Thyroid blood tests explained: TSH, T4, T3 and antibodies.
A thyroid blood test measures how well the gland in your neck is setting the pace of your body. TSH is the screening marker, free T4 and free T3 are the hormones themselves, and thyroid antibodies show whether your immune system is involved. Read together, they tell your doctor whether your thyroid is running slow, fast or normally.
Why women are tested more often
Thyroid problems are several times more common in women than in men, and the risk rises after pregnancy and through the perimenopausal years. The bigger reason for testing is overlap: an under-active thyroid produces tiredness, weight gain, low mood, brain fog, feeling cold, dry skin, hair shedding and heavier or irregular periods. Every one of those is also blamed on "hormones", stress or being a busy woman in her forties.
So a thyroid test is often about ruling a thyroid problem out before symptoms are attributed to something else. It is why TSH appears in our fertility, hormone and perimenopause panels, and why women are tested at four particular moments.
- Postpartum: the thyroid can swing over-active then under-active in the year after a baby (postpartum thyroiditis), easily mistaken for new-parent exhaustion.
- Perimenopause: low mood, poor sleep, palpitations and weight change are shared between the two, and they can coexist.
- Unexplained fatigue: thyroid sits beside iron on the shortlist of checkable causes.
- Cycle changes or fertility difficulty: both an under-active and an over-active thyroid can disturb ovulation.
What TSH, free T4, free T3 and antibodies each show
A thyroid function test is not one number. Each marker answers a different question, and the pattern matters more than any single value.
- TSH (thyroid-stimulating hormone): made by the pituitary gland in the brain, it tells the thyroid how hard to work. When the thyroid slows, the brain shouts louder and TSH rises. When the thyroid over-produces, the brain goes quiet and TSH falls. It is the most sensitive screening marker for a problem in the gland itself, and it can be misleading in the rare case of pituitary disease, which is one reason free T4 is read alongside it.
- Free T4 (thyroxine): the main hormone the thyroid releases, measured as the unbound fraction available to your tissues. It confirms whether TSH is responding to a genuine shortfall or excess.
- Free T3 (triiodothyronine): the more active hormone, mostly made by converting T4 in your tissues. It is most useful when an over-active thyroid is suspected, and adds less in the under-active picture.
- Thyroid peroxidase antibodies (anti-TPO) and thyroglobulin antibodies: markers of autoimmune thyroid disease. Positive antibodies mean your immune system is targeting the gland, which is the most common route to an under-active thyroid in Australia, and they raise the chance that a mildly abnormal TSH will progress.
Reference ranges, and the "TSH is normal but I feel awful" question
| Marker | Typical adult range | What it usually means |
|---|---|---|
| TSH | About 0.4 to 4.0 mIU/L | Above range suggests under-active; below range suggests over-active. Mild changes are often repeated before anything is decided. |
| Free T4 | About 10 to 20 pmol/L | Low with a high TSH confirms an under-active thyroid. High with a low TSH confirms an over-active one. |
| Free T3 | About 3.5 to 6.5 pmol/L | Raised in most over-active patterns. Usually normal in early under-active disease, so it rarely changes that decision. |
| Anti-TPO and thyroglobulin antibodies | Reported as negative or positive against a lab cut-off | Positive points to autoimmune thyroid disease. Levels do not track how you feel and are not usually repeated. |
Ranges vary by laboratory; your report shows the range your lab uses. Typical ranges on this page are drawn from Pathology Tests Explained (RCPA) and the sources listed below.
Now the hard question. If your TSH sits inside the range and you still feel exhausted and foggy, does that rule the thyroid out? Mostly, yes: a normal TSH with a normal free T4 means the gland is keeping up. It does not mean your symptoms are imaginary. It means the cause is more likely elsewhere, most often low iron, low B12 or vitamin D, sleep, mood or the perimenopausal transition.
Two nuances. TSH at the upper end of the range with positive antibodies is a picture doctors watch, because it can drift into an under-active thyroid over time. And TSH is not fixed: illness, time of day, high-dose biotin and pregnancy all nudge it, so a borderline result is usually repeated in six to twelve weeks (often around two to three months) before it is acted on.
High TSH and low TSH: what each pattern usually points to
Doctors read TSH and free T4 as a pair, giving four common patterns.
- High TSH, low free T4: an under-active thyroid (hypothyroidism). The brain is asking for more and the gland cannot deliver. Symptoms are usually present.
- High TSH, normal free T4: a "subclinical" under-active picture. The gland is still keeping up but needs more prompting. Often repeated, with antibodies checked, before any decision.
