THYROID
Normal Thyroid Levels: TSH, Free T4 & Free T3 by Age
QUICK ANSWER
A normal TSH for most UK adults sits between roughly 0.4 and 4.0 mU/L, with free T4 around 12 to 22 pmol/L and free T3 about 3.1 to 6.8 pmol/L. The healthy upper limit for TSH rises with age, so a result near 5 mU/L can be normal after 70. Labs differ; a qualified clinician should interpret any result.
Your blood test comes back. TSH 4.1. The letter says “normal,” or maybe “borderline.” You feel cold, flat, and your hair is thinning, so “normal” doesn't sit right. The honest answer is that one number on a lab range built for the whole adult population may not mean much for you specifically.
This guide sets out what thyroid levels actually look like, the UK reference ranges for each marker, how the “normal” range shifts with age, and the grey zone where most people who feel off will land. It draws on NICE guideline NG145, NHS guidance, and peer-reviewed research.
By the Helvy Medical Team · Reviewed by a qualified clinician · 14 min read
1. What your thyroid actually does
The thyroid is a small gland at the front of your neck. It produces two hormones, thyroxine (T4) and triiodothyronine (T3), that set the pace of nearly every cell in your body. They govern your metabolic rate, body temperature, heart rate, digestion, mood, and how quickly you think and recover.
The whole system runs on a feedback loop. The pituitary gland releases thyroid-stimulating hormone (TSH), which tells the thyroid how much T4 and T3 to make. When thyroid hormone is low, the pituitary pushes TSH up to compensate. When it is high, TSH drops. This is why TSH moves in the opposite direction to your thyroid hormones, and why a high TSH usually signals an underactive thyroid, not an overactive one.
The catch is the same one that affects most blood markers: UK lab reference ranges are wide and built from a broad population. A 27-year-old and a 78-year-old can both be told their TSH is “normal” at the same number, even though the healthy range is genuinely different for each of them. Understanding what your level means for your age and your symptoms is the point of this guide. For the full breakdown of what each marker measures and the antibodies behind autoimmune thyroid disease, our thyroid blood test guide goes deeper.
2. Normal thyroid levels: the UK reference ranges
The table below shows the reference ranges most UK laboratories use for a standard thyroid panel, drawn from British Thyroid Foundation guidance and common UK lab standards. Exact figures vary between labs, so always read your result against the range printed on your own report.
| MARKER | TYPICAL UK RANGE | WHAT IT REFLECTS |
|---|---|---|
| TSH | 0.4–4.0 mU/L | The pituitary's signal; the most sensitive first-line marker |
| Free T4 | 12–22 pmol/L | The main circulating thyroid hormone, unbound and available |
| Free T3 | 3.1–6.8 pmol/L | The active hormone your cells actually use |
| TPO antibodies | <34 kIU/L | Marker of autoimmune thyroid disease (Hashimoto's) |
A NOTE ON UNITS
UK labs report TSH in mU/L (sometimes written mIU/L; the two are identical), and free T4 and free T3 in pmol/L. Some US results use ng/dL or pg/mL, which do not convert cleanly across labs. If you are comparing your UK result to American content online, the numbers will not line up.
Some UK labs widen the TSH range to 0.27–4.2 mU/L, and a few use a lower upper limit. This inconsistency matters: a TSH of 4.1 could be flagged “high” at one lab and “normal” at another. It is one reason a single number is never the whole story.
3. Thyroid levels by age: why the TSH range shifts
Here is the part most people are not told. The upper limit of normal TSH genuinely rises as you get older. A large analysis of US population data found that the median TSH and its upper limit climb steadily with age, so an older adult can have a TSH that would look slightly raised in a 30-year-old yet sits perfectly within their own age-appropriate range.
The figures below are approximate age-specific upper limits (the 97.5th centile) for TSH, based on the pattern described in research on interpreting elevated TSH in older adults. Free T4 and free T3 stay relatively stable across adult life, so the age effect is mostly a TSH story.
| AGE | APPROX. TSH UPPER LIMIT | NOTES |
|---|---|---|
| 20–39 | ~3.5–3.7 mU/L | Tightest range; a TSH above 4 is more likely meaningful |
| 40–49 | ~3.8–4.0 mU/L | Close to the standard adult range |
| 50–59 | ~4.0–4.4 mU/L | Upper limit starts to drift up |
| 60–69 | ~4.5–5.0 mU/L | A mildly raised TSH is often a normal feature of ageing |
| 70–79 | ~5.0–6.0 mU/L | Treating to a young-adult target risks over-treatment |
| 80+ | ~6.0–7.5 mU/L | Highest upper limit; clinical judgement matters most here |
The practical takeaway: lowering the “normal” ceiling for everyone would label a large share of healthy older people as having an underactive thyroid when they are simply ageing. It also works the other way. A TSH of 3.8 at 30 deserves more attention than the same number at 75, especially alongside symptoms. These figures are for context, not self-diagnosis; a qualified clinician interprets a result against your full picture.
