Ideally, treatment of a hyperthyroid cat with a single dose of radioiodine should restore euthyroidism without inducing hypothyroidism. In other words, the goal of this treatment should be to irradiate and destroy all abnormal thyroid tissue (adenomatous or carcinomatous) but to leave the normal (nonadenomatous) thyroid tissue intact (1-4).
The optimal method for determining the amount of radioiodine required for effective treatment in cats remains somewhat controversial. Reported methods to determine the radioiodine dose for cats with hyperthyroidism are variable but can be divided into the following four general categories (4-9):
- Fixed or “one size fits all” dosing
- Individualized or “patient-specific” clinical scoring system for dose determination
- Individualized, advanced clinical scoring system, with calculation of thyroid tumor volume and thyroid radionuclide uptake
- High-dose I-131 treatment for thyroid carcinoma
Fixed-dose radioiodine therapy
The fixed-dose approach, the most common method of treating hyperthyroid cats, assumes that most of these cats can be successfully treated by administering the same fixed, relatively high dose of radioiodine to all cats (i.e., 4 to 5 mCi [148 to 185 mBq]), regardless of the severity of hyperthyroidism or size of the thyroid tumor (4-6).
Because no effort is taken to determine thyroid tumor size or to estimate severity of hyperthyroidism, this dosing method is the easiest, involving no calculations and requiring little in the way of nuclear medicine equipment. However, to attain a reasonable success rate with this method, a large number of cats end up being either overdosed or underdosed with radioiodine.
For example, in my clinic, the median individualized dose given to our hyperthyroid cats is now 2.5 mCi (100 mBq), much less than the dosages administered with the fixed-dose approach. In my opinion, a large number of cats treated using the fixed-dose method will receive too high of a radioiodine dose, exposing them to an unnecessary amount of radiation and greatly increasing the risk of iatrogenic hypothyroidism. Since I do not use this method, I can't quote exact incidence, but I wouldn't be surprised if over 75% of these cats become hypothyroid.
On the other hand, a few cats with very large, benign thyroid adenomas that I treat in my clinic will require calculated radioiodine doses that are much higher (i.e., 6 to 12 mCi; 222 to 444 mBq) than those typically administered with the fixed-dose approach. These cats will likely be treatment failures and show persistent or recurrent hyperthyroidism with the fixed-dose approach.
So, although the fixed-dose method is the simplest to employ, the incidence of both long-term hypothyroidism and persistent hyperthyroidism are higher with this dosing method.
Individualized or “patient-specific” clinical scoring system for dose determination
In the second method of dose determination that I first described almost 20 years ago (7), the dose of radioiodine administered to hyperthyroid cats is determined by a clinical scoring system based on 3 factors that take into consideration the severity of clinical signs, the size of the cat's thyroid gland (determined by palpation of the goiter on physical examination), and the pretreatment serum T4 concentration. Using this scoring system, a low, medium, or relatively high 131-I dose is selected (4,7,8).
For example, cats with mild clinical signs, small thyroid tumor(s), and only a slightly high serum T4 concentration would receive smaller doses of radioiodine (e.g., 3 mCi; 111 mBq); cats with severe clinical signs, very large thyroid tumor(s), and markedly high serum T4 concentrations would receive high doses of radioiodine (i.e., 5 mCi; 185 mBq); and cats that lie between these extremes would receive intermediate doses of radioiodine (e.g., 4 mCi; 148 mBq).
In contrast to the fixed-dose method, the total radiation dosage delivered to the cats with mild hyperthyroidism is lower and, thus, the prevalence of iatrogenic hypothyroidism is minimized. However, even with this scoring system, many cats develop low serum T4 values after treatment and some cats, especially those with severe disease, are not cured with a single dose of radioiodine.
Individualized, modified, clinical scoring system, with calculation of thyroid tumor volume and thyroid radionuclide uptake
In the third method that I now use, the dose of radioiodine administered is based on a refinement of the scoring system outlined above (4). Again, this refined scoring system takes into consideration the severity of the cat’s clinical signs, the size of thyroid tumor, and the pretreatment serum T4 and T3 concentrations. In addition, however, thyroid imaging is used to better estimate the volume of the thyroid tumor tissue, and the thyroid radionuclide uptake is determined (9-11). The dose of radioiodine is then calculated from these measurements.