- Low TSH, high free T4 or free T3: an over-active thyroid (hyperthyroidism). Anxiety, palpitations, heat intolerance, weight loss and lighter or absent periods are typical.
- Low TSH, normal free T4 and T3: a "subclinical" over-active picture, which can also appear temporarily in early pregnancy or during thyroiditis.
Thyroid and fertility, pregnancy and postpartum
Thyroid hormone is needed for regular ovulation; an under-active thyroid can lengthen cycles, cause heavier bleeding or make conception harder. It also raises prolactin in some women, which suppresses ovulation further. This is why TSH sits in the AMH and fertility panel and why doctors check it early in a fertility workup.
In pregnancy the reference ranges change. TSH naturally runs lower in the first trimester and laboratories use trimester-specific intervals, so a pregnancy result should never be read against the standard adult range. Your pregnancy care team manages this.
After a baby, postpartum thyroiditis can produce a short over-active phase around three to four months, then an under-active phase; some women stay under-active. Because the symptoms overlap so completely with looking after a newborn, thyroid testing is worth including in a postpartum blood check alongside iron, and is recommended for women with positive thyroid antibodies or symptoms.
When a GP orders only TSH, and when the full panel matters
In Australia, TSH is the accepted screening test and it is what a GP will usually order first. Under Medicare rules, free T4 and free T3 generally attract a rebate only when the TSH is outside the range or in specific circumstances such as monitoring a known condition, suspected pituitary disease, or taking certain medicines that are known to interfere with thyroid results. So a report showing TSH alone is not a shortcut; it reflects how the system is designed to work, and for many women it is enough.
The full panel earns its place when the first result is borderline, when symptoms are strong despite a normal TSH, when an over-active thyroid is suspected (free T3 matters here), when there is a family history, or when you want to know whether antibodies are present so a marginal TSH can be interpreted properly. KnowLuna includes all five markers so one collection answers the whole question.
Your KnowLuna thyroid panel
The KnowLuna thyroid panel measures TSH, free T4, free T3, thyroid peroxidase antibodies and thyroglobulin antibodies from one blood sample, collected at an accredited centre or with a home kit or a nurse visit. Collect in the morning if you can, pause high-dose biotin for 72 hours beforehand (it can interfere with the assay), and no fasting is needed.
Your report shows each marker against the laboratory range with a plain-English note and reads the pattern as a set; anything that needs attention is followed up by a doctor. Because the pattern matters more than any one number, the optional doctor consult is a sensible add-on if anything is borderline.
Common questions
What are the five tests for thyroid?
A full thyroid panel usually means TSH, free T4, free T3, thyroid peroxidase antibodies and thyroglobulin antibodies. TSH screens for a problem, free T4 and free T3 confirm the direction and severity, and the two antibodies show whether autoimmune disease is behind it. Most GP requests start with TSH and add the rest if it is abnormal.
What is a normal score for a thyroid test?
For TSH, most Australian laboratories use a range of about 0.4 to 4.0 mIU/L, with free T4 roughly 10 to 20 pmol/L. Ranges vary between labs and change in pregnancy, so read your result against the interval printed on your report. A value near either edge is worth discussing rather than dismissing.
What should be checked if TSH is high?
A high TSH is usually followed by free T4, to see whether the thyroid is still keeping up, and thyroid antibodies, to see whether autoimmune disease is the cause. Doctors commonly repeat a mildly raised TSH after six to twelve weeks, because illness, timing and supplements can push it up temporarily. Your doctor interprets the set together.
What does it mean if I test positive for thyroid antibodies?
Positive anti-TPO or thyroglobulin antibodies mean your immune system is targeting your thyroid, the pattern behind most under-active thyroid disease in Australia. With a normal TSH, it means a higher chance of developing a problem later, so your doctor may suggest periodic checks. With a raised TSH, it helps explain the cause. Levels themselves are not usually tracked.
Is a thyroid test covered by Medicare?
When a GP requests a TSH for a clinical reason, it is usually rebated and most laboratories bulk bill it, and free T4, free T3 and antibodies attract a rebate in defined circumstances such as an abnormal TSH. KnowLuna tests are self-pay with no Medicare rebate, the trade-off for ordering the full panel yourself without a referral.
Sources and further reading
- Pathology Tests Explained (RCPA): thyroid function tests
- Pathology Tests Explained (RCPA): thyroid stimulating hormone (TSH)
- Pathology Tests Explained (RCPA): free T4
- Pathology Tests Explained (RCPA): free T3
- Pathology Tests Explained (RCPA): thyroid antibodies
- Healthdirect: thyroid function tests
- Better Health Channel: Hashimoto's disease
General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.
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