How to prepare for a thyroid blood test
A few small things change thyroid results more than people expect. Follow these so your numbers reflect your thyroid, not your supplements or the time of day.
In the days before
- For a lab blood draw, stop biotin 3 days before. Biotin, common in hair, skin and nails supplements and many multivitamins, can make TSH read falsely low and free T4 and T3 falsely high on many lab tests.
- If you’re unwell, wait until you’ve recovered. A recent illness can lower free T3 and shift TSH for a few weeks.
- If your dose changed, wait 6 to 8 weeks before retesting. That’s how long thyroid levels take to settle on a new dose.
On the day
- Test in the morning, at about the same time each time. TSH is highest in the early morning and varies through the day, so a consistent time makes results comparable.
- For a lab blood draw, take your thyroid medication after the draw. A dose taken shortly before, especially one containing T3, can raise free T3 and free T4 for a few hours.
- You don’t need to fast for thyroid tests. Fast only if other tests on the same order need it, such as glucose or cholesterol.
What to ask for
- TSH, free T4 and free T3. TSH alone can miss problems with how much active hormone you have.
- TPO and thyroglobulin antibodies, at least once. They show whether Hashimoto’s is the cause.
- Reverse T3, if you have symptoms despite normal results. It helps show how T4 is being converted.
- Use the same lab each time when you can. Labs use different methods and ranges, so results compare best within one lab.
Ready to test? Test at home or at a Quest lab, then read your results with the thyroid lab analyzer.
Medically reviewed by Andrew Cunningham, MD, November 12, 2026.
Frequently Asked Questions
Do I need to fast before a thyroid test?
No. Fast only if other tests on the same order need it.
Should I skip my thyroid medication the morning of a lab draw?
Don’t skip it: take it after the blood draw instead. This matters most for medication containing T3.
How long before a thyroid test should I stop biotin?
For a lab draw, 3 days. Check multivitamins and hair, skin and nails supplements, which often contain it. Paloma’s at-home kit uses an assay where you don’t need to stop supplements.
Does coffee or exercise affect thyroid results?
Not meaningfully for thyroid tests. Coffee does reduce how much levothyroxine you absorb if taken together, which affects your levels over time, so keep your usual routine consistent.
Should I test if I’m sick?
Wait until you’ve recovered if you can. Illness can lower free T3 and change TSH for a few weeks, which can make results look worse than your thyroid really is.
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