How to read a thyroid panel: TSH, free T4, free T3 and antibodies
A thyroid panel is read as a pattern, not a row of single numbers. TSH is the pituitary’s reaction to how much thyroid hormone it senses, so it moves first and fastest; free T4 and free T3 are the actual hormones; reverse T3 and the antibodies (TPO, Tg) add context but rarely change a diagnosis alone. In the UK, NICE starts with TSH alone and adds the others only when TSH is out of range, which is efficient but misses some patterns. This guide explains what each marker means, why the “normal” and “optimal” ranges are different debates, and how to read the whole picture.
On this page
- Read the pattern, not the single number
- TSH: the pituitary’s thermostat
- Free T4 and free T3: the actual hormones
- Reverse T3: interesting, rarely useful
- Thyroid antibodies: TPO and Tg
- NHS reference ranges vs the “optimal” debate
- Subclinical hypothyroidism
- Why TSH alone can miss things
- Putting the pattern together
- What to ask your GP
- What to do next
Key facts
- NICE NG145 advises measuring TSH alone first in adults; if TSH is above range, add free T4; if below range, add free T4 and free T3 (“cascading”).1
- TSH and free T4 move in opposite directions on a roughly log-linear scale: a small fall in thyroid hormone produces a large rise in TSH, making TSH the most sensitive single marker.2
- Subclinical hypothyroidism means a raised TSH with a normal free T4. NICE only routinely treats it once TSH reaches 10 mIU/L or higher on two occasions three months apart.1
- Reverse T3 is not recommended by any major guideline for diagnosing thyroid disease; it does not reliably separate genuine hypothyroidism from the effects of unrelated illness.56
- In the Whickham survey, a raised TSH plus positive antibodies carried roughly a 4 percent per year risk of progressing to overt hypothyroidism in women.7
Read the pattern, not the single number
The single most useful idea when reading a thyroid panel is that the markers are linked by a feedback loop, so they only make sense together. The thyroid releases mainly T4 (thyroxine) and a smaller amount of T3 (the active hormone). The pituitary constantly samples how much hormone is around and adjusts its output of TSH (thyroid-stimulating hormone) to keep things steady: if thyroid hormone drops, TSH rises to drive the gland harder; if hormone is plentiful, TSH falls.
So you read the results as a direction of travel. A high TSH with low free T4 points to an underactive gland; a low TSH with high free T4 or free T3 points to an overactive one; a high TSH with a still-normal free T4 sits in between and has its own name. Reading any single value in isolation, especially TSH alone against a printed range, is where most misinterpretation begins.
TSH: the pituitary’s thermostat
TSH is not a thyroid hormone at all. It is the pituitary’s instruction to the thyroid, and it behaves like a thermostat reading: it tells you whether the body thinks it has enough thyroid hormone, not how much is actually present. That makes it a downstream signal, one step removed from the hormones doing the work.
TSH is the preferred first-line test because it is so sensitive. The relationship between TSH and free T4 is approximately log-linear: a modest fall in free T4, still within the normal range, can drive TSH up several-fold.2 So TSH usually moves outside its range before free T4 does, flagging early thyroid failure, which is why NICE recommends TSH alone as the opening test in adults when pituitary disease is not suspected.1
That sensitivity cuts both ways. TSH is easily nudged by things unrelated to long-term thyroid status: time of day (higher overnight, lower in the afternoon), recent illness, stress and certain drugs. It also lags real change, taking up to six weeks to settle after a dose adjustment and up to six months to normalise after prolonged severe hypothyroidism, even on correct treatment.1 A single TSH is a snapshot of a slow-moving system.
A practical consequence: NICE advises against testing thyroid function during an acute illness unless the illness itself is suspected to be thyroid-driven, because illness distorts the results. It also flags that high-dose biotin supplements can produce falsely high or low readings on many assays, so mention any supplement use before testing.1
Free T4 and free T3: the actual hormones
Free T4 and free T3 are the circulating hormones not bound to carrier proteins, the fraction available to tissues. T4 is the storage and transport form, relatively inactive until the body converts it, by removing one iodine atom, into T3, the hormone that actually binds receptors and drives metabolism. Most T3 in the blood comes from this conversion in the liver, kidney and other tissues, not from direct thyroid secretion.
