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Total T4 or free T4

by Asimina Kalaitzi
(Greece)

Reader Question:


Hi Dr. Coates,

I have a soon to be 13 year old female cat that suffers from GI (gastrointestinal) problems and is also diagnosed with HyperT.

Diagnosis was based on a high total T4 three months before treatment and also on a high free T4 three weeks ago (different vet).

We started treatment with methimazole 1.25 twice a day. After 3 weeks we retested both total and free T4. Total T4 was found below the low limit. Free T4 was found lower than three weeks ago but still higher that the upper limit.

My first vet thinks that total T4 should be considered, that my cat is now hypoT and that we should lower the dosage immediately so as to avoid kidney failure. New vet things that free T4 is the accurate hormone and that we should increase the dosage or at least keep it the same.

I am so confused. I do not know what to do. If my cat has another disease like IBD would that affect her hormones levels and which one, total T4 or free T4.

Kidney values were found OK, ALT was elevated.

I would appreciate an answer.

Best Regards,

Asimina

Suggestions from Dr. Coates

Hi Asimina,

I use T4 not free T4 to monitor cats receiving methimazole, and I believe this is relatively standard for two reasons:

  1. T4 testing is less expensive.

  2. A certain percentage of cats with illnesses that don’t involve the thyroid gland (in addition to being hyperthyroid) can have elevated free T4 levels.

Here’s a link to a relevant paper.

So, yes a condition like IBD could be playing a role in your cat’s disparate test results. It is also very important that your veterinarian(s) take into account your cat’s clinical signs when making methimazole dosing recommendations.

Best of luck,

Jennifer Coates, DVM

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