Thyroid Function Test Interpreter (TSH, FT4, FT3)
Thyroid Function Test Interpreter (TSH, FT4, FT3)
Enter where the TSH and free T4 fall against your laboratory’s reference intervals — and free T3 if measured — with the context that changes the answer: levothyroxine, acute illness, pregnancy, biotin, amiodarone. It names the pattern, including the discordant ones that most pages get wrong: a raised TSH with a raised FT4, a low TSH with a low FT4, and the result that is really a missed dose or a supplement.
Thyroid function tests
TSH + FT4 + context → patternTSH raised but below 10 mIU/L, FT4 above its interval, FT3 not measured, not on levothyroxine, not unwell, not pregnant, no biotin, no amiodarone
The patterns, side by side
| TSH | FT4 | Usual meaning | Common look-alikes |
|---|---|---|---|
| Raised | Low | Overt primary hypothyroidism | Under-replacement on levothyroxine |
| Raised | Normal | Subclinical hypothyroidism | Recovery from illness; poor adherence on levothyroxine |
| Low | Raised | Overt hyperthyroidism | Over-replacement; biotin; amiodarone |
| Low | Normal | Subclinical hyperthyroidism; T3 toxicosis if FT3 is raised | Illness, glucocorticoids, first-trimester pregnancy |
| Raised | Raised | Discordant — assay interference, TSH-secreting adenoma, thyroid hormone resistance | Erratic levothyroxine adherence; amiodarone |
| Normal | Raised | Discordant — as above, or heparin in the sample | Levothyroxine; amiodarone; familial dysalbuminaemic hyperthyroxinaemia |
| Low | Low | Central hypothyroidism | Severe non-thyroidal illness; recently treated hyperthyroidism |
| Normal | Low | Possible central hypothyroidism | Illness; phenytoin or carbamazepine; sample long after a dose |
The two fixed TSH lines this page uses
| Line | Where it comes from | What it changes |
|---|---|---|
| TSH 10 mIU/L | NICE NG145 rec 1.5.3 | Consider levothyroxine for subclinical hypothyroidism when TSH is 10 or higher on 2 occasions 3 months apart |
| TSH 0.1 mIU/L | ATA 2016 hyperthyroidism guideline; NICE NG145 rec 1.8.1 | Separates the more severe grade of subclinical hyperthyroidism; 2 readings below 0.1 prompt specialist advice |
Reading TSH and FT4 together
Thyroid function tests are read as a pair because TSH and free T4 are tied together by feedback. When the thyroid fails, FT4 falls and the pituitary pushes TSH up; when the thyroid is overactive, FT4 rises and TSH is suppressed. Most results follow that seesaw, and most pages stop there. The results that matter most to a laboratory are the ones that do not: a raised TSH beside a raised FT4, a normal TSH beside a raised FT4, and a low TSH beside a low FT4. Each means either that the feedback loop itself is broken, or — far more often — that one of the numbers is wrong.
This page asks where each result falls against your laboratory’s reference interval rather than for the number itself. That is deliberate. FT4 methods are not harmonised, so an FT4 of 22 pmol/L is raised on one platform and normal on another, and a calculator carrying a single interval would misread one of them. The only fixed TSH lines used are the two that guidelines act on: 10 mIU/L, above which NICE advises considering levothyroxine for subclinical hypothyroidism, and 0.1 mIU/L, which separates the more severe grade of subclinical hyperthyroidism. To change units, use the TSH unit converter, free T4 unit converter or free T3 unit converter.
The context questions are there because each one produces a characteristic false picture. Erratic levothyroxine adherence gives a raised TSH with a raised FT4. Biotin gives a low TSH with a high FT4 that looks exactly like Graves’ disease. Amiodarone raises FT4 in people whose thyroid is normal. Acute illness lowers TSH and FT3 and, in recovery, can briefly raise TSH, which is why NICE advises against testing during acute illness at all. Pregnancy changes the intervals themselves: use the thyroid function in pregnancy interpreter. For newborn screening, see the neonatal thyroid screening interpreter.
What the page cannot do is replace the conversation with the laboratory. When a result is discordant, the next step is almost always to ask for it to be repeated by a different method, with dilution or antibody blocking, before anyone is sent for a pituitary MRI or a genetic test.
Frequently asked questions
What does a high TSH and a high FT4 mean?
It is discordant: a raised FT4 should suppress TSH. The commonest explanations are erratic levothyroxine adherence, with tablets taken in the days before the test, and assay interference from heterophile or anti-T4 antibodies. Ask the laboratory to repeat on a different platform. Only if the pattern persists across methods do the rare causes — a TSH-secreting pituitary adenoma or resistance to thyroid hormone — need investigating.
