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Blood test showed enough Aza in the blood

So I had a phone call from my IBD nurse today saying my blood test results have shown I have more aza in my blood than I need, so from now on I only need to take 20mg a day now.
I find this a bit strange. 20mg is such a small dose! Everyone already comments on 40mg being a small dose.
So how much do you even need in the blood?
I don't understand why some people need to be on high doses and I'm on a tiny dose, whats the point in it at all?
I suppose at least that means I'm absorbing it properly, and I suppose its good to be on as little meds as possible.
I just started Aza a couple of weeks ago and I had some genetic testing done because everyone absorbs/metabolizes Aza differently and they wanted to make sure they were giving me the correct dose. Perhaps your body only requires a small dose. My test results showed "normal metabolism" so I am on 75mg a day and I weigh 63 kilos.


Do you remember if they did the (TPMT) test before you started AZA? It's usually a test they all do before giving us that type of medication. I have no idea if the following hypothesis makes sense but my guess would be you might have some mild form of Thiopurine Methyltransferase Deficiency. That would mean you body can't "clear" the thiopurines properly and hence, have a predisposition for it to be more toxic than it is for someone that does not have a reduced TPMT activity.
Thanks, yeah I am TPMT deficient. But I wasn't completely sure what it meant - just that I couldn't have high doses. But I don't see how only being on 20mg can do me any good. I weigh 45kilos.