Thyroid UK
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What is the maximum limit for B12?

I had my B12 tested in April and it was 484. I decided to take B12 supplements after reading B12 should be a minimum of 700. I've taken the supplements for nearly 3 months and had a blood test again (while on supplements, I think it's more accurate if I'm off the supplements for a month before having a blood test) and its now 1374! A big jump. Is this too high or ok?

Thanks x

FYI I have been diagnosed with hypothyroidism and am taking T3 only.

8 Replies

It's fine, high B12 is to be expected when on supplements or injections.

It is only an issue to be investigated further if you have a very high level without ever having had any supplements, see this article:

H x


Thanks H. That's good to know. I don't think that the latter would be the case with me, as I've had my B12 tested a few times before supplements and it was always around 500.

N x


The reference ranges for B12 vary a lot from country to country. You might find this link interesting :


Hope this is of interest. :)


Thank you humanbean. I read about Japan, but I didn't know what the other countries ranges are.

It's interesting as in Japan the upper limit perhaps is 1300 pmol/L. My B12 was measured in ng/L. 1300 pmol/L equals 1761 ng/L!


I have PA and at times my B12 level has been 2,000. (injections)


I have P.A. and my levels are also 2000+ on monthly injections.


Thanks Shaws and Margo. Do I continue with my B12 supplements or stop now?


It's more about how you feel, if they're doing something for you then keep taking them. I also need levels over 2000 to feel well (injections).


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