Your thyroid values together tell you whether your thyroid is working too hard, too gently 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 (Chaker et al., 2017). Reading one value in isolation rarely gives the whole picture.
A borderline result raises the most questions. Below you will read what each value measures, how to read them in combination and why reference ranges differ between laboratories.
What does TSH measure?
TSH (thyroid-stimulating hormone) does not come from your thyroid, but from your pituitary in the brain. It is the signal that drives your thyroid. A high TSH usually means your thyroid is working too slowly; a low TSH fits an overactive thyroid.
Because TSH is the control signal, it often changes earlier than the thyroid hormones themselves. That is why 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 is actually working, separate from what your brain asks for.
With an underactive thyroid free T4 falls; with an overactive one free T4 and T3 rise. Especially when your TSH is borderline, these values are decisive.
What does anti-TPO mean?
Anti-TPO are antibodies against a protein in your thyroid. A raised anti-TPO points to an autoimmune process, usually Hashimoto. It explains why a thyroid has started working slowly, even when the other values deviate only mildly.
Anti-TPO is especially useful if you have complaints and want 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 assay method and range, and pregnancy has its own limits. Always read your result alongside the range your own 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, especially if you have complaints.
What does a borderline thyroid value mean?
A borderline value, for example a TSH of 4.5 with a normal free T4, sits in a grey zone. Sometimes it fits subclinical hypothyroidism, sometimes it is a snapshot. A repeat measurement after a few weeks to months often helps more than re-interpreting one number.
How you approach such a result depends on your complaints and history. To find out which test suits you, read about thyroid testing: 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 provides context per value. This article belongs to our overview of thyroid symptoms in women, and connects to the explanation of an underactive thyroid.
Frequently asked questions
Is testing TSH alone enough?
TSH alone is a good first step, but it 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 between laboratories?
Yes. Each lab uses its own assay method and reference range. So do not compare loose numbers between labs; read each result alongside the range the relevant 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 assessed, to keep the context right.
References
- NHG-Standaard Schildklieraandoeningen (M31). Nederlands Huisartsen Genootschap. Available via nhg.org.
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
- Sheehan MT. Biochemical testing of the thyroid: TSH is the best and, oftentimes, only test needed. Clin Med Res. 2016;14(2):83-92. PMID: 27231117.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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