Using this protocol, very low doses of radioiodine (e.g., < 2 mCi; <75 mBq) will result in the cure of many cats with early hyperthyroidism that have small but hyperfunctional thyroid tumors on thyroid imaging. These radioiodine doses are much lower than the lowest dose given with the original scoring system (3 mCi; 111 mBq) or used with the fixed-dose methods (4-5 mCi; 148-185 mBq).In contrast, other cats with severe hyperthyroidism and large volumes of autonomously functional thyroid tissue (but without scintigraphic evidence of malignancy) may require up to 15 mCi (555 mBq) of 131-I to restore euthyroidism. These calculated radioiodine doses are much higher than the highest dose given with the original scoring system (5 mCi; 185 mBq) or used with the fixed-dose methods (4-5 mCi; 148-185 mBq).
Botton Line:
I believe that determining the thyroid tumor volume and percent 131-I uptake play key roles in calculating the best 131-I dose needed to completely ablate all tumor tissue but preserve any remaining normal thyroid tissue.
Physical palpation of the thyroid gland may yield equivalent information to that obtained from the thyroid scan in some cats, especially those with smaller, easily palpated nodules (7). However, the thyroid tumor volume will likely be greatly underestimated in cats with substernal or ectopic goiters based on physical evaluation alone, especially when those thyroid nodules cannot be palpated (4,11).
It is clear that we can achieve much better results (less hypothyroidism or persistent hyperthyroidism with use of an individualized dosing protocols. However, a percentage of hyperthyroid cats, especially those with long-standing, severe hyperthyroidism, will not have enough "normal" residual thyroid tissue left to maintain euthyroidism after we successfully ablate all of the thyroid tumor tissue. In those cats, mild to moderate degrees of iatrogenic hypothyroidism will result, no matter what dosing protocol is used.
In my opinion, curing the hyperthyroidism with radioiodine or surgery remain the ideal treatment options for cats. Even if the cat needs thyroid hormone supplementation, definitive treatment with either radioiodine or surgery still remain a better option than long-term medical or nutritional management. Without definitive treatment, the thyroid tumors remain and will continue to grow, and may potentially transform into thyroid carcinoma and metastasize (12).
References:
- Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
- Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;199-203.
- Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
- Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
- Meric SM, Rubin SI. Serum thyroxine concentrations following fixed-dose radioactive iodine treatment in hyperthyroid cats: 62 cases (1986-1989). J Am Vet Med Assoc 1990;197:621-623.
- Craig A. A prospective study of 66 cases of feline hyperthyroidism treated with a fixed dose of intravenous 131-I. Aust Vet Practit 1993;23.
- Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc 1995;207:1422-1428.
- Peterson ME. Radioiodine treatment of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:34-39.
- Forrest LJ, J. BC, Metcalf MR, et al. Feline hyperthyroidism: efficacy of treatment using volumetric analysis for radioiodine dose calculation. Vet Radiol Ultrasound 1996;37:141-145.
- Broome MR, Turrel JM, Hays MT. Predictive value of tracer studies for 131-I treatment in hyperthyroid cats. Am J Vet Res 1988;49:193-197.
- Broome MR. Thyroid scintigraphy in hyperthyroidism. Clin Tech Small Anim Pract 2006;21:10-16.
- Peterson ME, Broome MR: Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. J Vet Intern Med 20112: 26:1523.
24 comments:
Dr. Peterson, you said that the rate of hypo-t might be greater than 75% with method one. What is your guess regarding the second method? And what do you believe is the approximate rate of hypo-t with your method?
Thank you....
With the second dose method, the rate of overt hypothyroidism (low T4 or free T4 with high TSH value) is <5%. I would estimate that the incidence of subclinical hypothyroidism (low-normal T4 and free T4 with high TSH) would be about 20-30% (but that's a guess since those studies have not been done). It's clear, however, that use of T4 and nodule size alone is not the best approach for individualized dosing.
