Thyroid Function Test Explained: TSH, Free T3 and Free T4
What a full thyroid function test measures, what each marker means, and how TSH, Free T4, Free T3 and antibodies work together to show whether your thyroid is under or overactive.
Check your thyroidA thyroid function test (TFT) measures how well your thyroid is working. A full panel includes TSH (the pituitary signal), Free T4 and Free T3 (the actual thyroid hormones), and often thyroid antibodies. TSH shows how hard your body is pushing the thyroid, while Free T4 and Free T3 show how much active hormone it is producing. Read together, they reveal whether your thyroid is normal, underactive or overactive, and why. You can order a Full Thyroid Check privately with no GP referral.
- A thyroid function test measures TSH, Free T4 and Free T3, often plus antibodies
- TSH is the pituitary signal, Free T4 and Free T3 are the thyroid hormones
- High TSH with low Free T4 points to an underactive thyroid
- Low TSH with high Free T4 or T3 points to an overactive thyroid
- Thyroid antibodies reveal autoimmune causes like Hashimoto's or Graves'
- A single TSH test is a good screen, the full panel explains the why
What a thyroid function test includes
A thyroid function test (often shortened to TFT) is a group of markers that together show how your thyroid is working. Where a TSH test alone is a screen, the full panel explains what is actually happening and why. A complete thyroid function test usually measures:
- TSH (thyroid stimulating hormone), the signal from your pituitary gland
- Free T4 (free thyroxine), the main hormone your thyroid releases
- Free T3 (free triiodothyronine), the more active hormone your body converts T4 into
- Thyroid antibodies (such as TPO), which reveal autoimmune thyroid conditions
Our Full Thyroid Check covers all five of these markers in a single test: TSH, Free T3, Free T4, TPO antibodies (TPOAb) and thyroglobulin antibodies (TGAb), so you get the complete picture rather than a single number.
Get the complete thyroid panel
View the Full Thyroid CheckTSH: the signal
TSH comes from your pituitary gland and tells your thyroid how much hormone to make. It is the most sensitive single marker, which is why it leads the panel. Remember it runs opposite to thyroid activity: high TSH means your thyroid is underactive, low TSH means it is overactive. For a full breakdown of TSH on its own, see what a TSH blood test means.
Free T4: the main thyroid hormone
Free T4 (free thyroxine) is the primary hormone your thyroid actually produces. The word "free" matters: it is the unbound, available portion that your body can use, which is why Free T4 is more useful than total T4. Reading Free T4 alongside TSH is what separates the patterns, a high TSH with a low Free T4 confirms an underactive thyroid, while a low TSH with a high Free T4 confirms an overactive one.
Free T3: the active hormone
Free T3 (free triiodothyronine) is the most active thyroid hormone. Your body converts much of your T4 into T3, which is what actually drives your metabolism at the cellular level. Free T3 is especially useful in assessing an overactive thyroid and in cases where symptoms do not match the TSH and T4 alone. Not every situation needs it, but on a full panel it adds real depth.
Thyroid antibodies: the cause
Thyroid antibodies, most commonly TPO (thyroid peroxidase) and thyroglobulin (TgAb) antibodies, show whether an autoimmune process is behind a thyroid problem. Raised TPO antibodies are the hallmark of Hashimoto's thyroiditis, the most common cause of an underactive thyroid, and they can also appear in Graves' disease, a common cause of an overactive thyroid. Antibodies answer the "why," which is why they matter when TSH or T4 are out of range.
Reference ranges
Ranges vary by laboratory and assay, and pregnancy has its own. Use these as an indicative adult guide only, and always interpret with a doctor.
| Marker | Indicative adult range |
|---|---|
| TSH | ~0.4 to 4.0 mIU/L |
| Free T4 | ~10 to 20 pmol/L |
| Free T3 | ~3.5 to 6.5 pmol/L |
| TPO / thyroglobulin antibodies | Typically negative or low |
How to read them together
The power of a thyroid function test is in the pattern, not any single number. These are the common patterns:
| Pattern | TSH | Free T4 / T3 |
|---|---|---|
| Normal thyroid | Normal | Normal |
| Underactive (hypothyroidism) | High | Low |
| Subclinical hypothyroidism | High | Normal |
| Overactive (hyperthyroidism) | Low | High |
A doctor reads these alongside your symptoms and antibodies to reach a diagnosis. If your thyroid symptoms come with fatigue that could have other causes, it can be worth checking related markers too, iron, vitamin D and testosterone can all produce overlapping symptoms.
Check your full thyroid function
View the Full Thyroid CheckDo you need the full panel?
If you are simply screening and feel well, a TSH test is often enough as a first step. If your TSH is abnormal, if you have clear thyroid symptoms, or if you want the complete picture in one go, the Full Thyroid Check is the better choice, because it shows not just that something is off, but what and why. Explore the options under hormone blood tests.
Remember: thyroid results are best interpreted as a pattern by a doctor, alongside your symptoms. MediTests reports are reviewed by AHPRA-registered doctors and flag anything outside the reference range.
Frequently asked questions
A full thyroid function test measures TSH (the pituitary signal), Free T4 and Free T3 (the thyroid hormones), and often thyroid antibodies. Together they show whether your thyroid is normal, underactive or overactive, and why.
TSH is the signal from your pituitary telling the thyroid how much hormone to make. Free T4 is the main hormone the thyroid releases, and Free T3 is the more active hormone your body converts T4 into. TSH shows the signal, T4 and T3 show the output.
Indicatively, Free T4 is around 10 to 20 pmol/L and Free T3 around 3.5 to 6.5 pmol/L, though ranges vary by laboratory. They are best read alongside TSH and your symptoms.
Thyroid antibodies, usually TPO and thyroglobulin (TgAb) antibodies, reveal whether an autoimmune process is involved. Raised levels are the hallmark of Hashimoto's thyroiditis (underactive) and can appear in Graves' disease (overactive).
A TSH test is a good screen if you feel well. A full thyroid function test is better if your TSH is abnormal, you have symptoms, or you want the complete picture, because it shows what is happening and why.
Fasting is not usually required. Test in the morning for consistency, and pause high-dose biotin supplements for around 48 hours beforehand, as they can affect thyroid results.
- Australian Thyroid Foundation. Thyroid function tests and interpretation. thyroidfoundation.org.au
- Royal Australian College of General Practitioners (RACGP). Thyroid function testing and interpretation. racgp.org.au
- Healthdirect Australia. Thyroid function tests. healthdirect.gov.au
- Better Health Channel (Victoria). Thyroid gland. betterhealth.vic.gov.au
Get your full thyroid picture
The Full Thyroid Check covers TSH, Free T3, Free T4 and thyroid antibodies in one test. Walk into any of 3,300+ collection centres, results in 24 to 48 hours. No GP referral.
View the Full Thyroid Check