Friday, July 31, 2015

Diagnosing Feline Hyperthyroidism: Not Always as Simple as One Might Believe

Earlier this year, Dr. Mark Peterson participated in an Endocrinology course organized by the American College of Veterinary Internal Medicine (ACVIM). An overview of his lecture on "Diagnosing feline hyperthyroidism" was summarized by Dr. Jennifer Garcia and published in the July 2015 issue of Veterinary Medicine. To access this article online, click here.

Diagnosing feline hyperthyroidism: It's not always as simple as it seems

Don't rely too heavily on T4 concentrations since cats can have a false elevation.

In his presentation, “Diagnosis of hyperthyroidism: A critical evaluation of our current available tests,” Mark Peterson, DVM, DACVIM, discussed some of the pitfalls in relying too heavily on thyroid (thyroxine, or T4) testing alone. While a total T4 concentration will be enough to make an accurate diagnosis of hyperthyroidism in more than 90% of cases, he warned to always pay attention to the clinical signs and physical examination findings. There are cats that can have a false elevation in their T4 concentration, so supportive clinical signs as well as a palpable thyroid nodule will help rule in or rule out the diagnosis.

When it comes to successfully palpating for evidence of a thyroid nodule, Peterson detailed a few of his favorite techniques:
  • Stand behind the cat with the cat facing away from you—the cat feels less stressed if it can’t see you. Peterson also puts the cat in a basket with a towel so the cat feels more secure and is less squirmy. Use your thumb and index finger to gently run the length of the trachea from the larynx to the thoracic inlet.
  • Alternatively, with the cat in the same position, turn its head to the left and palpate. Repeat with the cat’s head turned in the other direction.
Examine the cat from behind, with the cat facing the owner.

For patients in which a thyroid nodule can be palpated but there are no clinical signs and there is no elevation in T4 concentration, he recommends monitoring signs at home and rechecking the level in six to 12 months.

Peterson also noted that there are different cut-off values from laboratory to laboratory. This means that a T4 concentration that is normal at one laboratory, may actually be elevated at another. This serves as another reminder of the importance of the physical examination and clinical signs when trying to diagnose hyperthyroidism.


DonaLa said...

After having lost 2 feline family members to hyperthyroidism within the past 2 years, I have read your blogs with great interest. If I may, I would like to comment in stressing the dire importance of monitoring blood pressure as well in feline hyperthyroid patients as the two often times run concomitant. I lost my one feline family member to a massive stroke which I strongly believe was due to unacknowledged high blood pressure on the part of the U.S.A. veterinary community as a whole.

Dr. Mark E. Peterson said...

Hypertension certainly can develop in hyperthyroid cats, but the prevalence is relatively low (10-15%), It most commonly develops 3-6 months after treatment and that is the most important time to monitor and treat.

Lookit said...

Dr. Peterson,

I have been reading your articles with great interest, as I have a 10-year-old cat recently diagnosed with severe hyperthyroidism – T4 level over 23 when first tested. Her vet put her on 5 mg/daily methimazole, but she became very lethargic and began excoriating her face badly about 3 weeks in. After almost a month, I decreased the dosage by half for three days right before her follow-up appointment hoping to alleviate the side effects.

We only managed to bring the T4 level down to 17 before methimazole had to be discontinued last week. Her vet has not encountered such a case before. She has a heart murmur, but her other levels seem good – kidney, liver, etc. I am wondering if she could still be a candidate for radioiodine therapy despite the inability to lower her T4 level in advance of the procedure, if she can pass the other screening tests.

BTW, I started giving her 250 mg daily of L-carnitine after she went off methimazole and it does seem to have restored her appetite and given her more energy.

Thank you so much for all of the information you have shared on your site!

Dr. Mark E. Peterson said...

Of course radioiodine can be used (and probably should) in this cat. I'd recommend thyroid scintigraphy to see what's going on and make sure that the underlying cause isn't huge thyroid disease or even thyroid carcinoma.

Lookit said...

Thank you so much for your swift reply. From talking to others with hyperthyroid cats, it seems vets in the Austin, TX area lean very heavily on methimazole as the preferred treatment. I have put a call in for a referral for thyroid scintigraphy.

I wish my high school biology teacher had explained things as lucidly as you do. I hated science classes back then but now I am fascinated with feline endocrinology.

Mark said...

I am wondering what to do if T4 is elevated but no thyroid nodule is apparent and only vague signs that could possibly point to hyperthyroidism?

My cat has had elevated T4 on two blood tests 3 months apart (worse on the second) There werent particular signs of ill health, but he does meow at night a fair bit and has lost a little weight in the last few years. He is 13 years old.

The vet wants to commence carbimazole.


Dr. Mark E. Peterson said...

I would ideally use thyroid scintigraphy to make the diagnosis or exclude it.

If not feasible, then I'd do a complete thyroid panel (T4, free T4 by dialysis, and TSH). If the free T4 is normal and the TSH is not undetectable, then your cat is likely not hyperthyroid. Some cats run a slightly higher T4 normally.

Mark said...

Thanks so much for your help!

Rachel said...

In your experience if a cat who is hyperthyroid but has never vomirred before but who has started vomiting 4 days after starting y/d diet - is this the diet?

Dr. Mark E. Peterson said...

Highly unlikely.

Anonymous said...

Hi Dr. Peterson - my cat started Vidalta (carbimazole) 10mg daily for mild hyperthyroidism. About 12 days into treatment she started to scratch inside her ears. She stopped taking the tablets but 5 days later she is still scratching to the extent she has sores inside her ears that she keeps re-opening with more scratching. I've put a cone collar on her to stop the scratching, but when can I expect this side effect to go away now that the medication has been stopped? Can a vet do anything to help? Thank you!

