Thyroid markers
Low Free T4 but Normal TSH: What the Pattern Means
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed September 20265 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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A low free T4 with a normal TSH is usually not a thyroid gland problem. UK primary care guidance names the common causes as another illness at the time of the sample, or medicines such as anti-inflammatories, water tablets, epilepsy drugs and high-dose steroids. Rarely the pituitary gland is the question. Repeating the test comes first.
Not sure which markers you need? Build your test →Your report has two thyroid lines on it. The free T4 is flagged low. The TSH above it sits comfortably inside its range. One is highlighted, the other is not, and nobody has explained how both can be true.
They can. This pattern has a short list of ordinary explanations, and for most people the thyroid gland is not on it.
1. Why do the two numbers seem to disagree?
Because they are measuring two different things.
Age is worth ruling out first, and free T4 levels by age explains why it rarely accounts for this.
Free T4, or free thyroxine, is the thyroid hormone circulating in your blood and available for your cells to use. TSH is not a thyroid hormone at all. It is the signal your pituitary gland, a small gland at the base of the brain, sends to tell the thyroid how hard to work.
So the usual pattern in an underactive thyroid is a low free T4 with a high TSH. The pituitary notices the shortfall and shouts louder. That is the shape our guide to underactive thyroid symptoms describes.
Your result does not have that shape. The T4 is low and the shouting never started. That is why it reads as odd, and why the explanation usually sits outside the gland.
2. What usually causes a low FT4 with a normal TSH?
Being unwell, or taking a common medicine. NHS Lothian’s guidance for primary care puts both in one sentence.
“A low Free T4 in the presence of a normal TSH is most commonly due to non-thyroidal illness or the use of NSAIDs, furosemide, anti-convulsants or high dose glucocorticoids.”
In plainer words, here is that list.
- Non-thyroidal illness. Any significant illness can push thyroid readings around while it lasts.
- NSAIDs. Anti-inflammatory painkillers such as ibuprofen and naproxen.
- Furosemide. A water tablet, prescribed for fluid retention.
- Anti-convulsants. Medicines used for epilepsy, and sometimes for nerve pain.
- High-dose glucocorticoids. Steroid tablets such as prednisolone.
None of that is rare. If you were ill in the week of the test, or you take one of those medicines, you already have a likely answer. Our guide on being ill before a blood test covers how long results take to settle.
3. Is your free T4 actually low?
Check it against your own lab’s range before you assume so.
UK labs print free T4 ranges that start anywhere from about 8 to 12 pmol/L and finish around 22 to 25. The same reading of 11 can be flagged at one lab and sit inside the range at another. Our guide to normal free T4 levels works through that spread.
One useful anchor. Where a specialist is replacing thyroid hormone after pituitary disease, the NHS Lothian guidance aims for a free T4 in the upper third of the range, quoted there as 16 to 21 pmol/L. That tells you where clinicians consider the comfortable part of the range to be.
A single reading also carries normal day-to-day variation. One number slightly under a line is a reason to repeat, not a conclusion.
4. When is the pituitary the question?
When the ordinary explanations have been ruled out and the pattern persists.
The same NHS guidance says secondary hypothyroidism should be considered in people with a low FT4 and a normal, or only slightly raised, TSH. Secondary here means the problem sits with the pituitary signal rather than the thyroid gland. It is uncommon, and it is a specialist question.
That is the order to hold in mind. Common things first, repeat the test, then refer if it stands up. A blood test measures the marker. Only a qualified clinician can weigh it against your history and how you feel.
If your TSH is the abnormal one instead, the reading is different again. Our guide to subclinical hypothyroidism covers a raised TSH beside a normal T4, which is the mirror image of your result.
5. What should you do next?
Take the printout back to your GP, and bring three facts with you.
- Were you unwell? Say so, and say when.
- What do you take? Include anything bought over the counter.
- Which lab ran it? Ranges differ, so the second test should ideally use the same one.
If you feel flat and the thyroid numbers look close to normal, iron, B12, folate and vitamin D are the usual next questions. Our guide on normal blood tests when you still feel ill works through them.
6. Which blood test reports free T4?
Helvy’s Thyroid & Vital Organs panel (£159) reports free T4, TSH and free T3 together, alongside liver, kidney and cholesterol markers. Seeing all three thyroid numbers at once is the point. Two of them cannot explain each other.
The General Energy & Wellness panel (£149) carries TSH and free T4, plus cortisol, vitamin D and B12 for a broader tiredness question.
Not sure which fits? The build-my-test tool walks you through it, and the thyroid hub collects the rest of our thyroid guides.
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See TSH, free T4 and free T3 on one report
Helvy’s home blood tests report the thyroid markers together, so no single line has to be read on its own. Samples are processed by UKAS-accredited UK labs.
Frequently asked questions
Can you have a low free T4 and a normal TSH?
Yes. NHS primary care guidance says the most common reasons are another illness at the time of the sample, or medicines such as anti-inflammatories, furosemide, anti-convulsants and high-dose steroids.
Does a low free T4 with a normal TSH mean hypothyroidism?
Not on its own. An underactive thyroid usually shows a low free T4 with a raised TSH. When the TSH is normal, the explanation is more often outside the thyroid gland.
What is secondary hypothyroidism?
It means the pituitary signal is the problem rather than the thyroid gland. UK guidance says to consider it when a low FT4 sits beside a normal or only slightly raised TSH. It is uncommon and needs a specialist.
Should the test be repeated?
Usually, yes. Illness and medicines move these numbers while they last. Ask your GP when to repeat, and use the same lab if you can, because ranges differ between them.
What free T4 level counts as low in the UK?
There is no single line. UK lab ranges start anywhere from about 8 to 12 pmol/L. Read your number against the range printed on your own report rather than one you found elsewhere.
Could ibuprofen really change my thyroid result?
NSAIDs appear by name in the NHS Lothian list of common causes. Tell whoever ordered the test what you take, including things bought over the counter, so the result is read in context.
Related guides
Normal Free T4 Levels UK
Why the same 12 can be flagged at one lab and not another.
Subclinical Hypothyroidism UK
The mirror pattern: a raised TSH beside a normal T4.
TSH Blood Test UK
What the pituitary signal is actually telling you.
Free T3 Blood Test UK
The third thyroid number, and when it adds something.
Illness Before a Blood Test
How long results take to settle after you have been unwell.
Blood Tests Normal but You Feel Ill
What else to rule out when the thyroid looks fine.
Related panels
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