Our preliminary results with my present method show a very low incidence of overt hypothyroidism (<1-2%) and a much lower prevalence of subclinical disease (<10%). As you may know, we are doing a prospective long-term follow-up study on the cats we treat, so I will have much more information concerning the rate of subclinical disease in the future.
At our clinic, however, this third method continues to evolve; as we tweak our methods and calculations for determining thyroid volume and % radioiodine uptake, I'm certain that we can do better as we go on with this study, especially for cats with very mild disease.
Dear Dr. Peterson!
I am a student of radiology technology in Austria, currently working on a Bachelor’s Thesis about the radioiodine therapy in cats, so I came across some of your amazing work. (You helped me to understand this topic a lot, since I usually work with humans and didn’t know much about feline diseases or anatomy in the beginning!)
I read in one of this blogpost that you calculate the dose using the iodine uptake, which I thought was very interesting. So I got curious about your work and wanted to ask you a few questions, if you don’t mind:
• How much dose of radioiodine do you give for the thyroid scintigraphy before the treatment? (since you only want to calculate the uptake, it won’t be much I guess)
• Would it also work if you would evaluate the uptake with 99mTc, or is there no correlation between the 131I- and the 99mTc-uptake of the thyroid gland?
• How exactly does your score look like?
• Do you apply the therapeutical dose immediately after the scintigraphy, or do you wait?
In Austria, we use a score using pre-treatment T4 concentration, size of the thyroid gland and severity of clinical signs to calculate the therapeutical dose. But since you include the uptake too, much higher or lower doses are possible, which would be worth trying in Austria.
As you wrote in another blogpost, a reason for treatment failure might be a low 131I-uptake because there isn’t enough radiation dose delivered to the thyroid gland. I’m trying to evaluate if it is possible to predict the outcome of the radioiodine therapy by using the 99mTc-uptake of the thyroid gland too. Theoretically, a high 99mTc-uptake should mean more therapeutical dose in the thyroid gland and vice versa. So a normal thyroid scintigraphy using this nuclide should be enough to calculate the therapeutical dose.
Thank you for all your hard work, you already helped me very much! I would be very happy if you could answer at least some of my questions, since my goal is to optimize the feline radioiodine therapy in Austria. Also, I would like to understand the whole concept a bit better!
I’m looking forward to your reply!
Your questions would take me about 6 papers to answer (which I am working on but very little on this published yet).
You might want to look at my profile on Research Gate where you can download PDFs of what I've published (https://www.researchgate.net/profile/Mark_Peterson11).
We use pertechnatate for thyroid scintigraphy and calculate the percent 99m-Tc thyroid uptake of that radionuclide. The higher uptake indicates a higher "autonomous volume" of tumor tissue. In other words, two cats with the same sized nodule may contain higher numbers of tumor cells packed into that tumor volume seen on the scan.
We give a low dose of I-131 that same day but measure I-131 uptake at 24-hours. If that number is not high enough (we hope for an I-131 uptake of 20-30%), then I may give an additional radioiodine dose.
My score is based on measured thyroid volume, the % 99m-Tc uptake, and serum T4 and T3 values. Way to complicated to describe here.
Although cats that have low % 99m-Tc uptakes will also commonly have low post-treatment I-131 uptakes (and show treatment failure), there is not a significant correlation between these 2 values. Cats with slightly high %99m-Tc uptakes and cats with very high %99m-Tc uptakes both will commonly show a post-treatment I-131 uptake of 20-30%.
Good luck... hope this helps. Now it's time for me to get back to my writing my research papers!
Thank you so much for your reply!
I find your method very interesting and will look into it a bit more, hopefully we can use it someday too.
Good luck with writing your papers, I am really looking forwart to them!
