T4 and T3 are the thyroid hormones that drive your metabolism. Free T4 is the storage form, free T3 the active form in your tissues. Together with TSH they give a complete picture of how your thyroid works.
I think T4 and T3 are often skipped too quickly. They explain exactly what your TSH alone does not show.
What are T4 and T3?
Your thyroid mainly makes T4, which your body converts into the more active T3. T3 does the real work in your cells: it sets how fast you burn energy. T4 acts as a store your body converts as needed.
So with a deviating TSH you look at free T4, and sometimes at free T3 for extra context.
Why do you measure free T4 and not total T4?
Most of your T4 is bound to proteins and not directly available. Free T4 is the part your tissues can actually use, and therefore a better measure than total T4.
| Value | What it measures | Why relevant |
|---|---|---|
| Free T4 (FT4) | The available thyroid hormone | Best measure next to TSH |
| Total T4 | Bound plus free hormone | Sensitive to protein fluctuations |
| Free T3 (FT3) | The active form in your tissues | In doubt or specific complaints |
Have your free T4 measured if your TSH deviates. This article belongs to our guide your thyroid and metabolism.
Free T3: when is it relevant?
Free T3 is not measured as standard, but it can help in doubt or with specific complaints. Because T3 is the active form, it sometimes gives context that TSH and free T4 miss.
Still, it is not a routine value. Many doctors make do with TSH and free T4, unless the picture stays unclear.
Reading TSH, T4 and T3 together
The strength sits in the combination. A high TSH with a low free T4 points to a slow thyroid, while a low TSH with a high free T4 points to a fast thyroid. A review in The Lancet describes how you read these values together to recognise a thyroid condition (Chaker, 2017).
The interpretation of TSH itself we cover in TSH levels explained.
Thyroid hormones and your recovery
Because T3 drives your metabolism, a thyroid out of balance touches your energy and recovery directly. A slow thyroid can make you sluggish and flat, a fast one restless. If you feel tired despite good training, read tired despite training.
A deviating value you discuss with your doctor. They decide whether treatment or further testing is needed, and follow your values over time.
What affects your T4 and T3?
Your T4 and T3 do not stand alone but are affected by your whole body. The conversion of T4 into the more active T3 happens in your tissues, and that step can come under pressure with illness, stress or a large energy deficit.
With long-term heavy load or strict dieting, your body can convert less T4 into T3, a pattern that sometimes stands out in athletes. That is usually an adaptation and not a thyroid disease, but it does explain why you feel flat.
The proteins that bind your hormone also play a part. That is why you look at free T4 and not total T4, because the free part best reflects what your tissues actually have available.
Reading these values stays bespoke. A deviating picture you discuss with your doctor, who weighs your whole situation.
References
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. The Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
- Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews. 2008;29(1):76-131. PMID: 17991805.
- Thuisarts.nl / NHG. Thyroid disorders. Accessed 2026.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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