4. TSH, free T4, free T3: what each one tells you
A complete picture reads all three together. Each marker answers a different question, and the pattern between them is what reveals what is going on.
TSH — THE SIGNAL
The most sensitive single test, because the pituitary amplifies small changes in thyroid hormone into large swings in TSH. A high TSH points to an underactive thyroid; a suppressed TSH (below 0.4) points to an overactive one. On its own, though, TSH cannot tell you how much actual hormone is in circulation.
FREE T4 — THE RESERVE
The main hormone the thyroid releases, measured in its free (unbound) form. A low free T4 with a high TSH confirms overt hypothyroidism. A high free T4 with a suppressed TSH points to hyperthyroidism. When TSH is borderline, free T4 is what tips the interpretation one way or the other.
FREE T3 — THE ACTIVE HORMONE
T3 is several times more potent than T4 and is what your cells actually act on. Most T3 is made by converting T4 in your tissues. Free T3 is most useful when investigating suspected hyperthyroidism or when symptoms persist despite a “normal” TSH and free T4.
Helvy's General Energy & Wellness panel measures thyroid function alongside iron, vitamin D, B12, folate, HbA1c and the wider markers that often explain symptoms a thyroid result alone cannot.
5. The grey zone: subclinical hypothyroidism (TSH 4–10)
Subclinical hypothyroidism means a mildly raised TSH with a free T4 still inside its range. It is common, and it is where most of the uncertainty lives. NICE NG145 sets out broad thresholds for what to do:
TSH IN RANGE, FREE T4 IN RANGE
Thyroid function is normal. If you have symptoms, the cause is usually elsewhere: iron, vitamin D, B12, blood sugar, sleep, or stress. This is where a broader panel earns its place.
TSH 4–10 mU/L — THE GREY ZONE
Subclinical. NICE advises repeating the test after 3 to 6 months before any decision, since a single mildly raised TSH often settles on its own. A trial of treatment may be considered if you are under 65 with clear symptoms, or if thyroid antibodies are positive. Age changes the picture, as the chart above shows.
TSH ABOVE 10 mU/L
Treatment with levothyroxine is usually recommended, even when free T4 is still in range, because the risk of progressing to overt hypothyroidism is high. This warrants a GP conversation.
The grey zone is exactly where many people are told “it's normal” while still feeling unwell. Knowing the framework lets you ask for a repeat test, antibodies, and a free T4 rather than accepting a single borderline number.
6. Symptoms mapped to thyroid status
Thyroid symptoms are notoriously non-specific, which is why testing matters rather than guessing. Broadly, an underactive thyroid slows everything down and an overactive one speeds it up.
| PATTERN | COMMON SYMPTOMS |
|---|---|
| High TSH (underactive) | Fatigue, weight gain, feeling cold, constipation, dry skin, hair thinning, low mood, brain fog |
| Low TSH (overactive) | Weight loss, racing or irregular heartbeat, anxiety, heat intolerance, tremor, loose stools, trouble sleeping |
Because these overlap with so much else, a thyroid result is most useful read in context. Persistent tiredness, for example, has many possible drivers; our guide to being always tired walks through the markers worth checking. Unexplained weight gain and hair loss are two other patterns where the thyroid is a common culprit but rarely the only one.
7. Thyroid antibodies: the hidden piece
A “normal” TSH and free T4 can still sit alongside an autoimmune process that is slowly damaging the gland. The most common cause of an underactive thyroid in the UK is Hashimoto's thyroiditis, in which the immune system makes thyroid peroxidase (TPO) antibodies that attack thyroid tissue over years.
Positive TPO antibodies with a TSH still in range mean your thyroid is compensating for now, but you are at higher risk of becoming hypothyroid in future. This is useful to know early, because it changes how a borderline TSH is interpreted and how often you should retest. It is also why two people with the same TSH can be given very different advice.
If your TSH is in the grey zone, asking whether antibodies were measured is one of the most valuable questions you can put to your clinician. The NHS overview of an underactive thyroid sets out how the condition develops and is managed.
8. Thyroid levels in pregnancy
Pregnancy changes thyroid demand significantly, and the standard adult ranges no longer apply. TSH tends to fall in the first trimester as pregnancy hormones stimulate the thyroid, then rises gradually. Thyroid hormone is essential for the baby's brain development, so getting this right matters.
| STAGE | APPROX. TSH RANGE |
|---|---|
| First trimester | ~0.1–3.5 mU/L |
| Second trimester | ~0.2–4.0 mU/L |
| Third trimester | ~0.3–4.0 mU/L |
These are general guides only. Pregnancy thyroid management should always be led by a clinician using trimester-specific, ideally lab-specific ranges. If you are pregnant or planning to conceive and have any thyroid history, this is a conversation to have with your GP or midwife rather than a home test alone.
