Timing and reasons for Thyroid blood tests

[SlowDragon]

Recommended by numerous research papers that thyroid testing should be in the morning and before taking daily levothyroxine

TSH has a circadian rhythm and is highest in early morning and lowest in afternoon

Links to a few research papers and articles

ncbi.nlm.nih.gov/pmc/articl...

thyroidpatients.ca/2020/07/...

oncquestlabs.com/blog/best-...

anchorwellnesscenter.com/te...

On Levothyroxine

Patient experience recommends testing as early as possible in morning, ideally before 9am (if possible) and last dose levothyroxine 24 hours before test.

Testing 24 hours after last dose will measure where Ft4 is at its lowest/most consistent point

see graphs here demonstrating Ft4 levels across 24 hours for patient on levothyroxine

europepmc.org/article/MED/3...

Research is unclear if eating before test affects results, but even if it doesn’t it’s best (if possible) to only drink water between waking and test so that you have empty stomach to take your levothyroxine immediately after test.

Patients experiences show these recommendations give the most consistent results.

If you normally take levothyroxine at bedtime/in night adjust timings prior to blood test so that it’s 24 hours between last dose levothyroxine and test

If testing Monday morning, delay Saturday evening dose levothyroxine until Sunday morning. Delay Sunday evening dose levothyroxine until after blood test on Monday morning. Take Monday evening dose levothyroxine as per normal

Research into bedtime dosage of levothyroxine

pubmed.ncbi.nlm.nih.gov/172...

If taking liothyronine (T3) or NDT

Even if you normally take a single dose T3, on the day before test split T3 as 2 or 3 smaller doses spread through the day, with last dose approximately 8-12 hours before test

See graphs for comparison of different T3 or NDT dosing regimes here

thyroidpatients.ca/2019/11/...

If you are personally looking to establish where your peak Ft3 is, you might wish to test 2-4 hours after taking last dose T3.

But if testing for medics, be aware testing this way might risk getting dose reduced, especially if you take single dose daily.

See also this pinned post on timings of thyroid tests

healthunlocked.com/thyroidu...

Reasons to test thyroid levels

Testing thyroid levels after any medication or other changes.

Thyroid levels should always be retested 6-10 weeks after any dose change in levothyroxine

Changing brand of Levothyroxine. Many thyroid patients notice differences between different brands of levothyroxine. If you notice changes in symptoms after brand change, bloods should be retested 6-8 weeks after changing brand.

gov.uk/drug-safety-update/l...

If increasing dose levothyroxine, ideally don’t change brand at same time. Only change one thing at a time

Retest thyroid levels 6-8 weeks after starting or stopping HRT

If vitamin levels tested very low, retest thyroid and vitamin levels 2-3 months after working on improving low vitamin levels

Similarly if starting (or stopping) gluten free and/or dairy free diet, retest thyroid levels 2-3 months after changing

Biotin

If taking any supplements with biotin in, stop these 5-7 days before testing, as biotin is frequently used lab testing equipment. Taking biotin supplements can falsely affect test results

[Buddy195 Administrator]

Thanks for posting SD 👍

[Capan24]

This is a little bit confusing to me. I take my thyroid medication at 10:00 p.m. before bed. I usually have to fast for my labs and do them at 7:00 a.m. When I'm having my fasting labs, should I skip my 10:00 p.m. thyroid medication or take it earlier in the evening at 8:00 p.m?

[SlowDragon Administrator in reply to Capan24]

If only testing TSH you don’t need to change

If testing Ft4, especially if looking to get dose increase in levothyroxine, last dose should be 24 hours before test

If you take normally take levothyroxine at bedtime

adjust timings as follows prior to blood test

If testing Monday morning, delay Saturday evening dose levothyroxine until Sunday morning. Delay Sunday evening dose levothyroxine until after blood test on Monday morning. Take Monday evening dose levothyroxine as per normal

[Capan24 in reply to SlowDragon]

Thanks. In the past when I've gotten my labs tested in the morning, my TSH has always been on the low side and my free t4 within the normal range.

[SlowDragon Administrator in reply to Capan24]

If only leaving 12-14 hours between last dose Levo and test Ft4 is falsely high

[Capan24 in reply to SlowDragon]

So what you're saying is there should be 24 hours between doses to get a more accurate lab. Maybe I just won't take my 10:00 pm dose and get my labs drawn in the morning

[SlowDragon Administrator in reply to Capan24]

That’s too long …then it’s 36 hours and get false low result

[FallingInReverse]

is this/can this be pinned?

[SlowDragon Administrator in reply to FallingInReverse]

It is pinned 👍

[CernCrystal]

Thank you so much, have put in my reference file for Dr visits and self care!

[Kingsway1]

Test was done after fasting on Wednesday morning I had last taken meds as usual on Tuesday am T4 and one dose of T3 no T3 on Tuesday night