Radioactive Iodine (I-131) for Cats: How It Works & Cost

Radioactive iodine, also called I-131 or radioiodine therapy, is a treatment for feline hyperthyroidism. Hyperthyroidism happens when abnormal thyroid tissue makes too much thyroid hormone, which can drive weight loss, a fast heart rate, high blood pressure, vomiting, and behavior changes in older cats. I-131 works because thyroid tissue naturally concentrates iodine. After a small injection, the abnormal thyroid cells absorb the radioactive iodine and are selectively damaged.

This treatment is widely considered a leading curative option because it usually treats the diseased thyroid tissue without surgery and usually without damaging nearby structures like the parathyroid glands. Many cats return to normal thyroid levels within 1-2 weeks, although full assessment of treatment success is usually based on follow-up over the next 1-3 months.

Unlike methimazole or an iodine-restricted prescription diet, I-131 is not daily management. It is usually a single treatment performed at a specially licensed hospital. Because cats are temporarily radioactive afterward, they must stay in the hospital until radiation levels fall low enough for discharge. Your vet or referral center will then give you home safety instructions for litter handling and close contact during the first few weeks.

I-131 itself does not have a "cause." It is a treatment used because a cat has hyperthyroidism. In most cats, hyperthyroidism is caused by benign adenomatous hyperplasia or adenoma of one or both thyroid lobes. These abnormal thyroid cells produce excess hormone and speed up the body’s metabolism.

A much smaller number of cats have thyroid carcinoma, which is malignant thyroid cancer. That matters because some cats with carcinoma may need a different I-131 dose, more advanced imaging, or a broader treatment plan through an internal medicine or oncology service.

Your vet may recommend I-131 when a cat is otherwise a good candidate for a curative approach, especially if daily pills are difficult, medication side effects have occurred, or pet parents want an option other than lifelong diet restriction. The decision is not one-size-fits-all. Kidney values, blood pressure, heart status, temperament, travel distance to a treatment center, and home radiation-safety logistics all affect whether I-131 is the right fit.

Cats are not diagnosed with "needing I-131" from one test alone. First, your vet diagnoses hyperthyroidism. That usually starts with a physical exam, including feeling for an enlarged thyroid gland in the neck, checking heart rate, and measuring blood pressure. Bloodwork commonly includes a total T4 level, chemistry panel, complete blood count, and urinalysis.

Most hyperthyroid cats have an elevated T4, but a small percentage can have a normal T4 early in disease or when another illness is present. If suspicion remains high, your vet may recommend repeat thyroid testing, free T4, T3, TSH-related interpretation, or referral testing. Screening for kidney disease is especially important because hyperthyroidism can mask reduced kidney function.

Before I-131, many hospitals also require recent lab work, blood pressure assessment, and sometimes chest radiographs, ECG, echocardiography, or abdominal ultrasound if there are concerns beyond uncomplicated hyperthyroidism. Some referral centers use thyroid scintigraphy in selected cases, especially if ectopic thyroid tissue or carcinoma is suspected. This workup helps your vet decide whether I-131 is appropriate and what level of monitoring your cat may need afterward.

There is no proven way to fully prevent feline hyperthyroidism, so there is no guaranteed way to prevent the need for I-131. What you can do is improve the chance of catching thyroid disease early, before weight loss, heart strain, or high blood pressure become more serious.

For most senior cats, that means regular wellness visits with your vet, routine bloodwork, urine testing, blood pressure checks, and tracking body weight at home. Hyperthyroidism is common in older cats, and early changes can be subtle. A cat who seems extra hungry, more vocal, or thinner may need testing even if they still act bright.

If your cat has already been diagnosed with hyperthyroidism, prevention shifts toward preventing complications. Give medications exactly as directed if your cat is on methimazole, feed any prescription thyroid diet exclusively if that is the chosen plan, and keep follow-up appointments. These steps do not replace I-131, but they can help stabilize your cat while you and your vet decide which treatment tier fits your cat, your home, and your budget.