Senior cats after radioiodine do well when regular blood tests catch a low thyroid

If your cat had radioactive iodine for an overactive thyroid, you already did the hard part. What comes next is simple: regular blood tests. A large study of senior cats found that most keep normal thyroid function after treatment. The cats whose thyroid slipped too low lived about as long as the others when it was treated.

So the blood test is your tool. It catches a change early, and the study shows what a difference that makes.

How the study worked

Researchers followed 1,344 cats treated with radioiodine for up to 12 years. They checked thyroid and kidney blood values 6 to 12 months after treatment, then again every 6 to 12 months. It was a prospective cohort study, published in September 2026 in the Journal of Veterinary Internal Medicine. Because the researchers watched cats over time rather than assigning treatments, the study shows strong links, and kidney disease, which is common in older cats, plays a part too.

What they found

Most cats did well. At the first check, 77.9% had normal thyroid function, nearly 8 in 10. Another 17.9% had a mildly low thyroid, and 4.2% had a clearly low one.

Normal stayed normal for most. Among cats that began with normal function, 89.2% were still normal at later checks, about 9 in 10. A few developed a low thyroid later, and a few had the overactive thyroid come back, which is why the rechecks matter.

The mildly low group had good news too. Among those cats, 42.5% went back to normal on their own.

The typical cat lived a long time after treatment. Median survival was 49.7 months for cats with normal function, 44.6 months for mildly low and 36.1 months for clearly low. Age, sex and kidney health explained more of the differences than the first thyroid result did. Older age, male sex and a higher creatinine level, a blood marker of kidney function, were each linked to shorter survival.

Here is the part that gives you something to act on. Cats with a persistent low thyroid that was not treated tended to live shorter lives. Cats with a low thyroid that were given levothyroxine, a thyroid hormone replacement, did not show that pattern. They lived about as long as cats that never had a low thyroid.

What is a low thyroid, and why check for it?

The thyroid is a small gland in the neck that sets the body’s pace. Radioiodine calms an overactive one, and in some cats it calms it a little too much. That is called hypothyroidism.

It comes in two forms. The mild form, called subclinical, often fixes itself, as the numbers above show. The clearly low form, called overt, usually sticks around. That is the one a daily levothyroxine pill is meant for.

Thyroid function can keep changing for years after radioiodine. The kidneys matter too, since low thyroid appears to make kidney trouble worse. Both show up on a routine blood panel, so one visit tells you a lot.

What you can do today

Pull out your phone and set a reminder to book your cat’s next thyroid and kidney blood test. If your cat had radioiodine, the study’s rhythm of every 6 to 12 months is a good one to ask your vet about. Call your vet’s office when the reminder goes off, and ask for the next open slot.