Thyroid markers
Free T4 Levels by Age: Does a Normal FT4 Change?
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed September 20268 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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UK labs print one adult free T4 range at every age. North Bristol NHS Trust uses 8.0 to 18.0 pmol/L, and other labs differ a little. Free T4 barely moves across adult life. TSH climbs from about 50 in women and 60 in men, so age shows up there instead.
Not sure which markers you need? Build your test →You are 71. Your free T4 reads 10.4, the range beside it starts at 8.0, and your TSH is flagged high. Somebody has mentioned an underactive thyroid. So you go looking for what free T4 should be at 71, and nothing you find is about your age at all.
That is because free T4 is unusual among blood markers. It stays put. The number that shifts as you get older is the one printed above it.
1. What is a normal free T4 level in the UK?
Roughly 8 to 18 pmol/L, depending on your lab. T4 is thyroxine, the main hormone your thyroid gland makes. Most of it travels stuck to proteins in your blood. The free part is the small fraction floating loose, and that is the part your cells can actually use.
North Bristol NHS Trust publishes these adult ranges, with no age column anywhere in the set.
| Marker | Range | Age bands? |
|---|---|---|
| Free T4 | 8.0 to 18.0 pmol/L | None for adults |
| Free T3 | 3.7 to 6.6 pmol/L | None for adults |
| TSH | 0.38 to 5.33 mU/L | None for adults |
Your own lab may differ. Some UK labs run up to 25 pmol/L at the top, which means the same 12 can look mid-range in one city and low in another. Our guide to normal free T4 levels lists the ranges several NHS trusts actually print. Always read your own report's range first.
What does a free T4 of 10, 12 or 18 mean?
Most people arrive holding one figure. Here it is, one value at a time, read against the 8.0 to 18.0 band above.
| Your FT4 | Label | How to read it |
|---|---|---|
| 7 | Low | Under the floor. Your TSH decides what happens next. |
| 9 | Normal, low end | Inside the range. Read it beside your TSH. |
| 12 | Normal | Mid-range. A common, unremarkable result. |
| 15 | Normal, upper half | Typical of people taking levothyroxine. |
| 18 | Normal, at the ceiling | On the top line, not past it. Some labs stop at 25. |
| 22 | Raised at many labs | Check your own range, then speak to your GP. |
One caveat covers every row. Free T4 alone settles very little, because the pair of numbers is the test. A low FT4 with a normal TSH means something quite different from a low FT4 with a high one.
2. Is there an official free T4 chart by age?
For children, yes. For adults, no. Newborns run far higher than adults, and paediatric samples are reported against age bands for that reason.
After 18 that stops. Whether you are 19 or 89, your free T4 is compared with the same band, and no UK lab adjusts it for the decade you are in.
That is not laziness. It is because the evidence for adjusting free T4 is genuinely weaker than the evidence for adjusting TSH, which is the subject of the next two sections.
3. Does free T4 change with age?
Hardly at all through most of adult life, then a little upward after 70. The largest recent study to look is worth quoting directly.
A 2024 paper in Thyroid pooled 7.6 million TSH results and 2.2 million free T4 results from 13 laboratories, collected between 2008 and 2022. It reported the two markers behaving differently:
“In adulthood, TSH URLs [upper reference limits] increased from 60 years in men, and from 50 years in women, while FT4 URLs increased from 70 years onward.”
— Thyroid, 2024
So free T4 does drift, and it drifts late and gently. The same authors were careful about what to do with that. They concluded that the free T4 data were “less uniform” and that more research was needed before age bands were put into practice.
Older work points the same way. A 1985 study of 7,248 people aged three months to 106 years found free T4 was “little affected by the age, the sex or the state of health of the subjects”. Free T3 moved with age. Free T4 sat still.
Two caveats. The 2024 data came from Dutch laboratories rather than UK ones, and both studies used lab records rather than a healthy volunteer sample. The direction is consistent, though, across forty years and four analyser brands.