Free T4 is the workhorse confirmatory test. When TSH is abnormal, it tells you whether thyroid output has genuinely shifted and how far: a high TSH with low free T4 confirms overt (clinical) hypothyroidism; a low TSH with high free T4 confirms hyperthyroidism; a high TSH with normal free T4 is the signature of subclinical hypothyroidism.
Free T3 is more selective. Because T4-to-T3 conversion is tightly regulated, free T3 is often the last thing to fall in hypothyroidism and can stay normal in someone genuinely underactive, which limits its diagnostic value there. Its real role is the opposite: confirming and grading an overactive thyroid. That is why NICE’s cascading rule adds free T3 when TSH is suppressed (where it can reveal isolated “T3 toxicosis”), and not routinely when TSH is high.1
Reverse T3: interesting, rarely useful
Reverse T3 (rT3) is an inactive molecule made when the body removes a different iodine atom from T4. It is a genuine physiological diversion: during starvation, serious illness, surgery or major stress, the body shunts more T4 towards reverse T3 and away from active T3, an apparent energy-conserving response. It is the biochemical hallmark of non-thyroidal illness syndrome (sick euthyroid syndrome): low T3, sometimes low TSH and free T4, and raised reverse T3, in someone whose thyroid is fundamentally normal.5
Reverse T3 has become popular in private and functional-medicine panels as a supposed marker of “poor conversion” or hidden hypothyroidism, but the evidence does not support that use. It does not reliably distinguish the genuinely hypothyroid patient from the simply unwell one: true hypothyroidism plus illness can give a normal reverse T3, and euthyroid people can have a low one.6 No major guideline, including NICE, recommends reverse T3 for diagnosing thyroid dysfunction.5
Evidence strength: the physiology of reverse T3 in illness is well established, but its use as a diagnostic test for thyroid disease is not supported by controlled evidence, and laboratory and endocrine bodies advise against ordering it for that purpose. Treat a reverse T3 result on a private panel as context at best, not a basis for diagnosis or for starting hormone.
Thyroid antibodies: TPO and Tg
Thyroid antibodies tell you about cause and risk rather than current function. They mark autoimmune attack on the gland, the mechanism behind Hashimoto’s thyroiditis (the commonest cause of UK hypothyroidism) and, in a different form, Graves’ disease.
- Thyroid peroxidase antibodies (TPOAb) are the most useful. They are positive in around 90 percent of Hashimoto’s cases, and NICE recommends measuring them when TSH is above the range to judge whether autoimmune disease underlies it.18
- Thyroglobulin antibodies (TgAb) are positive in roughly half of Hashimoto’s cases and add little once TPOAb is known, so they are not part of routine NHS first-line testing.8
- TSH-receptor antibodies (TRAb) are a separate test, used to confirm Graves’ disease in someone thyrotoxic, not to investigate an underactive thyroid.1
The key nuance is that antibodies predict risk, not present illness. You can be TPO-positive with a perfectly normal TSH and feel well, which is common in the general population; what antibodies do is raise the odds of trouble later. NICE is clear that TPOAb should be measured once and not repeated, because tracking the titre over time does not usefully guide management.1
NHS reference ranges vs the “optimal” debate
A reference range is a statistical range, built to capture roughly 95 percent of a reference population, not a definition of ideal health. UK laboratories set their own ranges depending on the analyser, so the numbers vary between trusts. Typical adult ranges look like the table below, but always read your result against the range printed on your own report.
