Friday, January 31, 2014

What's the Expected Time for Signs of Feline Hyperthyroidism to Resolve after Treatment?

My 16-year old cat was recently treated with radioiodine (I-131) for his severe hyperthyroidism. How long after radioiodine treatment will the thyroid values normalize and the symptoms of the hyperactive thyroid begin to resolve so he feels better? 

My cat lost about half of his body weight, much of it in muscle mass. Will his wasted muscles ever return to normal? If so, when can I expect to see improvement?

My Response: 

You have asked two good questions, which, on the surface seem simple enough to answer. The answers to "how long for clinical signs to resolve" however, depend on a number of factors. I'll do my best to explain why it's not possible for me to give you the definitive answers you want.

How long does it take for serum thyroid hormone levels to normalize after I-131?
Depending on the dosing protocol used, about 90% of cats will have serum thyroid hormone concentrations (e.g., T4 and T3) within reference range limits by 30 days after I-131 treatment. Most of the remaining cats will show a nice drop in T4 and T3 levels when rechecked in a month, but it will take longer to for their thyroid hormone concentrations to completely normalize.

In general, the full extent of the radioiodine treatment will be evident by 3 months after treatment, although a few cats continue to show even more (minor) improvement when rechecked at 4 to 6 months.

As the thyroid values normalize, the clinical signs we see also gradually resolve. Some signs, such as nervousness or rapid heart rate, generally resolve fairly quickly, whereas other signs, such as marked weight loss and muscle wasting, obviously take much longer.

How fast do we want the serum thyroid values to fall after radioiodine treatment?
My goal in treating hyperthyroid cats with radioiodine is to gradually normalize the high serum thyroid hormone concentrations— not lower the values too quickly. I'd rather that the thyroid values fall slowly over the first month after treatment, allowing the rest of the body to gradually get used to being euthyroid once again. This is especially true in cats with concurrent kidney disease, when a drastic fall in thyroid values can aggravate the serum kidney values and can even lead to severe renal failure.

To achieve this gradual fall in the high serum thyroid hormone levels, I administer the smallest dose necessary to cure the hyperthyroidism. By giving individualized, lower doses of radioiodine, we can also reduce the incidence of post-treatment hypothyroidism (underactive thyroid condition) in these cats.

How often does the radioiodine fail to cure the hyperthyroidism?
About 5% of cats that I treat will remain slightly hyperthyroid at the 3-month follow-up period. Many treatment facilities will claim a higher rate of cure (98-100%) than I do, which is made possible by administering higher doses of radioiodine to their cats. In addition to decreasing the incidence of persistent hyperthyroidism, the use of higher radioiodine doses will also hasten the rate of decline in the serum thyroid hormone concentrations.

So why not use this high-dose I-131 protocol instead of my lower-dose approach? Simple — the downside of administering higher radioiodine doses is that this method will lead to a higher rate of iatrogenic hypothyroidism as both the thyroid tumor, as well as most normal thyroid tissue, are irradiated and destroyed (1,2). As I've previously discussed (see my post, Estimating the Radioiodine Dose to Administer to Cats with Hyperthyroidism), more that 30% of cats will become hypothyroid using the standard high-dose treatment protocol, but this incidence could in fact be much higher, possibly up to 75%.

Diagnosing iatrogenic hypothyroidism
To monitor for iatrogenic hypothyroidism, we routinely run a serum thyroid panel (i.e., total T4, T3, free T4, and TSH) at 1 and 3 months after treatment (2,3). Most facilities recommend monitoring just the total T4 concentration, but this is not adequate for monitoring since many hypothyroid cats will maintain a low-normal total T4 value, despite being hypothyroid.  Based on our studies, it's becoming increasing clear that feline hypothyroidism can only be diagnosed by finding low to low-normal T4 and T3 values in conjunction with high TSH values.

Restoring lost body weight and muscle mass
Once euthyroidism is reestablished, most cats will gain weight within a few weeks (certainly by 2-3 months). If marked muscle wasting has occurred, it may not be possible to completely regain the lost muscle. Remember that it will help to feed a diet that's higher in protein (40-50% of calories), higher in fat (40-50% of calories), and relatively low in carbs (less than 15% of calories) (4). For more information, check out my post on The Best Diet to Feed Hyperthyroid Cats.

  1. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2014.
  2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: hypothyroidismCompend Contin Educ Vet 2013;35:E4. 
  3. Peterson ME. Diagnosis and management of iatrogenic hypothyroidism In: Little SE, ed. August's Consultations in Feline Internal Medicine: Elsevier, 2014;in press.
  4. Peterson ME. Nutritional management of endocrine disease in cats. Proceedings of the Royal Canin Feline Medicine Symposium 2013;23-28.

