Thyroid UK
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Blood test for B12. I am taking a supplement - when is best time for blood test?

I have been taking 1000ug of B12 for just over 2 months. It is the least effective sort (ie not the methyl one) and in tablet form. It is described as slow release. I normally take it in the evening before I go to bed. When is the best time to take a blood test? I am keen to see how I am doing as previously my B12 was at the bottom of the normal range.

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Hi. I seem to remember a discussion about this some time ago, and the message that I took away was that you really need to stop the B12 about 5 weeks before a test to get a true picture. Of course, I could have remembered this incorrectly, and am unable to find where I got this number from! Looking online, wikipedia says the elimination half life is 6 days, so I guess you should probably stop at least a week before the test to get a meaningful result, and longer to get a absolutely accurate result. Of course, the B12 test is not truly accurate anyway - but that's another story...!!!

I understand that all forms of B12 will show up the same in the blood test.

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I wonder if it might be ok not to bother with a test unless you really want one, as the supplement is cheap and the excess will be excreted if your body does not need it. I use patches bought on amazon and find them good, as you absorb better through the skin than the stomach if you have problems with intrinsic factor.

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If you are unable to absorb B12 the tablets will pass through your system and not make a difference which is why most people who are B12 deficient have jabs or use a B12 spray. These sites have a wealth of info that you might find helpful.

pernicious-anaemia-society....

b12d.org/

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