| Marker | Typical adult range | What it indicates |
|---|---|---|
| TSH | ~0.4 to 4.0 or 5.0 mIU/L | Pituitary signal; rises in underactive thyroid, falls in overactive.3 |
| Free T4 | ~9 to 22 pmol/L | Main circulating thyroid hormone; confirms direction and severity.3 |
| Free T3 | ~3.5 to 6.5 pmol/L | Active hormone; most useful for confirming an overactive thyroid.3 |
| TPO antibodies | Lab-specific cut-off | Marker of autoimmune (Hashimoto’s) thyroiditis; signals risk.8 |
The contested part is the TSH upper limit. In 2003 the US National Academy of Clinical Biochemistry noted that more than 95 percent of healthy people have a TSH between roughly 0.4 and 2.5 mIU/L, and some argued the upper cut-off should drop towards 2.5 to 3.0.4 Others, including Surks and colleagues, argued the conventional limit near 4.0 to 4.5 should stay, because the mildly raised values being reclassified often cause no measurable harm, frequently drift back to normal, and have not been shown to benefit from treatment.9 The debate has never fully resolved.
The UK position is pragmatic. NICE states that when treating hypothyroidism the aim is to keep TSH within the reference range, and that the evidence shows no benefit from targeting the lower rather than the higher end.1 So the “optimal TSH” targets popular online are not NICE-endorsed for the general patient, though the British Thyroid Association does suggest aiming towards the lower half of the range in specific circumstances, such as before a trial of liothyronine.10 Our health library covers this statistical-versus-functional threshold pattern across other labs too.
Subclinical hypothyroidism
Subclinical hypothyroidism has a precise definition: a TSH above the reference range with a free T4 that is still normal.1 It is common, affecting roughly 8 percent of women and 3 percent of men in the original Whickham survey, and sits in a grey zone where symptoms, age, antibodies and the actual TSH number all matter.7
NICE draws the line largely at a TSH of 10, on evidence-based reasoning: a TSH of 5 to 10 returns to normal untreated in around half of people, whereas a TSH above 10 rarely self-corrects and is more often genuinely symptomatic.1 The recommendations are:
- TSH 10 mIU/L or higher on two occasions three months apart: consider levothyroxine.1
- TSH above range but under 10, in an adult under 65 with symptoms of hypothyroidism: consider a six-month trial of levothyroxine, and stop it if symptoms do not improve once TSH is in range.1
- No treatment but monitoring otherwise: yearly if there are features of thyroid disease such as positive antibodies, or every two to three years if not.1
This is where antibodies earn their place. The Whickham follow-up found the annual risk of progressing to overt hypothyroidism was about 2.6 percent with a raised TSH alone, but roughly 4 percent when a raised TSH and positive antibodies occurred together.7 A positive TPOAb alongside a borderline TSH tilts the decision towards closer monitoring or treatment.
Why TSH alone can miss things
The TSH-first strategy is efficient and catches the great majority of thyroid disease, but it has genuine blind spots worth understanding.
- Secondary (central) hypothyroidism. If the problem is the pituitary or hypothalamus rather than the thyroid, TSH can be normal or even low while thyroid hormone output is inadequate. Here TSH alone is misleading, which is why NICE advises measuring TSH and free T4 together whenever pituitary disease is suspected.1
- Early or evolving disease. Because free T4 is added only once TSH is already abnormal, an early shift in free T4 within a normal TSH window can pass unremarked on a TSH-only test.
- Isolated T3 problems. Free T3 is not measured unless TSH is suppressed, so a rare T3-predominant picture relies on TSH being low enough to trigger the cascade.1
- Non-thyroidal illness. Acute illness suppresses TSH and lowers T3, which can mimic thyroid disease, which is why NICE says not to test during acute illness.1
- Assay interference. High-dose biotin and, occasionally, interfering antibodies can distort any of these results.1
None of this makes TSH a poor test. It is a screening signal that occasionally needs free T4, and sometimes free T3, to be read honestly. If you are organising several blood tests at once, our stack builder helps you track what has been measured and when.