Hi Dr. Peterson. I've searched and searched but I can't find answers to my questions online. My cat was just diagnosed with hyperthyroidism and at the moment the symptoms are mild (according to the vet), however, I first noticed signs for the past year or maybe two but didn't realize what it was. So it's possible my cat has had hyperthyroidism for a while; but it's hard to say. What I want to know is:
1. Although the consensus is to start treatment asap, how long is reasonable to wait if I can't do it now? My concern is the fact that the best therapy is I-131, but I don't want to do it while I am pregnant because of the radiation risks. I want to wait until after the baby is born but that's 4 months away plus maybe 1-2 months post-birth while I recover. Also, I'm the one who will be changing the litter, but if I am breastfeeding am I still risking my baby by having the procedure done? I wish treatment centers would keep the cat until she is safe for me to be around, but I am assuming they won't or it might be too expensive for that long.
2. If I wait until after the baby's born to start my cat's treatment, how bad could my cat's symptoms become? My vet told me this is a slow progressing disease but I would like more information about how slow and what's possible. I prefer not to do another form of treatment in the meantime like medication, because they have their own risks and aren't as effective.
3. The opinion of one facility (which is hours closer to me than the one that does scans before treatment; I'm in the DC/Maryland area) is that there is no need to do a thyroid scan before treating with I-131, but I know you feel otherwise. They said it's appropriate for research but not in a clinic and not necessary for diagnosis and treatment. Can you respond to this? I am really concerned about my cat developing hypothyroidism from being overdosed with I-131. And, on the off chance that it's the malignant kind, the treatment would potentially be different.
4. It seems that the thyroid scan also uses radioactive material, so does this procedure also pose a risk to me while I am pregnant? I was considering having it done at a separate facility if I can't find a closer one that scans their patients before administering I-131 treatment.
5. Should I get my cat's T3 and TSH levels checked? Only the total and free T4 levels were checked, but I read somewhere (maybe your site) that TSH levels that are off could be indicative of other problems.
Thank you.
If you are pregnant, I would use medical therapy to control the hyperthyroidism and do the procedure after your child is born. If you use a low dose, that medication is relatively safe and only very rarely causes side effects.
As time goes on, I do not feel that ANY cat should be treated without the benefit of a thyroid scan. I certainly do clinical research, but none of that has anything to do with proper diagnosis or dosing; that's why we do the scintigraphy. Those facilities that state that it's not needed are generally too cheap to invest in the purchase of a gamma camera or learn how to use it! I would encourage you to go elsewhere where they use the scans to help in dose adjustments.
Thyroid scintigraphy is done with a short-acting low-energy isotope. All of the radiation should be gone within 1-2 days so it's much safer; but this should be done right before the radioiodine treatment, not now. The tumor could grow over the next few months - probably not much, but I wouldn't want to treat based on a scan from 4-6 months before.
Serum T3s are not all that helpful for diagnosis. TSH levels aren't needed unless the total T4 and free T4 concentrations are borderline or normal. Serum TSH levels are very helpful (alone with followup thyroid scintigraphy) to diagnose hypothyroidism (under-active thyroid) after treatment with surgery or radioiodine.
I really appreciate your feedback. Balancing my concern for my cat versus my baby in this situation is challenging. Fortunately, I was able to find a facility close to me that does thyroid scintigraphy as a standard part of their I-131 treatment process. Regarding medication therapy, I know Methimazole is commonly prescribed. But I read in your blog that it could be absorbed through my skin or I could ingest residual powder if I am not careful. Is there an alternative drug that is just as safe? And, what would be considered a low dose? Lastly, do you think it would be ill advised to wait to do the I-131 treatment but not do medication therapy now? I'm more on the fence about the medication than I-131 from things that I read online. Thank you.
If you don't split the tablets, handling is not a concern (especially with the veterinary product, Felimazole). I can't advise you whether it is okay or safe to withhold treatment since I haven't seen your cat. Talk to your vet about that.
Hi Dr. Peterson,
thank you, as always, for your clear and helpful information. Could you talk about the cost of this treatment? I'm trying to get a sense of what an appropriate cost would be, and how that might compare to lifelong thyroid treatment. I am on a limited income due to a disability, but my companion cat is very special to me and I want to care for him as well as I possibly can. Thank you for your help!