Dr. Mark E. Peterson said...

This is an allergic reaction and means that you have to select another form of treatment.

Anonymous said...

Thanks for your reply! She has stopped the carbimazole 5 days ago, but is still scratching - how long could I expect the itching to last for her and can my vet do anything to help the itching in the meantime?

Dr. Mark E. Peterson said...

Should resolve within the next week or two.

Unknown said...

My cat has recently been diagnosed with hyperthyroidism (7.9 ref .8-4.0.)She is 17 years old. Is this too old to be considered for the radio iodine therapy?

Dr. Mark E. Peterson said...

No, I treat many cats that are geriatric, in the range of 15-19 years of age.

Unknown said...

Ginger had a slightly high T4 of 2.9 one and a half years ago. Oue vet said then that she might go hyperthyroid. So, we did another T4 and Free T4 6 months after that in August of 2015 and both tests were perfectly normal with a T4 of 2.5 (reference range 0.8 -4.7) and Free T4 of 1.6 (reference range 0.7 - 2.6). Then she had her yearly annual physical on August 1st of this year 2016 and her T4 was 5.0 and her Free T4 was 3.0
Her blood pressure is perfect and all her other blood work is within range.
I started her on Felimazole on August 2nd and then started calling Carol at your office. I stopped the Felimazole on August 15th and we are scheduled for a repeat T4 at our regular vet this Thursday August 25th. If that test is elevated (over 5.0?), then she has an appointment for the radioactive iodine treatment at your New York City office on Tuesday August 30th. Is over 5.0 on the T4 what you recommend to get the treatment?
Am I rushing things? My regular vet said she is glad we caught this early.
She just turned 12 and is a Calico and has always been "expressive" with her feelings. She has been drinking and peeing a little more for about a year.
I just worry about her staying away from me for 5 days as she has never been away from me and she is very affectionate with me (and strangers too, so don't worry), and the long drive there and back, and the cost which I have not quite saved up enough for, but almost.

Dr. Mark E. Peterson said...

A T4 of 5 is diagnostic. Better to treat hyperthyroid cats when young rather than geriatric if we can.

Unknown said...

Thanks, Dr. Peterson.

Merry said...

Hi Dr. Peterson,

I was hoping for some insight on dosage for radioactive iodine treatment. My cat was first diagnosed with hyperthyroidism in September of 2016 with routine senior bloodwork. Since his T4 was borderline at 4.5 and he was showing no visible symptoms I decided to hold on treatment and monitor him for 3-6 months or until he exhibited any symptoms. In February he began to consistently lose weight (1lb total loss from September to March) so we returned to the vet for bloodwork. T4 levels increased in 6.4 so it was time to pursue treatment. Kidney values were good so we went ahead and scheduled his i-131 treatment.

He is scheduled for May 1st, but the vet performing the procedure presented some newer research and thus more things to think and worry about. It is suggesting using a lower dose of radioactive iodine (2mci) which has less risk of making cats hypothyroid after treatment but was only 90% effective and could require a second dose. For the past 5 years this clinic has used a higher range of dosing based off T4 values with the lowest offered dose 3.5 Mci. The vet felt comfortable using either dosing option.

I'm not sure which way to go and am looking for some advice. I would like to avoid a second treatment for so many reasons, but if his values are low and the tumor is small, I don't want to create another problem that cannot be fixed and will need to be addressed with medication forever.

I was wondering if there are certain cats you would recommend these low doses for (2mci) and what type of clinical, lab or imaging criteria they tend to present with? And if you have used this dose, how are your results? This vet has not used this low dose yet.

Thank you so much in advance for any advice or assistance.

Dr. Mark E. Peterson said...

I wrote the paper your vet is talking about. I'd give 2 mCi or less for a cat with a T4 o only 6.4.

Zebra said...

Hi Dr. Peterson,

Thank you so much for all your research into feline hyperthyroidism as your papers usually show up prominently in feline hyperthyroidism web searches.

My (> 14 year old) female cat was recently diagnosed via routine "wellness" bloodwork with a Total T4 of > 8, and Free T4 4.6, and T4 by dialysis 61.8, all markedly elevated. She has always had normal thyroid tests, and since her normal bloodwork last year she actually had weight gain with no discernible appetite changes. My vet stated he did not palpate an enlarged thyroid, although that was before the bloodwork coming back and part of his normal physical exam. But I do wonder if he would be able to palpate it if we are actively looking for one. I think my question to you is that both my vet and myself cannot come up with any symptoms of hyperthyroidism, and I came across one of your compendium teaching papers for vets dated in 2013: .

To add to the context, I asked my vet if his T4 lab test had changed this year, and he acknowledged it did indeed change this year, and according to what I read in your paper, it may be prudent to retest her T4 using a different assay. However, it does seem her labwork is markedly high with all values being positive. I guess it was unclear to me if being asymptomatic was more important than all the values coming back markedly positive. Currently, I persuaded my vet to order a retest using a different assay, but I'm also thinking that maybe I should just skip the retest, and possibly move on to a thyroid scintigraphy since most likely she is hyperthyroid. My preferred treatment would be radioactive iodine which is best done with a scan first to adjust optimum dosage anyway.

I would be grateful for any input you may have regarding my cat.
Thank you!

Dr. Mark E. Peterson said...

We see cats with minimal symptoms all of the time, and those cat have clearly high thyroid values and definitive thyroid tumor disease on thyroid scanning.

Repeating the thyroid tests is never a bad idea, but I'd be surprised if they did not confirm hyperthyroidism. Your cat should be treated soon (within the next 6 months), but obviously there is not an emergency