Radioiodine is the only treatment for feline hyperthyroidism that destroys the overactive thyroid tissue instead of working around it, and one injection cures 85 to 95 percent of cats. The trade is time and money: 3 to 7 days in an isolation ward, 1,200 to 2,500 USD, and a 5 to 10 percent chance the cat swings the other way into hypothyroidism. For an owner who cannot pill a cat twice a day for the next 4 years, that is often the cheapest option in real terms.
Why a single injection replaces 730 pills a year
Iodine is a thyroid-specific element. The gland concentrates it, and a hyperthyroid gland concentrates it faster than any other tissue in the body. Radioiodine given by injection is taken up almost exclusively by that tissue, where its short-range radiation destroys the overactive cells from inside while the parathyroid glands 2 cm away receive a fraction of the dose. Healthy thyroid tissue is suppressed at the time of treatment, so it takes up little and survives.
Functional tissue is gone within 2 to 4 weeks. T4 usually lands in the reference range by 4 to 8 weeks, and the cat's appetite, weight, and behaviour follow over the next 2 to 3 months. Nothing is given daily afterwards.
Who is a candidate, and the 3 tests that decide
A confirmed diagnosis first: a total T4 above the reference range plus at least 1 clinical sign, usually weight loss with a good appetite. Cats with a normal T4 and a suspicious nodule need a free T4 or a T3 suppression test before anyone books a treatment slot.
Then kidney function, because normalising thyroid hormone drops the glomerular filtration rate. Between 20 and 30 percent of hyperthyroid cats are masking chronic kidney disease (CKD), and it shows up within 3 months of treatment. A creatinine, urine specific gravity, and blood pressure before therapy tell you whether the cat will still have acceptable kidney function once the thyroid is normal. A cat already at IRIS stage 3 is usually treated with medication and monitored instead.
The third test is temperament and logistics. The cat must be isolated at the clinic for 3 to 7 days and kept at a distance at home for a week after. A cat that panics in a cage, or a household with a pregnant person or a child under 12 who cannot keep 1 m away, is a poor fit.
The 3 to 7 day stay, in order
Day 1 is admission, a physical exam, and the injection. The dose is set by the vet from the size of the gland and the measured iodine uptake, so 2 cats of the same weight can receive different amounts. No sedation is needed for the injection itself.
Days 1 to 3 are radiation decay and observation. Staff handle the cat with gloves and a dosimeter, litter is collected as radioactive waste, and visiting is usually refused or limited to 15 minutes behind a shield. Discharge depends on the measured surface dose rate, not on how the cat looks, which is why 1 cat goes home on day 3 and the next on day 7.
Ask at admission for 3 numbers: the activity administered, the discharge dose rate limit the clinic works to, and the plan if the cat will not eat in the ward. A cat that refuses food for 48 hours in isolation comes home thinner than it arrived.
Home rules for the first 7 days
Residual radioactivity leaves through urine, faeces, saliva, and shed hair, and it is measurable for about 7 days. Keep the cat in one room, use clumping litter, scoop twice a day with gloves, and double-bag the waste. Wash hands after every contact. Sleep in a different room for 7 nights, and keep cuddling to under 30 minutes a day at arm's length.
Pregnant women and children under 12 should not handle the cat or the litter for the full 7 days, and a cat that sleeps on a pillow needs a different bed for that week. These rules come from radiation-safety regulation, and every clinic hands out its own written version. Follow the clinic sheet if it is stricter than this page.
What the 1,200 to 2,500 USD buys
| Item | Typical range (USD) | Notes |
|---|---|---|
| Radioiodine treatment | 1,200 to 2,500 | Includes 3 to 7 days of boarding |
| Pre-treatment panel | 150 to 350 | T4, creatinine, urine, blood pressure |
| T4 at 1 and 3 months | 80 to 150 each | 2 visits, 4 to 12 weeks |
| T4 at 6 months | 80 to 150 | Confirms the final result |
Total first-year cost usually lands between 1,550 and 3,300 USD once the panels are counted. Methimazole costs 20 to 40 USD a month plus 150 to 250 USD a year in bloodwork, so the pill route runs 400 to 700 USD a year and never stops. Radioiodine breaks even at about year 4 and is cheaper after that, which matters more for a cat diagnosed at 9 than for one diagnosed at 16.
The 5 to 10 percent who become hypothyroid
Overshoot is the main complication, and it is manageable. Between 5 and 10 percent of treated cats end up with a T4 below the reference range at 3 or 6 months. Signs are easy to miss in an old cat: 2 or 3 extra hours of sleep, a coat that looks dull, weight gain of 300 to 500 g on the same food, and reluctance to go out in cool weather.
Treatment is a small daily tablet of levothyroxine, with the dose set by the vet and a T4 recheck 4 weeks later. Most cats stabilise on 1 dose. A cat with iatrogenic hypothyroidism and untreated CKD can also show a rising creatinine, so both values are read together at every recheck.
Recheck schedule and the 3 signs to report
T4 at 4 to 6 weeks, again at 3 months, then at 6 months, then once a year with the senior panel. Blood pressure at every visit, because hypertension persists in roughly 1 in 5 cats even after the thyroid is normal, and a systolic above 160 mmHg needs treatment.
Report 3 things early: a cat that stops eating within 72 hours of coming home, a cat that is more lethargic at week 3 than at week 1, and any weight loss that continues past 8 weeks. Continued weight loss after a cured thyroid usually means the kidney disease was unmasked, not that the treatment failed. If medication or surgery is on the table instead, the 3 routes compared sets out the trade-offs, and monitoring methimazole covers the day 14 and 3-week blood draws. Budget the follow-up year with the annual cost estimate before you commit.
Book it in 4 steps
- Confirm the diagnosis with a total T4 and a free T4 if the value is borderline.
- Run the kidney and blood pressure panel, and ask what the plan is if CKD unmasks.
- Reserve the 3 to 7 day slot, and put 1,550 to 3,300 USD aside for year 1.
- Set the calendar: T4 at 4 to 6 weeks, 3 months, 6 months, then yearly.
Terms mentioned in this article
Frequently asked
Does radioiodine always cure hyperthyroidism in one treatment?
Between 85 and 95 percent of cats are cured by a single injection, with T4 normal by 4 to 8 weeks. The rest need a second treatment, usually because the gland was large or uptake was uneven. A second course is uncommon, and clinics usually quote it separately rather than including it in the first price.
How long does the cat stay radioactive after treatment?
Measurable radioactivity lasts about 7 days. Expect 3 to 7 days in the clinic until the surface dose rate falls under the regulatory limit, then 7 days at home in one room, gloves for litter, and under 30 minutes of close contact a day. Pregnant women and children under 12 stay away for that week.
What happens if the cat becomes hypothyroid afterwards?
Five to ten percent of treated cats end up with a low T4 at 3 or 6 months. The fix is a small daily levothyroxine tablet, dosed by your vet, with a T4 recheck 4 weeks later. Most cats stabilise on 1 dose, and the condition is far easier to manage than the hyperthyroidism it replaced.