--Joyce
Cost of radioioidine will vary tremendously depending on location and exactly what is done, but generally is in the range of $1200-2500. The main cost of methimazole is the recheck fees, so you can calculate what the cost of a CBC, chemistry panel and T4 is for each visit (I'd recommend testing at least 3-4 times yearly). But the main reason we use I-131 is not for cost, but rather to cure the tumor.
Thank you. Yikes. Yes, I know that it would cure the thyroid issue. I will do some research and find out might do this treatment in Tucson what the cost would be.... Just like with ourselves, it is awful when we can't do the right treatment for our beloved kitties because of cost. Hopefully someday the cost will come down! Thanks so much for the information!
Hi Dr. Peterson
My cat was diagnosed with Hyperthyroidism on 28 May with T4 >129 (19-62 in-house test). Given by a overdose Vialdta (15mg/day) for 2 weeks to 12/06/2015, Frankie had a terrible vomiting, and we stopped the medication,and started Methimazole gel with a lower does around 1.25mg twice a day from 18/06- 10/07/, new blood test T4 was 25(19-62,inhouse) and T4 was 16.2 （9.5-48 outside lab) and chemistry panel numbers are good when T4 decreased to normal. Another 2 weeks methimazole gel used, and stopped for 10 days on 24/07 for I-131 treatment. Frankie received his I-131 treatment on 03/08/2015.In our small country, only one does fits all cats, it is 150 MBq.
30 days post treatment on 03/09/2015, T4 is 67 (19-62 inhouse )and 51.5 (9.5-48 outside lab)
45 days post treatment on 17/09/2015, T4 is 68 (9.5-48 outside lab only)
I am still waiting for the 90 days post treatment test, but the facility said it appears the treatment failed.
I have contacted a new facility, they suggested to wait and see 90 days post treatment test result and then decide redo is necessary or not, and due to the limited radiation supplied for animal in this small country, the next available redo slot may be in February or March 2016, May i ask a few more questions here:
1. Do you agree the first treatment failed?
2. What the redo time frame you recommended? 60 days? 90 days? or even longer than 6 months?
3. do i need to back to methimazole gel if the result of 90 days post treatment still in the up trend ? Frankie is doing well now, no eating too much, no diarrhea, weights keeps at 4.3-4.4kg since he went back from 131 facility, but vomited once a week since last month.
4. do you suggest use alternative product such as L-carnitie to reduce his symptoms during the waiting time?
5. In regard to the dose, do you suggest we should negotiable with new facility to increase from 150 MBq to a little bit higher?
Sorry too many questions here, i know you are the foremost specialist in Hyperthyroidism for cats, I even plan to bring my cat to your clinic for treatment, is it possible?
I really appreciate your valuable comments and thoughts.
Kind Regards
Yoko
You need to wait at least 3 months and 6 months is best before any I-131 retreatment. Giving 250 mg of L-carnitine can't hurt anything and may help.
Hi Dr Peterson,
Thanks for your above response, sorry for later thanks message as I do not want bother you frequently.
My cat Frankie did his blood and urine tests of 90 post 131 treatment yesterday, we did CBC/full chemistry panel/T4 and sent sample to outside labs, and for T4 we used two outside labs to compare the results.
His USG is 1.047
All numbers in CBC are normal, but most of numbers are in the lower normal range, is that a matter?
ALB in chemistry panel is low at 31 (33-43) compares with 34 on 17 September,CRE is 104 (67-150)compares with 101 on 17 September, other numbers are in the lower normal range as well.
His T4 is 46.2 (9.5-48) compares with 68 on 17 September and 51.5 on 3 September, it is 90 days post treatment, have you seen the same case like Frankie, with T4 was in a uptrend after 2 months treatment and then in a downtrend in the test of 90 days post treatment?
Another outside lab result of T4 is 42 (20-40).
Can we still say the first treatment failed as the vet in facility said so when he saw two times result of post treatment. As i need to swap the new facility, so i didn't send above new test result to them.
Would you mind giving me some comments on it? regarding the low ALB, did low albumin level in Frankie’s blood presented us with false lower hormone levels?
