The short answer: TSH is a good screening test. For the full picture of your thyroid blood test results you also need free T4 (fT4), free T3 (fT3) and, if you have symptoms, the thyroid antibodies. Only together do they show whether your thyroid makes enough hormone, whether it reaches your tissues and whether an autoimmune process is behind it.
Your thyroid results at a glance
| Marker | What it shows | Lab reference range (example) | My functional guide value |
|---|---|---|---|
| TSH | Control hormone from the pituitary gland; rises when there is too little thyroid hormone | 0.22–4.46 mIU/l | below 1.5 mIU/l |
| fT4 | free thyroxine, the storage form | 0.9–1.7 ng/dl (about 12–22 pmol/l) | 1.1–1.4 ng/dl (about 14–18 pmol/l) |
| fT3 | free triiodothyronine, the active form | 2.3–4.2 pg/ml (about 3.5–6.5 pmol/l) | above 3.0 pg/ml (about 4.6 pmol/l) |
| TPO antibodies | Antibodies against the enzyme TPO; the most important Hashimoto’s marker | below 35 IU/ml | below 9 IU/ml |
| Tg antibodies | Antibodies against thyroglobulin | below 20 IU/ml | ideally below the detection limit |
| TRAb | Antibodies in Graves’ disease (overactive thyroid) | below 1.75 IU/l | negative |
Lab reference range: typical example values; ranges differ between labs and methods. Always go by the reference range printed on your results. Functional guide values: based on experience from functional medicine and my coaching, not official normal values.
What is a normal TSH level?
Most labs set the upper limit at around 4 to 4.5 mIU/l. Experts have long pointed out that more than 95 per cent of people with a healthy thyroid have a TSH below 2.5 mIU/l; many of those with higher values have early Hashimoto’s thyroiditis.[1] In a group in which Hashimoto’s was very rare, the average was 1.18 mIU/l.[1]
Two long-term studies fit this picture. In an English study over 20 years, the risk of later developing an underactive thyroid rose with every TSH value above 2 mIU/l, and rose further with positive antibodies.[2] In a Chinese study, people with a baseline TSH of 1.0 to 1.9 mIU/l were the least likely to have abnormal thyroid results over the following five years.[3]
Other professional bodies keep to the wider range. The European Thyroid Association distinguishes between a raised TSH of 4 to 10 and above 10 mIU/l, and recommends repeating a raised result after two to three months together with fT4 and TPO antibodies.[4] My functional guide value of below 1.5 mIU/l is not a guideline standard.
Why thyroid antibodies matter so much
Hashimoto’s is an autoimmune disease. The antibodies can show up long before your hormone levels change. In the English long-term study, women with positive antibodies had about an eightfold higher risk of developing an underactive thyroid over the following 20 years. When a raised TSH and positive antibodies occurred together, the risk was around 38 times higher.[2]
If you have symptoms and so far only know your TSH, it is worth asking about TPO and Tg antibodies.
DIO1 and DIO2: where T4 becomes active T3
Your thyroid mainly makes T4. T4 is a precursor. It only becomes active when enzymes remove one iodine atom, turning it into T3. These enzymes are called deiodinases. Two of them activate the hormone (DIO1 and DIO2), a third inactivates it (DIO3). All three need selenium at their active site.[9]
| Enzyme | Where it is found | What it does |
|---|---|---|
| DIO1 | Thyroid, liver, kidneys; at the cell membrane | makes T3 that passes quickly into the blood, within about 30 minutes[9] |
| DIO2 | Pituitary gland and hypothalamus, brain, muscles, heart, bone, brown fat, inner ear; close to the cell nucleus | makes T3 right where it is needed; this T3 stays in the cell for longer[9] |
| DIO3 | mainly brain and skin | switches T4 and T3 off; T4 is turned into inactive reverse T3 (rT3)[9] |
Why this matters
All your cells receive the same T3 from the blood. But how much active hormone actually reaches a cell also depends on how much it makes itself via DIO2.[9] In the brain, a large share of T3 comes from this local conversion; animal studies suggest up to 80 per cent.[10] That is why your blood results don’t automatically show how well supplied your organs are.
Stress, inflammation and lack of oxygen shift the balance: DIO1 and DIO2 then become less active and DIO3 more active. The result is less T3 and more rT3.[9] Your thyroid also needs iron to make its hormones in the first place.[5] That is why, when there are symptoms, I always check fT3, selenium, ferritin and inflammation markers too.