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Melissa Brown said...

I have a question about this article. My cat who is 15 recently underwent i-131 on January 25. It's been about 19 days since he went in for treatment.

His case is very unusual as he has had hyperthyroidism since he was 10 - so for 5 years. His was ectopic. He had been well controlled on 5mg am and 5mg pm. Until about April of 2015 when he went in for his blood work check to find his T4 had risen to 20! He was getting his meds so we thought it was the manufacturer. We tried a different manufacturer and it went down to 8. Retested in 4 weeks and it was back up to 11.7. So since he was diagnosed also with IBD we thought it was an absorption issue. So we started on trandermal. After 4 weeks he was at a t4 of 29. No increase was done until this point. My vet recommended i-131. Then I immediately increased his meds to 7.5mg am and pm. When going for initial evaluation for i-131 they found his thyroid adenoma was ectopic and quite large (obviously he's had it for 5 years). However at that evaluation which was only 7 days after getting the t4 reading of 29, his thyroid values were normal at 1.1. Kidney's were good as well. So in preparation for his procedure after several days off the methimazole they wanted to test his T4 to gain an idea of how much to give. They said they had standard doses of .3 or .6. Well when they tested his T4 it was at 54! They wanted him off of it for 7 days. After reading your article about methimazole usage prior to i-131 I told them that I would continue methimazole because who knew what his thyroid number would increase to without it. And it could cause more harm than good. However after while at facility he was not on any thyroid medication but was put on two heart medications to protect his heart. He came home in a week and seemed very lethargic laid around. Then a few days ago he was laying around the water dish and hunching over his appetite went down a bit. So yesterday 2/12/16 I took him in for blood work because my initial fear was his now hypo and it could be stressing kidneys that might now not be working so well anymore. Well I had them test the t4 for curiosity and it was still high at 43! So now my question is lets say a cat has severe hyperthyroidism and he had super high numbers how long before the thyroid concentration in his body would normalize? While T4 is less than it obviously was previously, it is still quite high. Would you recomend supplementation of small doses of methimazole to keep that number somewhat lower and keep testing every 30 days? How long will it take to normalize his T4 in a cat with severe hyperthyroidism with super high thyroid values? And when will I know if it has worked or not?

Dr. Mark E. Peterson said...

So the facility did scintigraphy to diagnose the thoracic tumor? Sounds like they didn't calculate dose based on tumor volume.. Did they only give 3 or 6 mCI? If so, that's unlikely to be enough given the high T4. These cats can be very difficult to treat and commonly need 10-30 mCi and sometimes repeated doses to cure them. Talk to your vet about starting methimazole fore 2-3 months. Then you can stop it for a week to see what the effect of the treatment had on the tumor.

Melissa Brown said...

They did not do the scintigraphy as the facility that did it does not have the equipment to do so. They gave him a dose based on his t4 which at that time was 54. They did say having the mass aspirated would tell if it had turned malignant or not. But from some of your blogs usually if it's malignant it will not respond to medication, and his is responding to the medication and it does so quickly. But I did not have the money for i-131 and the aspiration. So I had them do the i-131 and they gave him 6mci. They only give two doses as standard.

If I do restart him on Methimazole what dose would be recommended? Or is it start him back on 7.5mg and 7.5mg pm based on the current t4 reading or start him at a lower dose and see how it handles the t4? I assume I'll have to still test once a month to see where it's going. And starting him back on methimazole will not affect what the i-131 is doing?

Thank you!

Dr. Mark E. Peterson said...

Not possible to come up with a certain dose of methimazole. I'd pick a dose and recheck in 1-2 weeks and then adjust as needed. Then you will have to monitor every month or so and continue to adjust the dose (or stop it) as needed.

Melissa Brown said...

Saw the vet that did the i-131 today, Dr. Kelly Balog (she said she studied under your tutelage when getting into i-131) and she said that his thyroid nodule in his chest is extremely large. When there is ectopic thyroid tissue that is extremely large does that in itself indicate that it is malignant or has turned malignant? Have you seen any ectopic hyperthyroidism that had a very very extremely large adenoma and it wasn't cancerous but just had grown large over the many years the animal has had it?

Thank You!

Dr. Mark E. Peterson said...

If the nodule is truly ectopic (extra piece of thyroid), that would not increase the chance of malignancy. If it's a lobe that grew into the chest from the neck, that certainly would increase the chance of cancer.