I am writing from Michigan State where we just started doing I-131 therapy at this past year, and I have a question about one of my own cases. I recently attended the endocrine lectures you presented at the ACVIM specialty forum in Las Vegas and loved all of your hyperthyroidism stuff!
I have a cat that we treated with 4 millicures of I-131 SQ on September 21, 2010. 12 yr old cat, otherwise healthy that I had been trying to manage medically for about a year but could not find the right daily dose that either made him hypothyroid or was not enough. Finally owner agreed to I-131.
He was treated on 9/21/10. On 11/5/10 his first post tx thryoid panel was T4 > 156 nmol/L, TT3 >4.7nmol/L, Free T4 > 127 pmol/L, FT3 elevated at 10.1 (1.5-6.0 pmol/L). Clinically he was doing OK, had gained some wt but was still thin and was tachycardic at 220.
We rechecked another thyroid panel on 12/1/10 and the T4 was still > 156, TT3 elevated at 4.3 (.6-1.4), FT4 >127 and FT3 elevated at 11.6. Physical exam was about the same, he had gained about 4 oz of weight, still tachycardic at 220 bpm but no murmur.
I am probably lucky, but I have never had a case that did not come down by the time of the first recheck and would like to know how long you recommend waiting before proceeding with further diagnostics or treatments. There is a palpable goiter which feels exactly the same as it has since he was diagnosed; chest radiographs prior to tx were normal other than mild cardiomegaly that he has had for years and is unchanged. No murmur.
I know some cats can take months to come into the normal range, and would like to get your thoughts on these values and how long you would wait before retesting and how long you would give him in total before reevaluating or retreating. I was just made aware of another case by a referring vet where the T4 is still elevated at 1 month post, but it is at 86 after having been >156 prior to tx.
Thanks so much for taking the time to read this and offer any thoughts!!
Most cats become euthyroid again normal within a month, and retreatment should be strongly considered if the serum T4 concentration is still high at 3 months post-treatment. This cat obviously isn't going to get any better and needs treatment.
Can you do a thyroid scan? What's the sex and color of this cat? Some cats appear to have a "sex-linked resistance" to the radioiodine treatment so that may be important.
In general, I give at least a 50% higher amount of I-131 at time of retreatment. So, in this cat, that would mean at least 6 mCi. It's quite possible that this cat needs a higher dose than that if his thyroid volume is huge, and I routinely give 7-15 mCi to cats with extremely large goiters. It's difficult to determine the extent of the thyroid mass (especially thoracic invasion or ectopic thyroid nodules) without a thyroid scan in these cats. If the cat has thyroid carcinoma, we generally treat with a much larger dose of 30 mCi.