I keep coming across comments about not taking hypoT meds before a blood test to get more accurate readings. Can someone explain this to me? Don't we want to know whether the meds are helping us and therefore need to know the levels while taking them? I'm confused. Thanks.
When you take any medication the blood levels will spike shortly after the drug enters your bloodstream. Then once it circulates for a while and some enters the cells you will have a more accurate reading of the blood levels that particular dose is providing (and this also depends of the half-life of the medication). When you test for thyroid hormone levels you want to find out what the lowest average amount of hormone in your blood is (or the "trough" of the medication cycle) not the spiked level, which can give the impression that you've got higher levels circulating than really are there all the time. So it's often recommended you test your thyroid hormones in the morning before taking your first pill of the day. This gives you an idea of your levels when the amount circulating in your bloodstream are at their lowest average level and will ensure you have adequate amounts available during this time.
It's also really important to always test at the same time with the same lab and the same fasting status every time you test so as to be able to compare results accurately.
T4 is slow-acting. The amount of it you took today doesn't go to work for you today. It won't do that until about 2 weeks from now. When blood is drawn within about 4 hours after taking T4, the level will reflect the temporary spike in blood-borne T4 before it gets absorbed at the cellular level. You want to know the average level of T4 that is normally there most of the time, not the artificially high level that's there just after it's absorbed into the bloodstream from the gut. Many people have had their doses unnecessarily and mistakenly reduced based on that high spike and have gone hypo again due to it.
My doctor always stresses the importance of not taking the medication that I am having the test for, for the 24 hours before the test. She is a stickler for this and is always adamant that I take the medication exactly 24 hours prior to the test, and not again until after the test. So in your case, you would take the medication at 2 pm on the day before and not again until after the test. I have also been told that thyroid levels should be checked first thing in the morning. Could you go earlier to have your test?