Thyroid UK
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Stop supplements before doing blood tests?


I've ordered thyroid and vitamin (B12, ferritin, fplate and D3) test from medichecks. I've just read on a post today that you should stop iron supplements for a week before you test your ferritin to get any iron out of your system. As I also supplwmwnt D3 and good vitamin B complex daily, does anyone know if I also need to stop these supplements for a week before I do the bloods?

Advice much appreciated. I am new to all this and was diagnosed with Hashis three weeks ago. Having gone gluten free a couple, of weeks ago and started a shedload of supplements, I do feel much better. I'm reluctant to stop the D3 and B vitamins for a week if it's not necessary.

Many thanks in advance.

4 Replies

Kipsy Bluebug is the iron guru and she always advises to leave off iron supplements for a week, so I would definitely do that.

B Complex - they usually contain Biotin (B7) and biotin can give false readings for thyroid tests so it's best to leave that off for a few days.

D3 - City Assays (Birmingham's NHS City Hospital who do home fingerprick tests for Vit D) say you don't need to stop taking D3 for any length of time, just don't take it on the morning of your test, take it after the blood draw.

By the way, as you're taking D3, are you also taking it's important cofactors, particularly K2-MK7 and magnesium


Huge thanks SeasideSusie- such an informative and helpful reply. I will follow that advice about stopping iron and biotin for a week and not taking D3 on the day of testing. Thank you!

Thank you for the link too- I've read it and will get some vitamin K asap and think about boron too. I'm taking magnesium and zinc already. They seem to think vitamin A is best sourced from food although I wonder if that applies to Hashis. It's all getting so expensive!


I agree about stopping iron, although you can stop it for just 5 days if you want.

STTM has this to say on the subject :

For iron, we learned to be off for 5 days based on information from the Iron Institute, i.e. to see what we are “holding onto”.

Source :

I've tried tracking down the Iron Institute comment STTM refer to for this 5 days off iron, but can't find it. But when I was taking iron myself, I stopped iron because I saw no harm in doing so.

The NHS has this to say on the subject - they don't suggest waiting for 5 days, they just suggest 24 hours and a morning, fasting blood test :

Iron blood test

Iron blood tests are usually taken in the morning before you eat anything. You should also avoid taking iron pills or tablets for 24 hours before your test. Your body absorbs iron very quickly from food or pills, so this can raise your iron levels and affect the test results.

Iron blood tests help diagnose conditions such as iron deficiency anaemia (lack of red blood cells caused by low iron levels).

Source :

As for stopping B vitamins :

stopping them for 5 days is a good idea too because of the biotin contained in most B Complex tablets/capsules.

Vitamin D - I've never been sure of the ideal time to stop this.

I decided to make things easy for myself by stopping all my supplements apart from the ones I personally consider absolutely essential for me, for 5 - 7 days before testing. As long as I'm consistent then it works for me.

The ones I consider absolutely essential for me are my 5-HTP (an anti-depressant), magnesium citrate, and my occasional use of potassium supplements. The last two help with muscle spasms and cramps in my legs.


Thank you so much for taking the trouble to reply.


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