What levothyroxine does to DIO2
Levothyroxine is pure T4. Your body has to convert it into T3 itself. But T4 slows DIO2 down: the enzyme is tagged for breakdown as soon as there is a lot of T4 around.[9,11] This hardly happens in the control centre in the hypothalamus. There, DIO2 stays active, the control centre “sees” enough hormone, and TSH returns to normal. In other tissues, however, DIO2 is slowed down. In an animal study, this led to relatively low T3 with a normal TSH on levothyroxine.[11]
A large analysis in humans fits with this: in 1,811 people without a thyroid gland who had a normal TSH on levothyroxine, fT4 was higher and fT3 lower on average than in healthy people. 15.2 per cent had an fT3 that was too low, and in more than one in five people overall, fT3 or fT4 was outside the reference range.[12]
Genetics plays a part too. In one study, a common variant in the DIO2 gene was linked to poorer wellbeing on T4 alone; people with this variant benefited more from a combination of T4 and T3. The authors stress that the result still needs to be confirmed.[10]
The European Thyroid Association provides a route for this: if you have persistent symptoms on levothyroxine despite a normal TSH, you can be offered a trial of combined T4 and T3, as an experimental treatment and with a review after three months.[13]
Typical patterns
| Pattern | Typical values | What it means |
|---|---|---|
| Subclinical hypothyroidism | TSH above the lab limit, fT4 normal | have the test repeated after 2–3 months with fT4 and TPO antibodies[4] |
| Hashimoto’s thyroiditis | TPO antibodies raised, often Tg antibodies too; hormone levels often normal at first | diagnosis by your doctor, usually with an ultrasound; regular check-ups |
| Functionally borderline | TSH between 2.5 and the lab limit, fT3 and fT4 in the lower third | not a diagnosis; together with symptoms, a reason for full testing (functional assessment) |
| On levothyroxine | TSH normal, fT4 on the high side, fT3 on the low side | a known pattern;[12] discuss with your doctor whether conversion is sufficient |
| Overactive thyroid | TSH low, fT3 and/or fT4 raised | always have it checked by a doctor, usually an endocrinologist |
How to prepare for a thyroid blood test
- Time of day: TSH follows a daily rhythm and is markedly higher in the early morning than in the afternoon and evening.[6] So that your results are comparable, always have your blood taken at the same time of day, ideally in the morning.
- Thyroid hormones: If you take levothyroxine, don’t take your tablet before the blood test that morning; take it straight afterwards. In the hours after you take it, fT4 rises markedly; in a small study it was around 40 per cent above the starting value three to four hours later.[16] If you take the tablet before the test, your fT4 looks better than your levels really are over the rest of the day. Many doctors’ surgeries don’t tell you this. The only exception is when your doctor deliberately wants to measure the value after the dose. Either way, tell your doctor’s surgery when you took your last tablet.
- Biotin: Biotin can interfere with some lab tests: fT3 and fT4 then appear too high, TSH too low.[7,8] This applies to high doses. Experts consider this a problem from about 5 mg a day; no interference has been reported for multivitamin and B-complex products with up to 1 mg.[14] High amounts are found mainly in hair and nail capsules or in collagen powders with added biotin. In one study, 10 mg a day, a common dose in such products, distorted several thyroid tests.[15] In my coaching, I already factor this in from about 3 mg. Tell your doctor’s surgery if you take a product like this: after 5 to 10 mg, at least eight hours should pass before the blood test, and up to 72 hours for some tests.[14]
- Medicines: Some medicines change your results without anything changing in the thyroid itself. Metformin, for example, can lower TSH.[17] You can read what slows the absorption of your tablet, and how far apart to take it from food and other medicines, in “How to take levothyroxine properly”.
Where to start
- My thyroid calculator puts your results into context and converts units. Your lab values stay in your browser.
- My free self-check shows you whether your signs point more towards your thyroid, perimenopause or the stress axis. You’ll then receive the starter guide that matches your result.
- Your free clarity call: 20 to 30 minutes on Zoom. Together we look at where you stand and which next step suits you.
Frequently asked questions
Is a TSH test alone enough to check your thyroid?
As a screening test, yes; for the full picture, usually not. TSH shows the control signal. fT3 and fT4 show whether enough active hormone is getting through; TPO and Tg antibodies show whether an autoimmune process is present.
What do raised TPO antibodies mean?
They are the most important sign of Hashimoto’s. In a long-term study, women with positive antibodies had about an eightfold higher risk of a later underactive thyroid.[2] The diagnosis is made by your doctor, usually together with an ultrasound.
Why is my free T3 low on levothyroxine when my TSH is normal?
