How Does Radioiodine Really Work to Treat Hyperthyroidism?

I had a general question regarding radioiodine (I-131) treatment for hyperthyroid cats. Although I have referred a number of feline patients for this treatment, I do not know that much about the details of the treatment.

Do all cats receive the same dose or do you vary the dose based upon the severity of the cat's hyperthyroidism? How does one actually administer the radioiodine dose? 

A little explanation would be appreciated. Thanks!

My Response:

Route of I-131 administration
We've been giving the radioiodine subcutaneously since 1986 (1). Prior to that, we initially tried the oral route, but that meant handling radioactive capsules (and hoping the cats wouldn't chew them) or stomach tubing the cats (and hoping the cats wouldn't vomit). In human patients, they generally put the radioiodine solution into a juice drink to cover up the "iodine taste" and the people just drink the solution. Obviously, that wouldn't work in cats.

Administering radioiodine
subcutaneously to a cat

From around 1980 to 1986 we gave all of the doses IV, which worked fine. However, that meant that two people always needed to be exposed when the dose is administered (now I generally do it by myself) and the cat needed to have a catheter placed for the injection. The IV administration worked well, but occasionally, I saw anaphylactoid reactions (rather terrifying!) upon treatment. Obviously, there is something in the solution that the cats don't like when the drug is given more than once intravenously. I have NEVER seen an anaphylactoid reaction when the radioiodine solution is given subcutaneously.

Calculation of the I-131 dose
As far as dosing goes, I dose the hyperthyroid cats differently than most treatment facilities, which use a fixed dose of 4 to 5 mCi administered to all cats, no matter how mild or severe the cat's hyperthyroidism.  I really do believe that facilities that use a fixed dose are overdosing most of the cats, and under-dosing others. Cats with thyroid carcinoma generally require much larger radioiodine doses, generally in amounts of around 30 mCi but sometimes even more (4,5).

Thyroid scintigraphy (scans) in 3 hyperthyroid cats. The cat on the far left has a very small unilateral nodule, the middle cat has small, assymetrical bilateral nodules,
and the cat on the right has larger bilateral disease. The calculated I-131 doses
that we administered to these cats ranged from 1.5 mCi to 3.5 mCi.

I currently give a range of doses from 1.5-10 mCi to cats with benign adenoma (adenomatous hyperplasia). This is based on severity of hyperthyroidism (both clinically and biochemically, ie, the T4 level), size of thyroid tumor(s) both on palpation and thyroid scintigraphy, age of the cat, and known concurrent diseases (2-5).

Goals of I-131 therapy: cure hyperthyroidism but avoid hypothyroidism
My goal of therapy is to 'cure' the hyperthyroid state without causing hypothyroidism. Everyone talks about the fact that they can cure 98% of hyperthyroid cats. Well, that's easy; anyone can order a big dose for all hyperthyroid cats and cure them, but many will become hypothyroid.

It becomes more difficult to titrate the doses because one has to think about the whole cat and it's thyroid uptake and iodine kinetics, but I do believe it's so very important. That's where this whole treatment issue becomes more tricky.

Hypothyroidism and the kidney
It's becoming increasing clear that both hyperthyroidism and hypothyroidism are bad for the kidneys (6), so the last thing we want to do is cure the hyperthyroidism but create iatrogenic hypothyroidism. And that is especially true if the owners cannot give oral medication or if the cat already has mild renal disease.

Well, this is probably much more than you ever wanted to know, but I hope it answers your questions.

Follow-up Question:

Thanks for the great explanation. Can I assume cats that receive a small dose of I-131 will be able to leave isolation earlier than cats that receive more substantial doses? Is there a required minimum stay for cats post treatment or is their stay based on their "radioactivity"?

My Response:

The NRC rules have a minimum stay of 3 to 5 days, depending if there are children or pregnant women at home. Once that minimum stay is met, then the cat's radiation "reading," measured by a survey meter at a meter from the cat's neck must be less than 0.5 milliRoentgens per hour (mR/hr).

So, if a cat is treated with 2 to 3 mCi, we can almost guarantee that the radiation levels will fall to below 0.5 mR/hr by 3 days. If a cat gets, 4 mCi or more, it may take a few more days, depending on the effective half-life of the radioiodine and the residence time of I-131 in the individual cat's thyroid tumor. And if a cat with thyroid carcinoma is treated with 30 mCi, they may have to stay in the hospital for 2 to 3 weeks.

Some cats excrete the radioiodine from their thyroid gland —and therefore their kidneys—faster than other cats. Such faster clearance is not always good — we need to have the radioiodine within the tumor for enough time to deliver the radiation dose to the cat's thyroid tumor!

References:

1. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1995:207:1422-1428. 
2. Peterson ME: Radioiodine treatment for hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:34-39.
3. Peterson ME: Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC (eds): Current Veterinary Therapy XIV. Philadelphia, Saunders Elsevier, 2008, pp 180-184.
4. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110.
5. Peterson ME, Broome MR: Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC (eds): Current Veterinary Therapy XV. Philadelphia, Saunders Elsevier, 2012; in press.
6. Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092.