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 : stopthethyroidmadness.com/l...
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 : nhs.uk/chq/Pages/1018.aspx?...
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.