I've been looking at old posts about Vitamin B12 blood tests. I read that blood tests are no use if you are supplementing, and even then, they only measure what's in the blood, not what's in the cells.
So, if I want to find out whether I'm deficient (after 16 years without a thyroid gland, and at least 14 years under-medicated), what do I need to do?
Do I need to stop my supplement for a while first?
Is there a partucular type of test that I need?
When I first tested B12 (taking no supplements) that gave me my baseline. I supplemented with sublingual methylcobalamin and when I retested some time later it showed over 1000. I adjusted my dose (take fewer days per week) and result now stays between 900-1000. So it looks to me that testing when supplementing tells you if you are taking enough or too much, so you can adjust accordingly.
Up to now I've only had serum B12 test done, I may do Active B12 at some point.
Which of those tests is the most important?
I don't know.
The advice I eventually received was to wait four months after stopping supplements. Personally, I waited until after three. If your GP tells you two weeks is long enough, it's simply not. My B12 deficiency history led me to see for myself that you need to leave it longer than two weeks. I initially had a B12 test with Medichecks and it came back very low (something like 92). I panicked and supplemented tsraight away, sending the report off to my GP, who only wanted to confirm the result by doing his own test. I stopped supplements for two weeks as per his advice. Of course, because I'd supplemented (3000 IU sublingual methyl B12 for five days), my results were skewed and came back as being 250 (still not great, but 'in range' so he didn't want to treat).
Sorry if this seems like I'm retelling my story - I just want to say that from personal experience I know it's definitely more than two weeks that's needed and that I waited three months eventually to be sure I was getting the true result I had started with.
Active B12 test rules out further complicating factors in the form of conversion issues, I believe, but it isn't currently accepted as a 'gold standard' test. Basically, serum B12 can show normal results but active B12 may be low (so the normal serum result may be made up of 'inactive' B12, which we don't use). Conversely, active B12 can be normal whilst serum B12 be low. This happened to me. What it means, I still have no idea.
Oh dear, that's all a bit complicated. I suspect that my GP will think it's easy, and I'll get whatever is the bog-standard test (if I get anything).
Maybe try joining the B12 deficiency group here on Healthunlocked?
Your GP will offer you one test - serum B12 - which may be all you need for a diagnosis. It's complicated by the fact that some people can have normal serum B12 but still have low active B12, or that you can be normal in both but the ranges are too leniant. Even with active B12, there's a 'grey area' of 'normal' where they recommend further testing if you fall into it.
If you're going through your GP, just accept the serum B12 first. As I say, that might be all you need. You should also push for the two tests making up the pernicious anaemia screen. Then if necessary you can do a private test for active B12 (NHS GPs won't do this - only St. Thomas' Hospital do this test in the UK on the NHS) and take it from there. One step at a time 
Adelew,
It takes four months for red blood cells to die off and renew which is why you should wait 4 months after supplementing if you want a baseline results.
That's helpful. I'm seeing my GP tomorrow about my blood pressure, but I'll ask him for a B12 test in 4 months time, and that will be just in time dor my next endo appointment. Then I can share the results with him - no doubt he'll be very interested - or not!
Dolphin5,
No reason why endo should be interested in B12 levels except for B12 deficiency being a cause of persistent unwellness in patients optimally dosed on Levothyroxine.
I believe it is quite likely, after 16 years without a thyroid gland that I lack B12 due o low stomach acid. So my feeling is that an endocrinologist treating people in my situation should be interested in more than T4, surely???
Personally I wouldn’t do without a vitamin I needed for 4 months
Endo unlikely to be interested anyway. Many don’t consider vitamins at all
Dolphin5,
If you have any of the conditions in Box 1. Some possible causes of persistent symptoms in euthyroid patients on L-T4 towards the bottom of onlinelibrary.wiley.com/doi... your endo is likely to say it's nothing to do with your thyroid and will expect your GP treat you or refer you to relevant specialists.
Ah ok, good to know. Like I say, mine went back to being low enough to o get treatment within three months, but had I waited the four I'm sure it would've been back to my original score.