What does a low TSH with a low FT4 mean?
The pituitary is not responding to a low thyroxine, which is central hypothyroidism unless another explanation fits. Severe non-thyroidal illness and recent treatment of hyperthyroidism, after which TSH can stay suppressed for months, both mimic it. Check cortisol before giving thyroxine to anyone who may have pituitary disease.
When is subclinical hypothyroidism treated?
NICE NG145 advises considering levothyroxine when the TSH is 10 mIU/L or higher on 2 separate occasions 3 months apart, and a 6-month trial for adults under 65 with symptoms and a TSH above the reference range but below 10 on 2 occasions 3 months apart. A single mildly raised TSH should be repeated first, because many return to normal.
Can biotin affect thyroid function tests?
Yes, on assays that use biotin–streptavidin capture. It makes TSH read falsely low and FT4 and FT3 falsely high, which looks like Graves’ disease, and can give a false positive TSH receptor antibody. Tell the laboratory, stop the supplement and repeat. Manufacturers commonly advise at least 8 hours off supplement doses and longer for high doses; the FDA says there is not enough information to be sure any interval is sufficient.
Should thyroid function be checked in hospital?
Not routinely. NICE advises against testing during an acute illness unless the illness may itself be due to thyroid dysfunction, because illness lowers TSH and FT3, and TSH can rise transiently during recovery. Results taken while unwell should usually be repeated after recovery before anything is started.
Why does this page not ask for the actual TSH and FT4 values?
Because FT4 methods are not harmonised and reference intervals differ between laboratories, so a single built-in interval would misread results from many of them. Where your result falls against your own laboratory’s interval is what carries the meaning. The two TSH lines guidelines act on, 0.1 and 10 mIU/L, are built into the TSH options.
Related calculators
References
- Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745–762. — the discordant patterns: interference, TSH-secreting adenoma, resistance to thyroid hormone, adherence on levothyroxine, non-thyroidal illness.
- National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE guideline NG145, 2019 (updated since). Rec 1.2.6: "Do not test for thyroid dysfunction during an acute illness unless you suspect the acute illness is due to thyroid dysfunction"; rec 1.5.3: consider levothyroxine for subclinical hypothyroidism with "a TSH of 10 mlU/litre or higher on 2 separate occasions 3 months apart"; rec 1.5.4: a 6-month trial for adults under 65 with TSH above the reference range but below 10 on 2 occasions 3 months apart and symptoms; rec 1.8.1: specialist advice for "2 TSH readings lower than 0.1" at least 3 months apart with evidence of thyroid disease.
- Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. — subclinical hyperthyroidism graded as TSH below 0.1 versus 0.1 to the lower limit of the reference range.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670–1751.
- Cardenas GA, Leslie CA, Cabral JM. Amiodarone-induced thyrotoxicosis: diagnostic and therapeutic strategies. Cleve Clin J Med. 2003;70(7):624–631. — in euthyroid patients on amiodarone: slightly raised free T4, normal or low T3, raised reverse T3 and transiently raised or lowered TSH.
- Bartalena L, Bogazzi F, Chiovato L, Hubalewska-Dydejczyk A, Links TP, Vanderpump M. 2018 European Thyroid Association (ETA) guidelines for the management of amiodarone-associated thyroid dysfunction. Eur Thyroid J. 2018;7(2):55–66. doi:10.1159/000486957. — type 1 (iodine-induced) and type 2 (destructive) amiodarone-induced thyrotoxicosis.
- US Food and Drug Administration. UPDATE: The FDA warns that biotin may interfere with lab tests. Safety communication, 2017, updated 5 November 2019.
- Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 guidelines for thyroid disease in preconception, pregnancy, and postpartum. Thyroid. 2026;36(5):481–544. doi:10.1177/10507256261445624. — lab- and trimester-specific TSH and FT4 intervals preferred; if unavailable, TSH 0.1–4.0 mU/L in the first and second trimesters and the non-pregnant interval in the third; new overt hypothyroidism with TSH below 6 mU/L may be confirmed within 3 weeks, and with TSH 6 or more, or persisting, treated; persistent first-trimester subclinical hypothyroidism should be treated, and subclinical hypothyroidism diagnosed in the second or third trimester may not require treatment; TPOAb status no longer guides treatment of subclinical hypothyroidism; levothyroxine target TSH 0.5–2.5 mU/L preconception and in pregnancy (good practice statement).
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/