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.

Unknown 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.

chocochoco 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.

chocochoco 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.

dorymbasd 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.

Dyana Keen 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.

Dyana Keen said...

Thanks, Dr. Peterson.