Feline hyperthyroidism- T3 suppression test

Re: Feline hyperthyroidism- T3 suppression test

Ian Ross — Thu, 29 Nov 2012 19:09:52 GMT

Kate,

I have just had a similar case. A young cat (4 y o) (which was the one doubt in my mind) severe weight loss, amazing appetite (climbing onto dining room table to steal from plates, climbing into fridge etc) which occured at the same time as weight loss, no diarrhoea. TT4 normal and Free T4 low. Then started with loose stools, and suspect is an IBD. The owners had had an older cat with hyperthyroid and said all the signs were exactly the same! This one didn't have a palpable goitre or particularly high heart rate.

Ian

Re: Feline hyperthyroidism- T3 suppression test

Kate Richardson — Thu, 29 Nov 2012 11:15:18 GMT

[quote user="Martin Atkinson"]Another point though, how much of an indicator does everyone find raised liver enzymes to be in hyperthyroid cats?: I've found it almost a universal indicator but had one last week with nailed on symptoms, a palpable goitre and a TT4 of 167 yet its liver enzymes were normal.[/quote]

Most of my hyperthyroid cases have had elevated ALT levels (mnore than other liver enzymes) but have seen cases with normal liver enzymes.

Have never had to do a T3 suppression test; generally either run a free T4 if strongly suspicious or retest and reassess clinical signs/weight etc 4-8 weeks later and confirm diagnosis or not. Have had several cases where I would have put money on them being hyperthyroid but they turn out not to be and have had internal parasites/IBD/malabsorptive git disease/hepatic disease/lymphoma/EPI instead. The only thing in these cases that went against them being hyperthyroid was none of them had a palpable goitre though, but most of the other classic hallmarks.

Re: Feline hyperthyroidism- T3 suppression test

Robin Grimmer — Thu, 29 Nov 2012 10:53:10 GMT

[quote user="Martin Atkinson"]

[quote user="David Mills"]In my experience, t4 is one of those measurements that people get terribly hung up on the numbers about.[/quote] Indeed we are all at times guilty over treating numbers rather than symptoms/condition. IME fluctuations with 'normal' range can be dramatic, I have a couple of cats which fluctuate from below to well above normal with minor changes in dosage which it is difficult to increment given the fact we're not supposed to break the tablets. If surgery/change of medication/radio-iodine are not treatment options I've now stopped busting a gut over it, given that compliance may always be an issue, if the patient looks normal then I go with the flow and maintain an average dose.

[/quote]

Felimazole is my drug of choice as there are so many dose combinations. My own cat is on just 2.5mg once daily as she has concurrent CKD and this just keeps her in the upper end of the normal range.

Re: Feline hyperthyroidism- T3 suppression test

Martin Atkinson — Thu, 29 Nov 2012 10:40:46 GMT

Re: Feline hyperthyroidism- T3 suppression test

David Mills — Wed, 28 Nov 2012 21:20:44 GMT

There are a number of papers out there that detail hypert4 with concurrent renal dz, and they indicate that with compatible clinical signs, t4>40 is hyperthyroid. In my experience, t4 is one of those measurements that people get terribly hung up on the numbers about.

Different labs will quote different ranges for all sorts of things depending on their normals - for instance, Idexx's creatinine normal for creatinine is too high compared to IRIS normals (150 is IRIS - Idexx is ?190); many lower limits for PCVs for cats are far too high from other labs. Any lab normals are necessarily calculated from a small sample size, usually around 100 animals. Around 1 in 20 parameters measured will be abnormal due to statistical error in defining lab normals i.e. will be normal for the individual tested but abnormal when compared to the 'normal' cohorts.

Re: Feline hyperthyroidism- T3 suppression test

ashvetenry — Wed, 28 Nov 2012 19:12:32 GMT

"however now the generally accepted upper normal limit has been lowered (40nmol/l) this takes care of most of them" Is this the now accepted range?

Re: Feline hyperthyroidism- T3 suppression test

Graham Bilbrough — Wed, 28 Nov 2012 14:57:34 GMT

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In general, my preference is for the new VfT4 assay: it is more precise, faster, cheaper and with comparable accuracy (for deciding whether a dog has hypothyroidism or not or deciding whether a cat has hyperthyroidism or not).

There are two exceptions:

1. For a dog with a need to assess thyroid function when there is concurrent significant non-thyroidal illness (NTI)—this is a dog with more profound symptoms than you would encounter with “just suspect hypothyroidism”!—then I would prefer the free T4 after E.D. Today, we have more familiarity with how the older assay performs with concurrent disease.

However, we should keep in mind that free T4 after E.D. is not as good as the 1990’s papers made it look. Indeed, many people believe the free T4 is independent of euthyroid sick syndrome, but if the animal is sick enough, the free T4 can be low as well. From my experience, over 40% of dogs with severe NTI have free T4 (after ED, Nichols-successor reagent) that would be consistent with hypothyroidism.

2. 2.If the VfT4 result is elevated, and the dog is not receiving synthetic T4, I would suspect T4 autoantibodies and would then use the free T4 after E.D.

On your somewhat separate subject, I still regard the TSH stimulation test (using re.hTSH) as the gold-standard to diagnose canine hypothyroidism. But, that is really pricey and there seems to be a constant struggle to obtain the required injectable product.

Thanks

Graham

Re: Feline hyperthyroidism- T3 suppression test

ashvetenry — Wed, 28 Nov 2012 13:04:33 GMT

Hi Graham,

thank you- I have spoken to Idexx on a couple of occassions about this, I have always been put through to a clinical advisor who haven't readily been able to advise which of the freeT4 tests to request, your advice and knowledge is appreciated. If I understand correctly VfT4 is the way to go, with little indication for fT4ED any longer.

