SlowDragon, I understand your logic about continuing to take all vitamins and medications before testing. But some pills and potions will alter blood tests so much and so quickly that giving them up briefly is a good idea, particularly if you intend to come off them permanently at some point.
Iron supplements can alter blood levels very quickly. The NHS suggests stopping iron supplementing for 24 hours and testing while fasting :
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.
Source : nhs.uk/chq/Pages/1018.aspx?...
STTM suggests stopping iron supplements for 5 days :
TO PREPARE FOR DOING LAB WORK:
a) For most of the below, we learned from our more progressive doctors that you’ll need to be off what you are testing for a minimum of 12 hours, i.e. take nothing the morning of the test.
b) 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”.
c) For saliva, we learned to be off any cortisol-containing or cortisol-changing supplement for up to two weeks, “if possible” and in working with your doctor. Some things we may not be able to get off of, though.
Source : stopthethyroidmadness.com/l...