Q & A: How is Radioiodine (I-131) Administered and Dosed in Hyperthyroid Cats?

I have a general question regarding how veterinarians administer and dose the radioiodine ( I-131) as treatment for hyperthyroid cats. Although I have had a number of my hyperthyroid patients treated successfully with radioiodine, I do not know how one administers and doses it.

A little explanation would be appreciated.

My Response:

We've been giving the radioiodine subcutaneously since 1986. Prior to that, we initially tried the oral route, but that meant handling radioactive capsules (and hoping the cat's 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.

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 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 that cats don't like when the drug is given more than once intravenously.

Today I still administered the radioiodine to all of my hyperthyroid cats by the subcutaneous route. In many of these cats, I can perform the treatment by myself, thereby not exposing another member of my staff to the full dose of radiation that is contained in the syringe. I have NEVER seen an anaphylactoid reaction when the radioiodine solution is given subcutaneously.

As far as dosing goes, I really do believe that facilities that use a 'fixed dose' are overdosing most of the cats, and under-dosing others.

I give a range of doses from 2-10 mCi to cats with benign adenoma (adenomatous hyperplasia). This method of dose determination is somewhat more complicated, and is based on the following factors:

• Clinical severity of hyperthyroidism
• Magnitude of the serum T4 level
• Size of thyroid tumor(s) on palpation
• Result of thyroid scintigraphy (thyroid scanning)
• Age of the cat
• Known concurrent diseases

Cats with thyroid carcinoma generally require much larger radioiodine doses, generally in amounts of around 30 mCi but sometimes even more. These cats generally have larger thyroid masses, generally invading soft tissue and extending into the thoracic cavity.

Hyperthyroid cat with thyroid carcinoma (on left) demonstrating the massive, multinodular tumor, invading and extending into the thoracic cavity. The horizontal line is the region of the thoracic inlet. 

My goal of therapy is to 'cure' the hyperthyroid state without causing hypothyroidism. Many treatment facilities boast 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. I agree it's more difficult to titrate the doses because one has to think about the whole cat and what we are doing, but I do believe it's so very important. That's where this whole treatment issue become more tricky.

It's becoming increasing clear that both hyperthyroidism and hypothyroidism are bad for the kidneys, so that 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.