You are not speaking of holding off on a multivitamin too for 4 months(?)
Clarrisa,
If you want a baseline B12 result you shouldn't take any supplement containing B12.
Multivitamin generally not recommended anyway.
Too little of things we need and stuff in them that we don’t need
If you have Hashimotos then avoiding any extra iodine is recommended and that’s in most multivitamins
My Internist told me to stop them, they got something right!
You can also test for levels of homocysteine and MMA via Medichecks
Using test of Homocysteine levels to check for low b12
Good explanation on homocysteine
Oral may be as good as injections
aafp.org/afp/2003/0301/p979...
B12 generally
drjockers.com/warning-signs...
Methylmalonic acid and B12
serum B12 measures the amount of B12 in your blood - it can be useful for telling if you have a problem absorbing B12 - less useful for saying if you are B12 deficient - which is about what is happening in your cells where B12 is actually used.
Waiting 3-6 months is actually about the amount of time that it generally takes for excess B12 to be removed from your blood by your kidneys.
If you don't have an absorption problem then you will be able to access quite significant stores of B12 in your liver - sufficient to mean that a lack of B12 in your diet takes years and even decades to manifest as a deficiency.
Things that can help with the diagnosis of an absorption problem is actually having a time line for B12 showing a drop over time, either in or without the presence of supplementation.
As well as an absorption problem you may also have a problem with metabolising B12 - active B12 can help with identifying this because it is looking at the amount of a specific form of B12 - TC11 - that is able to get from your cells to your blood.
There isn't a direct test for what is happening in your cells - but looking at some waste products that your body can't recycle if you don't have enough B12 - homocysteine and MMA - but both of these can also be elevated by other things so they need to be done in a context that checks what is happening with other things (eg folate levels and liver/kidney problems).
The most common cause of B12 deficiency is an auto-immune condition - PA - in which the body attacks the mechanism that allows B12 to be absorbed. If this isn't working properly then it also means that you can't effectively rebuild and use reserves of B12 in the liver as the mechanism for releasing it is to release it in bile into the ileum for reabsorption ... the mechanism that now isn't working. There is a test for this specific cause of B12 deficiency - IFA - but this gives false negatives 40-60% of the time - so it's good for confirming PA as the cause but not very useful for ruling out PA as the cause. This is the only test in relation to B12 deficiency that doesn't require 3-6 months refraining from B12 supplementation - only 2days - 2 weeks (depending on the assay method).
It seems to be a bit of a minefield. Part of my interest now is that I've learned that people with no thyroid gland often have a problem absorbing B12 due to low stomach acid.
I think I'l just have to wait several months, and then see what I can get my GP to test.
the reasons why people who are hypothyroid seem to have problems absorbing B12 are far from clear and I think the possible reasons (as there is probably more than one mechanism involved) have been confused.
Low stomach acidity suggests that you have PA and low stomach acidity will affect multiple vitamins and minerals. PA is an autoimmune disorder. Its quite likely that if you have one auto-immune condition (eg hashi's) that you will develop others - and one theory for the high cross over between PA and hashis is that the cells that form the gut develop from the same cells as the thyroid meaning that there is quite a high chance that antibodies that affect the gut can become antibodies that also attack the thyroid and vice versa. This is known as thyro-gastric syndrome ... and it will also affect your ability to absorb levo.
Another possible explanation for the high incidence of absorption problems and thyroid problems is that hypothyroidism will result in muscle weakness, which can affect the muscles in the wall of the gut, meaning that the transit of food through the gut is slower. This can result in higher rates of bacterial build up, such as h pylori or SIBO, which will inhibit the absorption of minerals and vitamins in general and can affect B12 as a result. Generally this seems to affect iron levels first.
There are other theories out there as well.
Well, who knows? I've never had hashis, so there's nothing like that going on. I have been undermedicated for many years, and had gut problems, so all sorts could be going on there. It will probably never all become clear, but I keep exploring, one day at a time!
vitamin B12
Query re iron deficiency blood tests when hypothyroid and low B12
Refused vitamin and nutrient blood tests 
blood tests
Seven blood tests anyone over fifty MUST have