As an aside, does Idexx have a view point on the need for dynamic thyroid testing in the dog? Is it considered completely unnecessary now with T4/TSH and autoantibodies.

Many thanks,

Ashley.

Re: Feline hyperthyroidism- T3 suppression test

Graham Bilbrough — Wed, 28 Nov 2012 11:56:45 GMT

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Hi, Ashvetenry

IDEXX Labs UK currently offers two free T4 assays:

1. 1. free T4 _after_ equilibirium dialysis (e.d.) and

2. 2. the new Veterinary free T4 assay.

For the after e.d. assay, in the UK, we use what I would call the successor to the Nichols Institute reagent. It is not the exactly the same system as was used for the original publications and, in my opinion, performs differently. The “original formula” Nichols reagent is no longer available. The post-e.d. part is done with a radioimmune assay (RIA) and these are far from perfect: inherently imprecise, radioactive materials, expensive etc.

Our new Veterinary free T4 assay—a vast improvement on the old analogue assays used elsewhere—is more precise and (importantly) much cheaper. Its accuracy for calling cats as hyperthyroid or not (taking a technetium scan as the gold standard) is every bit as good as the post-e.d. assays we evaluated. We have the results from a nice clinical study, that involved Mark Peterson, that I can talk to you about on the ‘phone, but not print as we await publication in a peer-reviewed journal. In reality, none of what is offered today is published in detail.

According to Richard W. Nelson, DVM, DACVIM,* of University of California, Davis: “Results of recently completed clinical trials clearly show the new Free T4 Test is as accurate as the Free T4 (ED) Test for assessing thyroid gland function. Given the superior precision and faster turnaround time, the Free T4 Test should be the methodology of choice for most patients.”

* By means of full disclosure, he serves on an advisory board for IDEXX Labs and on occasion receives compensation.

Free T4 is a complex area of testing and I would be happy to chat on the ‘phone.

All best wishes

Graham

Re: Feline hyperthyroidism- T3 suppression test

Mark Holmes — Wed, 28 Nov 2012 11:29:10 GMT

Alkp and Alt are a useg guide but I have had it go both ways. Lots of cats hyperT4 yet liver figures normal. Also high liver figures presumed to be due to thyroid disease turn out to something else.

Re: Feline hyperthyroidism- T3 suppression test

Martin Atkinson — Wed, 28 Nov 2012 10:46:28 GMT

Also rarely have an issue with diagnosis in cats by symptoms and T4 alone especially once I realised that some hyperthyroid cats had T4 levels in upper end of normal and not all have palpable goitres, however now the generally accepted upper normal limit has been lowered (40nmol/l) this takes care of most of them. I've done an occasional Idexx Free T4 by immunoassay and picked up one hyperthyroid cat with equivocal TT4 results but never thought to question the validity of the test and never felt the need for T3 suppression.

Another point though, how much of an indicator does everyone find raised liver enzymes to be in hyperthyroid cats?: I've found it almost a universal indicator but had one last week with nailed on symptoms, a palpable goitre and a TT4 of 167 yet its liver enzymes were normal.

Re: Feline hyperthyroidism- T3 suppression test

ashvetenry — Wed, 28 Nov 2012 09:23:26 GMT

Thanks everyone

Re: Feline hyperthyroidism- T3 suppression test

Ian Ross — Wed, 28 Nov 2012 08:47:52 GMT

If I have enough clinical suspicion and a high normal Total T4 (anything less than 30 tends to rule out) then I get a free T4 by ED done on the same sample, it saves stressing the cat out for multiple blood samples and doesn't upset the owners having to bring the cat back in for more bloods.

In a study published in 2001, Mark Peterson found that of hyperthyroid cats, 91.3% had high total T4, whilst 98.5% had high free T (by equilibrium dialysis). Of the 80 hyperthyroid cats with normal total T4, 76 (95%) had high free T4. He felt that the T3 suppression test and TRH stimulation test both had disadvantages. For the cats with "milder" disease, free T4 was still high in 93%. However, his cases were selected as being hyperthyroid based on clinical signs, goitre, high serum total T4, or in (80 cases) T3 suppression or TRH stimulation. Maybe there are a population out there that would not be diagnosed by this route.

I tend to do total T4, followed by free T4, and if both of these are negative and I stil think its hyperthyroidm, then I retest in a month, or hunt for non-thyroid illness.

Re: Feline hyperthyroidism- T3 suppression test

Michael Woodhouse — Tue, 27 Nov 2012 23:17:15 GMT

Hello and welcome!

I'd never even heard of the test but NWL have a protocol online. We send most of our stuff to Idexx and I don't remember seeing the protocol there, but I never looked for it.

It does sound like a lot of work and unless you hospitalise the cat there could be issues with compliance, and then they need piling 3 times per day. Then the product, Tertroxi, isn't available from the wholesalers and isn't listed on the BNF. I suspect by the time you have found some and imported it, stressed the cat to hell, it probably isn't worth it!

In some of these equivocal cases re-sample in a month gets you your answer. There's not a great rush to diagnose.

I'd actually prefer a single sample test to diagnose HAC, Addison's etc asI hate functional testing. For mixed vets they are too tying. Loving the point ACTH for equine Cushings.

Re: Feline hyperthyroidism- T3 suppression test

Rob Loxley — Tue, 27 Nov 2012 22:54:12 GMT

Have never found the need in a cat, T4 and freeT4 seem to do enough for me