I did use L-carntine follow you suggestion, and Frankie is doing very well at home, no vomiting and diarrhea, eating and drinking as normal, heart rate is from lowest 120 bpm to highest 160bpm, and he is put on weight from 4.27kg (03 September) to 4.45kg (2 November) using vet scale.Vet also said "grade 2 L-sided holosystolic murmur present - stress murmur?"
What do you suggest we should do in the next?
Much appreciate your time and help.
Regards
Yoko
I'd wait and retest in 2-3 more months. I would not retreat him now (too soon), especially since he is doing well. I'm assuming that the pretreatment T4 value was much higher.
I don't think the chemistry numbers are anything to be concerned about.
Hi Dr Peterson
Thank you so much for taking your valuable time to answer my above questions promptly.
May I ask one more question here, as we used L-carnitine for nearly 20 days since October, is dropped T4 influenced by L-carnitine intake? Do we need to continue use it for Frankie?
Regards
Yoko
L-carnitine doesn't change circulating thyroid levels. It is very safe and can be continued indefinitely, even if not hyperthryoid.
My cat just received the radiation iodine treatment 3 weeks ago it cost me 2,750 after tests, x rays and the 48 hour stay in the hospital. He hasn't gone to his follow up yet which is next week but he did suffer some kind of small seizure after he was home 2 days, he still eats like a pig always wanting food and is still skin and bones. Does he need another treatment? Please help I feel our vet only cares about my wallet but they have his history already.
It can take 1-3 months for the thyroid values to normalize. Seizures are not part of hyperthyroidism or the treatment. Talk to your vet and consider rechecking sooner.
Hi Dr Peterson
Thanks for your great site and great wisdom. All your info left me better equipped for my first appt with the internist at Gulf coast Vet Specialists in houston with my cat Chloe who has a T4 of 6.1 and many of the hyperT symptoms. I found out that this place uses standard dosing, a 'small' dose of 3.5 and a 'large' dose of 7. I have requested individualized dosing based on the scintigraphy etc.
I've been trying to access your dosing protocol to give their radiologist--whom, it appears, is open to discussing dosing options, but can't access. i will try your research gate, but if it's not on there if there is any other link or advice, please let me know if you can. Thank you!
Martica Heaner PhD
We are currently conducting clinical research studies investigating the usefulness of thyroid volume, % thyroid uptake, and serum T4/T3. So my dosing protocol is not a simple one to follow or copy and it's still a work in progress (fine tuning it).
That all said, most cats that I treat with a T4 of 6.1 receive a dose of 1.8-2.0 mCi.
Thank you so much. I just found your chapter on ResearchGate and downloaded it--and forwarded it to the internist and radiologist. (Hopefully they are not annoyed my my trying to dictate their protocol!) But I will also forward your info here to them. Thanks SO MUCH for being so generous with your knowledge and expertise for us lay folks out here!
Dr. Peterson, I like your low-dosing approach. I have not read your stuff on diabetes, but intend to. But am a lay diabetes-expert having had many cats with diabetes. One thing that I discovered was, without an exception, every single vet I dealt with seriously overdosed the cats with Lantus. (The only insulin I'm familiar with.) One vet I KNEW was causing a Somogyi rebound--that was not being detected because he was only testing BG BID at each shot time. He doubled the dose within 3d based on increasingly high numbers. This was a cat I had saved from the kill shelter. Once I realized this I insisted with my rescue group that I bring him home--the vet didn't want him home till his BG was 'stable' so I had to get permission and sign a medical waiver that I was taking the cat home 'against medical advice'. The day I brought him down I tested him immediately upon getting home--and after his 2.5U shot of Lantus that morning--3h later his BG was 40!
What I found with 4 of my own diabetes cats was that I was able to get ALL of them down to ~ 0.25U a day of lentos only ONCE a day..and this kept them regulated...(along with low carb, grain free canned diet.) The key I found was that if there was a higher number at shot time, I would give the teeny dose, but WAIT to feed a few hours until it started kicking in...up to 2 or 3...this worked on 4 cats. I think it's a big scary thing that the vets are dosing with up to 2-4U BID of Lantus on cats. I cringe every time I hear it cause I know the cat is probably in danger, and then often her hypo and death stories.
best
Martica
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