Putting the pattern together
With the individual markers understood, the patterns become readable. The table below summarises the common combinations.
| TSH | Free T4 | Free T3 | Likely picture |
|---|---|---|---|
| High | Low | Low or normal | Overt (primary) hypothyroidism.1 |
| High | Normal | Normal | Subclinical hypothyroidism; treat per the TSH-10 rule and antibody status.1 |
| Low | High | High | Overt hyperthyroidism (e.g. Graves’, toxic nodule).1 |
| Low | Normal | Normal | Subclinical hyperthyroidism.1 |
| Low or normal | Low | Low | Central hypothyroidism, or non-thyroidal illness; do not test during acute illness.15 |
| Low | Normal | High | T3 toxicosis; this is why free T3 is added when TSH is suppressed.1 |
Two principles tie it together. First, treat symptoms and numbers jointly: NICE notes that people can feel well with results slightly outside the range and unwell with results inside it, so the panel guides rather than dictates.1 Second, repeat before acting on a borderline result, because TSH fluctuates and a one-off mild abnormality often settles. Our insights articles apply the same logic across other tests.
On treatment: levothyroxine (T4) is the NICE first-line option for hypothyroidism. NICE does not routinely recommend liothyronine (T3) or natural (desiccated) thyroid extract, because evidence of benefit over levothyroxine is insufficient and long-term safety uncertain; natural thyroid extract has no UK marketing authorisation.1 These are specialist decisions, not self-prescribing territory.
What to ask your GP
- My TSH is raised but I was only told it is “borderline”: what is the actual number, and was free T4 measured in the same sample?
- Given my symptoms, can we check TPO antibodies, since positive antibodies change how closely we should monitor or whether to treat?
- If my TSH is between 5 and 10 with symptoms, would a time-limited trial of levothyroxine be reasonable under the NICE guidance?
- Should we repeat the test before making any decision, given that a single mildly abnormal TSH often settles on its own?
- Is there any reason to suspect a pituitary cause, in which case TSH alone might not be enough and free T4 should be measured too?
References
- National Institute for Health and Care Excellence. Thyroid disease: assessment and management (NG145). Published 2019, updated 2023. nice.org.uk/guidance/ng145.
- Hoermann R, et al. Homeostatic control of the thyroid-pituitary axis: perspectives for diagnosis and treatment. Front Endocrinol. 2015;6:177. On the log-linear TSH to free T4 relationship. PMC4653296.
- North Bristol NHS Trust, Severn Pathology. Thyroid function tests (test information and reference ranges). nbt.nhs.uk, accessed 2026.
- Baloch Z, et al. (National Academy of Clinical Biochemistry). Laboratory medicine practice guidelines: laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid. 2003;13(1):3-126. PMID 12625976.
- Fliers E, Bianco AC, et al. Non-thyroidal illness syndrome: laboratory aspects and clinical significance (narrative review). Eur J Endocrinol / NCBI. Euthyroid Sick Syndrome, StatPearls, accessed 2026.
- Burmeister LA. Reverse T3 does not reliably differentiate hypothyroid sick syndrome from euthyroid sick syndrome. Thyroid. 1995;5(6):435-441. PMID 8808092.
- Vanderpump MPJ, Tunbridge WMG, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol (Oxf). 1995;43(1):55-68. onlinelibrary.wiley.com.
- Fröhlich E, Wahl R. Thyroid autoimmunity: role of anti-thyroid antibodies in thyroid and extra-thyroidal diseases. Front Immunol. 2017;8:521. TPOAb positive in ~90% and TgAb in ~50% of Hashimoto’s. PMC5418333.
- Surks MI, Goswami G, Daniels GH. The thyrotropin reference range should remain unchanged. J Clin Endocrinol Metab. 2005;90(9):5489-5496. PMID 16148346.
- Okosieme O, et al. (British Thyroid Association). Management of primary hypothyroidism: statement by the British Thyroid Association Executive Committee. Clin Endocrinol (Oxf). 2016;84(6):799-808. onlinelibrary.wiley.com.
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.