4. Why does your TSH carry the age signal instead?
TSH is not a thyroid hormone. It is the instruction your pituitary gland sends to your thyroid, which makes it a measure of demand rather than of supply.
As you get older, that instruction gets louder for the same amount of hormone. Your TSH creeps up while your free T4 holds steady. Nothing has broken. The set point has moved.
This matters in a practical way. A raised TSH with a normal free T4 is the pattern clinicians call subclinical hypothyroidism, and it becomes far more common with age. When the 2024 study applied age-specific TSH limits, the number of such results fell in women over 50 and men over 60.
In other words, some older results look borderline mainly because they are being judged against a range built around younger people. That is a conversation for your GP rather than a conclusion you can draw alone. Our guide to TSH levels by age goes through the numbers in full.
5. What moves free T4 more than your age does?
Several things, and all of them outweigh the decade you are in:
- Levothyroxine, which tends to sit free T4 in the upper half of the range.
- When you took that tablet, since a dose taken hours earlier lifts the reading.
- Pregnancy, which shifts the whole thyroid picture trimester by trimester.
- Recent illness, which can push results either way for a few weeks.
- High-dose biotin supplements, which interfere with some thyroid assays.
That last one catches people out. Our guide to supplements that affect blood tests lists how long to leave them before a sample.
None of this is a diagnosis. It is the list a qualified clinician runs through before reading anything into a single result.
6. Which blood test includes free T4?
Not every one. An NHS thyroid check often measures TSH first and only adds free T4 if the TSH comes back outside its range. That is why plenty of people have a TSH history and no FT4 at all.
Helvy's Thyroid & Vital Organs test measures free T4 and free T3 alongside TSH every time, plus liver and kidney markers and a full cholesterol breakdown. It is a home finger-prick kit, processed by UKAS-accredited UK labs. It reads thyroid hormone levels rather than thyroid antibodies, which are a separate step to take with your GP.
If you are not sure which panel fits, the build-my-test tool walks you through it in about a minute.
READY TO TEST?
See your free T4 and TSH together, not one at a time
Helvy's home blood tests report the full thyroid set with plain-English context, so a borderline number never sparks a needless panic. Samples are processed by UKAS-accredited UK labs.
Frequently asked questions
What is a normal free T4 level by age in the UK?
There is no adult age chart. North Bristol NHS Trust uses 8.0 to 18.0 pmol/L for every adult, and other UK labs run up to 25 at the top. Only children are reported against age bands.
Does free T4 fall as you get older?
Not meaningfully. A 2024 study of 2.2 million free T4 results found the upper limit edged up from 70 onward rather than down. A 1985 study of 7,248 people found free T4 was little affected by age at all.
Is a free T4 of 12 normal at 70?
Against a range of 8.0 to 18.0 pmol/L, 12 sits mid-range at any adult age. The figure that decides what happens next is the TSH printed beside it. Always read your own lab's range.
Why is my TSH high when my free T4 is normal?
That pairing is what clinicians call subclinical hypothyroidism, and it becomes commoner with age. TSH measures demand rather than supply, and the set point rises over the years. Your GP reads the pair together, often with a repeat test.
Do UK labs adjust free T4 for your age?
Only up to 18. Adults are all compared with one band. The 2024 Thyroid study argued for age-specific TSH ranges but said the free T4 evidence was less uniform and needed more work first.
What is the difference between T4 and free T4?
Total T4 counts all the thyroxine in your blood, including the large share bound to proteins. Free T4 counts only the loose fraction your cells can use. UK labs report the free measure because it tracks how you feel more closely.
Should I take levothyroxine before a thyroid blood test?
A dose taken shortly before a sample lifts your free T4 reading. Most clinics ask you to take the tablet after the blood draw, or to note the timing. Follow the instruction your own clinic gives you.
Can a supplement change my free T4 result?
High-dose biotin can interfere with some thyroid assays and skew the number. Leave it well before a sample and tell whoever reads your results what you take.
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