Should you take levothyroxine before a blood test?
The short answer
Usually no: have your blood drawn first, then take your levothyroxine. Free T4 and total T4 rise for several hours after a dose, peaking about 4 hours later in one study, so a draw soon after your pill can read higher than your usual level.
What to do
- Book a morning draw and bring your levothyroxine with you.
- Take your dose right after the draw, unless your prescriber has told you otherwise.
- Do not skip or change your dose for the test.
- After a dose change, ask your prescriber when to retest; guidance is usually 6 to 8 weeks.
- Taking biotin (often in hair, skin and nail supplements)? Read our biotin guidance before you test.
- Keep the same timing every time you test, so results are comparable.
Which results it changes
| Marker | Effect | How long |
|---|---|---|
| Free T4[1][2] | Rises after a levothyroxine dose (about 40% above the pre-dose level in one study) | Peaks about 3 to 4 hours after the dose |
| Total T4[1] | Rises after a levothyroxine dose | Peaks about 4 hours after the dose |
| TSH[3] | Used to adjust the dose; rechecked after a dose change | Retest 6 to 8 weeks after a dose change |
| TSH (with biotin)[4][5] | Can read falsely low on biotin-based assays | Greatest about 2 hours after a biotin dose |
| Free T4 and Free T3 (with biotin)[4][5] | Can read falsely high on biotin-based assays | Greatest about 2 hours after a biotin dose |
Why the timing of your dose matters
Levothyroxine is T4. After you swallow it, the T4 level in your blood goes up for a few hours. In a study of 143 people taking their usual daily dose, both total and free T4 rose, with the peak about 4 hours after the pill. In another small study, Free T4 was about 40% above the pre-dose level 3 and 4 hours after a dose.
So a draw taken a few hours after your pill can show a higher Free T4 than you have for most of the day. Drawing before your daily dose avoids that and gives a steadier result to compare over time. Many people take levothyroxine first thing in the morning, which is also when many draws are booked, so it is easy to take the pill out of habit. Put it in your bag instead and take it once you are done.
What about TSH?
TSH is the main test used to adjust levothyroxine. Guidance is to check it 6 to 8 weeks after starting levothyroxine or changing the dose, then once a year when it is at goal. If your dose just changed, ask your prescriber when they want the next test.
Our TSH test instructions say to take your blood sample before your daily dose, or at least 4 hours after. If you are also checking Free T4, drawing before your dose is the simplest rule, since Free T4 can still be near its peak around 4 hours after a dose.
Which tests this applies to
The timing rule matters most for Free T4 and total T4, which measure the same hormone your pill contains. It also keeps TSH results consistent. Thyroid antibody tests (TPO and thyroglobulin antibodies) are not covered by the studies above.
If you take a medicine that contains T3 (liothyronine) or a desiccated thyroid product, ask your prescriber how to time your draw. This page covers levothyroxine only.
Biotin can also skew thyroid results
Many thyroid tests use a biotin-based step. Biotin from supplements can falsely raise Free T4 and Free T3 and falsely lower TSH on these tests, a pattern that can look like an overactive thyroid.
In a study where adults took 10 mg of biotin a day, results for TSH, Free T4, total T3 and thyroglobulin were affected on some lab platforms, and interference was greatest about 2 hours after a dose. See our blood test preparation guide for how long to pause biotin before testing.
Make your results comparable
Test at a similar time of day, at the same point relative to your dose, each time. Note the time you last took levothyroxine and any biotin, and share it with whoever reviews your results. Only your prescriber should change your dose.
Frequently Asked Questions
More on preparing
Sources
- Caron P, Tudor C, Grunenwald S. Levothyroxine Absorption Test With the Daily Levothyroxine Dose in Patients With "Refractory Hypothyroidism". J Endocr Soc. 2025 Mar 3;9(4):bvaf017. doi: 10.1210/jendso/bvaf017. PMID: 40046102. PubMed
- Marina M, Ceda GP, Aloe R, Gnocchi C, Ceresini G. Circulating concentrations of free thyroxine after an oral intake of liquid LT4 taken either during fasting conditions or at breakfast. Acta Biomed. 2016 Jan 16;87(3):247-252. PMID: 28112689. PubMed
- Chaker L, Papaleontiou M. Hypothyroidism: A Review. JAMA. 2025 Nov 18;334(19):1750-1760. doi: 10.1001/jama.2025.13559. PMID: 40900603. PubMed
- Dasgupta A. Immunoassay design and biotin interference. Adv Clin Chem. 2022;109:165-183. doi: 10.1016/bs.acc.2022.03.005. PMID: 35953126. PubMed
- Ylli D, Soldin SJ, Stolze B, Wei B, Nigussie G, Nguyen H, Mendu DR, Mete M, Wu D, Gomes-Lima CJ, Klubo-Gwiezdzinska J, Burman KD, Wartofsky L. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin. Thyroid. 2021 Aug;31(8):1160-1170. doi: 10.1089/thy.2020.0866. PMID: 34042535. PubMed
Content reviewed for accuracy by the JustLabs editorial team.
Educational information, not medical advice. Don’t stop or change a prescribed medicine without talking to your prescriber. If your lab order gives different instructions, follow your lab order.