Levothyroxine only provides T4. Conversion into T3 is mainly done by the enzymes DIO1 and DIO2. T4 slows DIO2 down in many tissues, but hardly at all in the control centre in the brain. That is why TSH can be normal while fT3 is low.[11,12] Talk to your doctor about it.
Should I take levothyroxine before a blood test?
No. On the morning of the test, take your tablet only afterwards. In the hours after you take it, fT4 rises markedly and makes your levels look better than they are.[16] Tell your doctor’s surgery when you took your last tablet.
When is the best time for a thyroid blood test?
Ideally in the morning and always at the same time of day, because TSH varies considerably over the day.[6] If you take high-dose biotin, for example from hair or collagen products, pause it after checking with your doctor’s surgery.[14]
Read on
This article does not replace medical advice. Diagnoses are made by your doctor; changes to medication belong in medical hands.
Sources
- Wartofsky L, Dickey RA. The evidence for a narrower thyrotropin reference range is compelling. J Clin Endocrinol Metab 2005;90:5483–5488. doi.org/10.1210/jc.2005-0455 · Review
- Vanderpump MP et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol 1995;43:55–68. doi.org/10.1111/j.1365-2265.1995.tb01894.x · Cohort study
- Teng W et al. Effect of iodine intake on thyroid diseases in China. N Engl J Med 2006;354:2783–2793. doi.org/10.1056/NEJMoa054022 · Cohort study
- Pearce SHS et al. 2013 ETA Guideline: Management of Subclinical Hypothyroidism. Eur Thyroid J 2013;2:215–228. doi.org/10.1159/000356507 · Guideline (ETA)
- Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism. Thyroid 2002;12:867–878. doi.org/10.1089/105072502761016494 · Review
- Delanghe JR et al. Interpretation of TSH results can be improved by reference values fluctuating in time. Horm Mol Biol Clin Investig 2024;46:21–26. doi.org/10.1515/hmbci-2024-0043 · Laboratory study
- Bhimireddy N, Na S, Cho E. The implications of biotin supplementation on multiple sclerosis, hair loss conditions, and thyroid disorders. Mult Scler Relat Disord 2026;116:107974. doi.org/10.1016/j.msard.2026.107974 · Review
- Campi I et al. 2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function. Eur Thyroid J 2026;15(4). doi.org/10.1530/ETJ-26-0011 · Guideline (ETA)
- Sabatino L et al. Deiodinases and the three types of thyroid hormone deiodination reactions. Endocrinol Metab (Seoul) 2021;36:952–964. doi.org/10.3803/EnM.2021.1198 · Review
- Panicker V et al. Common variation in the DIO2 gene predicts baseline psychological well-being and response to combination thyroxine plus triiodothyronine therapy in hypothyroid patients. J Clin Endocrinol Metab 2009;94:1623–1629. doi.org/10.1210/jc.2008-1301 · Genetic analysis of an RCT cohort (n = 552)
- Werneck de Castro JP et al. Differences in hypothalamic type 2 deiodinase ubiquitination explain localized sensitivity to thyroxine. J Clin Invest 2015;125:769–781. doi.org/10.1172/JCI77588 · Experimental study (animal model)
- Gullo D et al. Levothyroxine monotherapy cannot guarantee euthyroidism in all athyreotic patients. PLoS One 2011;6:e22552. doi.org/10.1371/journal.pone.0022552 · Retrospective study (n = 1,811 vs. 3,875 controls)
- Wiersinga WM et al. 2012 ETA Guidelines: the use of L-T4 + L-T3 in the treatment of hypothyroidism. Eur Thyroid J 2012;1:55–71. karger.com/etj/article/1/2/55 · Guideline (ETA)
- Li D et al. AACC Guidance Document on Biotin Interference in Laboratory Tests. J Appl Lab Med 2020;5:575–587. doi.org/10.1093/jalm/jfz010 · Guidance document (laboratory medicine society)
- Li D et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA 2017;318:1150–1160. doi.org/10.1001/jama.2017.13705 · Non-randomised crossover study (n = 6)
- Marina M et al. Circulating concentrations of free thyroxine after an oral intake of liquid LT4 taken either during fasting conditions or at breakfast. Acta Biomed 2016;87:247–252. ncbi.nlm.nih.gov/pmc/articles/PMC10521879 · Comparative study (n = 21)
- Lupoli R et al. Effects of treatment with metformin on TSH levels: a meta-analysis of literature studies. J Clin Endocrinol Metab 2014;99:E143–E148. doi.org/10.1210/jc.2013-2965 · Meta-analysis (7 datasets, n = 206)
