland. Cancer of the thyroid gland can occur in cats, however it is rare.
Symptoms of hyperthyroidism
  • Weight Loss
  • Increased Appetite
  • Vomiting
  • Diarrhea
  • Hypertension
  • Polydipsia – Increased Thirst
  • Polyuria – Increased urination
  • Nervousness
  • Hyperactivity or lethargy
  • Behavioral changes
  • Cardiac Issues:
    • Tachycardia – rapid heartbeat
    • Heart Murmurs
    • Arrhythmias – irregular heartbeat
additional treatment options

Medical treatment consists of giving oral or topical medication two or three times daily for the remainder of the cat’s life. The medication does not cure the disease; it merely controls the release of thyroid hormone. There is an incidence of side effects such as vomiting, diarrhea, poor appetite, and skin lesions. Missed medication will result in relapses.

Surgical removal of the affected thyroid gland(s) will cure the disease. Potential anesthestic risks in older, frail cats may eliminate surgery as a viable option.

Does My Cat Have To Stay In The Hospital?

Yes. Cats treated with radioactive iodine must remain hospitalized for 4-7 days following the injection. This is when they are most radioactive. Their level of radioactivity is checked daily with a Geiger counter (see below). When it reaches an acceptably low level, as determined by the Nuclear Regulatory Commission (NRC), they can safely go home.

Department Doctors

John Lucy, DVM, Diplomate, ACVIM (Internal Medicine)

Dr. Lucy grew up in Ocean County, and developed a fondness for working with animals at an early age. After completing his undergraduate training at Rutgers University, he traveled to upstate New York and received his DVM from Cornell’s world-renowned College of Veterinary Medicine. He completed his rotating internship in medicine and surgery at Oradell Animal Hospital and residency in small animal internal medicine at Cornell. Dr. Lucy's medical interests include endocrinology, hepatology, immune-mediated disorders and minimally invasive diagnostics and therapeutics. Dr. Lucy is the director of our Radioiodine Clinic for the treatment of hyperthyroid cats. He was the 2015 recipient of the Society for Comparative Endocrinology’s early career award and his research on feline hyperthyroidism was awarded the prestigious Oxford Award for clinical endocrine publication of the year in 2018. In his spare time, Dr. Lucy enjoys running, swimming, travel and watching copious amounts of HGTV.


Reena Shah, DVM (I-131)

Dr. Shah graduated from Cornell University College of Veterinary Medicine. She completed an internship in small animal medicine and surgery at Oradell Animal Hospital and since then has remained on staff. Dr. Shah is involved with Oradell Animal Hospital's radioactive iodine program for the treatment of hyperthyroidism in cats. She likes to cook and spend time with her family.
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