Your result is in and a row of numbers sits on your screen: TSH, free T4, maybe anti-TPO. Fine, but what do they mean together? Your thyroid values tell whether your thyroid works too hard, too softly or just right. The four that matter are TSH, free T4, free T3 and anti-TPO. TSH is usually the most sensitive: it reacts first, often before you notice anything. Reading one value alone rarely gives the whole picture.
A borderline result raises the most questions, and that is exactly where interpretation makes the difference. Below you will read what each value measures, how to read them in combination and why reference ranges differ per laboratory.
What does TSH measure?
TSH (thyroid-stimulating hormone) does not come from your thyroid, but from your pituitary gland in your brain. It is the signal that drives your thyroid. A high TSH usually means your thyroid works too slowly; a low TSH fits an overactive thyroid.
Because TSH is the control signal, it often changes earlier than the thyroid hormones themselves. So a thyroid test almost always starts with TSH.
What do free T4 and free T3 say?
Free T4 and free T3 are the thyroid hormones themselves. Free T4 is the storage form; your body converts it into free T3, the active form your cells use. Together they show how much hormone actually works, apart from what your brain asks.
With an underactive thyroid free T4 falls; with an overactive one free T4 and T3 rise. Especially when your TSH is borderline, these values can be decisive.
What does anti-TPO mean?
Anti-TPO are antibodies against a protein in your thyroid. A raised anti-TPO can point to an autoimmune process, usually Hashimoto. It can explain why a thyroid has started working slowly, even when the other values deviate only mildly.
Anti-TPO is most useful if you have complaints and seek an explanation, or if thyroid disease runs in your family.
Thyroid values: indicative reference ranges
The table below gives commonly used reference ranges for adults. Important: laboratories differ in method and range, and pregnancy has its own limits. Always read your result alongside the range your lab states.
| Value | Indicative range (adults) | Unit |
|---|---|---|
| TSH | 0.4 to 4.0 | mIU/L |
| Free T4 | 10 to 23 | pmol/L |
| Free T3 | 3.5 to 6.5 | pmol/L |
| Anti-TPO | below about 35 | kIU/L |
These numbers are a guide, not a diagnosis. A value just outside the range does not automatically mean disease. Discuss your result with a doctor, certainly if you have complaints.
How do you read the values together?
The strength lies in the combination. A high TSH with a low free T4 points to an underactive thyroid; a low TSH with a high free T4 fits an overactive one. If your TSH is slightly raised while free T4 is normal, we speak of subclinical hypothyroidism, a grey area that often calls for a repeat. Anti-TPO added tells whether an autoimmune process is involved.
The NHG (Dutch College of General Practitioners) thyroid guideline uses TSH as the starting point and expands when in doubt. That is exactly why a panel with several values often says more than a standalone TSH. On Thuisarts.nl, the public information of the NHG, you read that a result should always be judged in the context of your complaints, not as a loose number.
What does a borderline thyroid value mean?
A borderline value, for example a TSH of 4.5 with a normal free T4, falls in a grey area. Sometimes it fits subclinical hypothyroidism, sometimes it is a snapshot. A repeat measurement after a few weeks to months often helps more than reinterpreting one number.
How you approach such a result depends on your complaints and history. Want to know which test suits you? Read about testing your thyroid: self-test, home test or blood test.
All values in one test
At Lunara the Thyroid Complete measures TSH, free T4, free T3 and anti-TPO together, so you can read the values in context. A BIG-registered doctor gives context per value. For a baseline, the Thyroid Function looks at TSH and free T4. This article belongs to our overview of thyroid symptoms in women.
Frequently asked questions
Is testing only TSH enough?
TSH alone is a good first step, but misses nuance if you have complaints or if TSH is borderline. Free T4 and anti-TPO make the picture fuller. Discuss with your doctor which combination suits you.
Do thyroid values differ per laboratory?
Yes. Each lab uses its own method and reference range. So do not compare loose numbers between labs, but read each result alongside the range that lab states.
Do medication or the pill affect my thyroid values?
Some medicines and hormonal contraception can affect thyroid values. So mention your medication when your result is reviewed, so the context is right.
Sources
- NHG Guideline Thyroid Disorders (M31). Dutch College of General Practitioners. Available via nhg.org.
- Thuisarts.nl, Results of thyroid blood testing. Dutch College of General Practitioners. Available via thuisarts.nl.
Every blood test result via Lunara includes a professional review by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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