9. What can skew your thyroid result
Before you act on a borderline number, rule out the things that quietly distort thyroid readings:
TIME OF DAY
TSH follows a daily rhythm and is highest in the early morning, falling through the afternoon. An afternoon sample can read lower than a morning one, which occasionally masks a mildly raised result. For consistency, test in the morning.
BIOTIN SUPPLEMENTS
High-dose biotin, found in many hair, skin and nail supplements, interferes with the lab assay and can produce falsely low TSH or falsely high thyroid hormone readings. Stop biotin for at least two days before testing.
ACUTE ILLNESS
A serious infection or recent hospital stay can temporarily disturb thyroid readings (sometimes called non-thyroidal illness or “sick euthyroid”). Testing during or just after a significant illness can mislead, so retest once you have recovered.
MEDICATIONS
Levothyroxine timing, steroids, amiodarone, lithium and some other drugs all affect thyroid results. If you take levothyroxine, take your blood sample before your morning dose so the reading reflects your steady state.
10. When and how to test
For the most reliable thyroid result, take a morning sample, avoid high-dose biotin for a couple of days beforehand, and if you already take thyroid medication, sample before that day's dose. A single borderline TSH should be repeated after a few months before any conclusion, since mild elevations often resolve.
What a thorough thyroid check should include:
- TSH— the first-line screening marker
- Free T4— to confirm and grade any abnormal TSH
- Free T3— useful for suspected overactivity or persistent symptoms
- TPO antibodies— to detect autoimmune thyroid disease early
- Iron, ferritin and vitamin D — deficiencies that mimic thyroid symptoms
If your TSH is normal but you still feel unwell, the answer is often in the markers around the thyroid rather than the thyroid itself. That is the case for a surprising share of people who feel foggy or tired despite a clear thyroid result, and it is why a broader panel tends to be more useful than a TSH on its own.
Frequently asked questions
What is a normal TSH level in the UK?
Most UK labs use a TSH reference range of about 0.4–4.0 mU/L, though some go up to 4.2 mU/L. The healthy upper limit also rises with age, so a slightly higher TSH can be normal in older adults. Always read your number against the range printed on your own report, and have a qualified clinician interpret it.
Is a TSH of 4 too high?
A TSH of 4 sits at the top of the standard adult range and is often reported as normal, particularly in older adults. In a younger person with clear symptoms it may be worth a repeat test and a free T4 and antibody check. A single borderline reading is not enough to diagnose anything on its own.
Can your thyroid levels be normal but you still feel unwell?
Yes, and it is common. If TSH and free T4 are both in range, the cause of fatigue, low mood or brain fog is often elsewhere: iron, vitamin D, B12, blood sugar, sleep or stress. Positive thyroid antibodies with a normal TSH can also explain symptoms and signal future risk, which is why antibodies are worth including.
Do thyroid levels change with age?
The upper limit of normal TSH rises gradually with age, so an older adult can have a mildly raised TSH that is normal for them. Free T4 and free T3 stay relatively stable. Applying a young-adult target to an older person risks over-treatment, which is why age-aware interpretation matters.
What is the difference between TSH, free T4 and free T3?
TSH is the pituitary's signal telling the thyroid how hard to work, and it is the most sensitive screening test. Free T4 is the main hormone the thyroid releases. Free T3 is the active form your cells use, mostly converted from T4. Reading all three together gives a far clearer picture than TSH alone.
Should I test my thyroid at home?
Home finger-prick tests can accurately measure thyroid markers when processed by a UKAS-accredited lab. Take the sample in the morning, pause high-dose biotin beforehand, and if you take thyroid medication, sample before your dose. A home test is a useful first step; any abnormal result should be reviewed with your GP.
Check your thyroid levels
Our General Energy & Wellness panel (£149) includes thyroid function alongside iron, vitamin D, B12, folate, HbA1c and 11 more markers. Home finger-prick kit, results in 5 days, from UKAS-accredited UK laboratories.
Medical disclaimer:This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Reference ranges cited here are based on British Thyroid Foundation guidance, NICE NG145 and published research, and may differ from the ranges used by your local NHS laboratory. Do not make changes to medication, supplementation, or treatment based solely on information in this article — consult your GP or a qualified healthcare professional. All Helvy blood tests are processed by UKAS-accredited UK laboratories to ISO 15189.
Last updated: June 2026 · By Helvy · Medically analysed at UKAS-accredited UK laboratories