Supplements and blood tests: Hi I am not feeling... - Thyroid UK

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Supplements and blood tests

MiniMum97 profile image


I am not feeling great and have got myself confused. I am testing Vitamins D, B12 and B9, Magnesium, Selenium, Zinc, Ferritin, Sodium and Calcium tomorrow.

I take iron, vitamin d, fish oils, selenium, magnesium, vitamin c.

I also take a b complex but stopped this a few days ago as it contains biotin. I usually stop everything for 4-5 days but got myself confused.

Should I have left off any of the above before blood testing tomorrow? I can't remember at all.

Many thanks

15 Replies

I think that for iron, it is best to leave it out for a week. I got some very peculiarly top / over range results when I only stopped for a couple of days. For B12 it is almost impossible to tell unless you have stopped supplementing for several months. This is one reason why low B12 is rarely noticed by GPs as they rely on blood tests rather than symptoms. For Biotin I am guessing that after 5 days you are probably ok though. Just be aware that your ferritin may be a bit skewy.

milkwoman profile image
milkwoman in reply to HLAB35

Hi - can you explain the effect biotin has on a ferritin result?

MiniMum97 profile image
MiniMum97 in reply to milkwoman

Biotin can give false blood test results on any blood tests that use an immunoassay. Thyroid blood tests often use an immunoassay but so do blood tests for other things. It’s therefire best to avoid biotin for 4-5 days before a blood test.

HLAB35 profile image
HLAB35 in reply to milkwoman

The biotin should not effect the ferritin result after a 5 day break but Iron supplements can effect ferritin quite a lot as I seemed to have 'overdosed' on iron for a test I had a while ago, then, for the next one I stopped iron for a week and my results were more realistic.

milkwoman profile image
milkwoman in reply to HLAB35

I’ve supplemented iron and biotin off and on and no doctor has ever said anything. I’ve struggled with low ferritin values for a long long long time. My iron profile results have always been in the “ normal range” with ferritin either below range or near below. My last test showed a %sat up near top of the range. I’m seeing my GP on the 18th. I’m going to stop biotin and iron supps and ask to be tested again. I dont think it will make a difference as I truly believe I have an absorption issue and probably need injections. Iron deficiency would explain how no matter what I do, I cannot seem to overcome my symptoms of fatigue and weakness. Thanks to everyone who has responded - I now feel better educated to talk about this with my physician.

MiniMum97 profile image
MiniMum97 in reply to HLAB35

Well I took iron yesterday and my ferritin was tested this morning and had only moved up from 24 to 28 since the last test so it doesn’t seem to have had an effect in this case. I intend to stop the iron and retest so it will be good to see the comparison.

HLAB35 profile image
HLAB35 in reply to MiniMum97

I'm finding that I absorb iron well these days as I follow a few simple rules... Take with Vit C and away from anything that'll bind with it... eg. Tea, Cereals, Garlic or compete with it eg. Dairy. I had a result that was huge for me, then I read that I really should not have supplemented in the week prior. My next result was far more sensible.

Your B12 won’t be accurate, if you’ve supplemented at all in the past four months. You’re best to go by symptoms of low B12.

MiniMum97 profile image
MiniMum97 in reply to Tnhundley

Hi I am more looking to check the effect if supplements rather than baseline levels.

Tnhundley profile image
Tnhundley in reply to MiniMum97

I understand that, but if you are supplementing, a B12 test will show high levels no matter what.

For example- a friend of mine was bedridden from her B12 deficiency. She didn’t know she couldn’t absorb B12 properly and was supplementing. Her levels were very high but she kept getting sicker.

She got B12 shots -she self injects every day- and is now able to care for her children again. Her body can absorb it from shots.

If your levels are high and you’re feeling great, then you’re fine. But if you feel crappy and your levels are high but you’re still having any if these symtoms, please consider shots.

MiniMum97 profile image
MiniMum97 in reply to Tnhundley

Yes I understand that thanks.

SeasideSusie please see my post above. I opted not to take the private test to test the above vitamins today (just went along for the NHS tests - thyroid and ferritin). I’ll book in for another blood draw next week. I am looking to test the effect of the supplements I am taking (plus a few extra vitamins as you can see). Please can you advise if you can when I should stop supplements prior to testing. Many thanks

SeasideSusie profile image
SeasideSusieAdministrator in reply to MiniMum97


If you want to know if you are supplementing at the right level, you can continue taking your supplements up to the day before, fhen on the day of the test take them after the blood draw.

The exception is Biotin, either in a B Complex or as a stand alone supplement. This is because biotin is sometimes used in the testing procedure, but we wouldn't know if the lab carrying out our test uses it, so it's safer to leave biotin off for 3-5 days before the test. If biotin is used in the testing procedure and we continue to take a biotin supplement then it gives false results.

One other exception is iron supplements if testing iron/Ferritin and I understand that should be left off for a week (and don't eat liver for a week).

Thank you that is a perfect reply 😊

For info on why it is important to stop biotin before doing testing for all sorts of things, see these threads :

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