Thyroid UK
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Timing of blood test when on Methimazole

I realize this is a bit different but I would appreciate your opinions.

My cat is hyperthyridic! I give her Methimazole (that's similar to Carbimazole) twice a day, and have taken her to the vet every month for a blood test to get her dose right - pretty much what we humans have to do. When I took my cat last month I was chastised for giving her a pill that morning, I was told to go away for 6 hours and return to collect her - I did that. So this morning I took her for her monthly blood test and when asked when did she last take her pill, I said, last night. "OH NO!" said the vet, a different one this time, "We need to take the blood four hours after the pill". So, here I am sitting waiting for the phone again, my day completely taken up because I live a long way away from the vet - and waiting to collect my cat.

So, what are the rules exactly, does any one know? It looks as if Methimazole doesn't hang around in the system like T4 does. Shouldn't my cats system be empty of medication to measure what is going on when no meds are present?

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As someone with Graves disease, who was on Carbimazole, I was never told to not take my medication before a blood test. That said, vets seem to know much more than GP's!

I think Carbimazole has a 7 hr half life, if that helps?

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Thanks for your info Silver Fairy. I did a bit or research and went back to the vet. It seems that they try to hit the center of the half life of Carb/Meth, which, as you say is about 7 hours. In the literature you can see 12 hours and 6 hours. Their measurements are probably calibrated for this mid point. This is because if you measure too close to the dose (<4 hours) you are measuring the medicine but if you measure too far away (>12 hours) you will get the disease level. If you hit it in the middle and you have a calibration curve then you can make sense of the reading.

It is different with Levo and NDT because the T4 hangs around for a week but the T3 is highly variable (which is probably why doc's are reluctant to measure T3).

And yes, vets are very good in this area, my cat has to go in every 4 weeks and if I don't take her, they call me! I didn't return to a stupid Endo and, one year later she still hasn't noticed, fortunately during that time I found a good one.

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