Free T4 and Free T3 Explained
The two thyroid hormone measurements that sit underneath TSH, what a normal Australian range looks like, and how TSH, free T4 and free T3 are read together.
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· · Reviewed against the sources listed at the end of this page.
The Quick Answer
Free T4 (free thyroxine) and free T3 (free triiodothyronine) are the two active thyroid hormones your GP measures directly in a blood test. They sit below TSH (thyroid stimulating hormone) in the testing hierarchy: TSH comes from the pituitary gland and rises or falls in response to how much thyroid hormone the pituitary can detect, while free T4 and free T3 are the hormones the thyroid gland itself is actually producing and releasing.
Typical Australian adult reference intervals are approximately 10 to 20 pmol/L for free T4 and 3.5 to 6.0 pmol/L for free T3, alongside a TSH of roughly 0.4 to 4.0 mIU/L. Every pathology provider sets its own range depending on the assay platform it uses, so the numbers printed on your own report always take priority over these typical figures.
All three are read together. No single one of these numbers settles anything on its own. TSH, free T4 and free T3 combine into a specific pattern of thyroid function, from mild subclinical change through to overt over- or underactivity. See the patterns table below.
Total Hormone vs Free Hormone: Why Australian Labs Report the Free Fraction
Most of the thyroxine and triiodothyronine circulating in your bloodstream is bound to carrier proteins, chiefly thyroxine binding globulin, along with transthyretin and albumin. Hormone stays inactive while it is attached to a carrier protein. Only the small unbound fraction, the free hormone, can leave the bloodstream and act on your tissues.
Older total T4 and total T3 tests measure bound plus free hormone added together, which means the result moves whenever the amount of carrier protein changes, even if the thyroid itself has not. Pregnancy, the oral contraceptive pill, hormone replacement therapy, liver disease and severe illness can all raise or lower thyroxine binding globulin and therefore distort a total hormone result. Measuring free T4 and free T3 sidesteps this problem almost entirely, which is why Australian pathology providers report the free fraction by default and total hormone tests are now rarely used.
Typical Australian reference ranges
TSH (thyroid stimulating hormone)
The most sensitive first-line test. Produced by the pituitary gland, not the thyroid itself. Rises when the thyroid is underactive, falls when it is overactive.
Free T4 (thyroxine)
The main hormone the thyroid releases. Mostly converted to the more active free T3 inside tissues. Reflects current thyroid output.
Free T3 (triiodothyronine)
The biologically active hormone, made mostly by conversion of T4 in the liver, muscle and kidney. Falls fastest of the three during acute illness.
Reading TSH, Free T4 and Free T3 Together
The same free T4 result means something quite different depending on whether TSH is high, normal or low alongside it, which is why the three results are always interpreted as a set rather than one at a time.
High TSH + Low Free T4
The thyroid gland itself is underactive, and the pituitary has raised TSH in an attempt to drive it, without success.
Common causes: Hashimoto thyroiditis is by far the most common cause in Australia. Also seen after thyroid surgery, after radioactive iodine treatment, in iodine deficiency, and with medications such as lithium or amiodarone.
High TSH + Normal Free T4
Early or mild thyroid underactivity. The pituitary is working harder, and for now that extra push is keeping free T4 inside the normal range.
Common causes: Usually early Hashimoto thyroiditis. Can also be transient after an episode of thyroiditis. Treatment depends on how high the TSH is, thyroid antibody status, symptoms, and pregnancy or fertility plans.
Low TSH + High Free T4 or Free T3
The thyroid is producing too much hormone, and the pituitary has switched off TSH in response. When only free T3 is raised and free T4 is normal, this is called isolated T3 toxicosis.
Common causes: Graves disease, toxic multinodular goitre, a single toxic adenoma, subacute or postpartum thyroiditis, or taking more thyroxine than prescribed.
Low TSH + Normal Free T4 and Free T3
Mild overactivity. Free hormone levels are still within range, but the pituitary has already sensed enough activity to reduce TSH output.
Common causes: Early Graves disease, a nodular goitre, or, in someone already taking thyroxine, a replacement dose that is a little higher than they currently need. Often rechecked in 6 to 12 weeks before acting.
Low or Normal TSH + Low Free T4
The thyroid gland itself may be entirely normal. The problem is upstream, in the pituitary or hypothalamus, which is not releasing enough TSH to drive it. TSH can look falsely reassuring here because it is not appropriately raised.
Common causes: Pituitary tumour, pituitary surgery or radiotherapy, Sheehan syndrome after major postpartum haemorrhage, or other pituitary and hypothalamic disease. Uncommon, but important not to miss, particularly alongside other pituitary hormone symptoms.
High TSH + High Free T4
This combination does not fit either an underactive or an overactive thyroid and usually means something has affected the sample or the test, rather than a new thyroid disease.
Common causes: Laboratory assay interference (biotin or heterophile antibodies), blood drawn soon after a recent thyroxine dose or after catching up on several missed doses at once. Rarely, a TSH-secreting pituitary adenoma or inherited resistance to thyroid hormone.
Match your own TSH, free T4 and free T3 to one of these patterns
Upload your pathology PDF and SmarterBlood will line up your TSH, free T4 and free T3 against Australian reference ranges, flag the pattern above that matches, and track every future thyroid test on the same timeline.
Upload Your Thyroid ResultsWhy Free T3 Is Often Left Off, and Rarely Changes Hypothyroidism Management
In Australia, TSH is the Medicare-rebated first-line thyroid test. If TSH comes back outside its reference range, most laboratories automatically add a free T4 measurement to the same sample as a reflex test, at no extra cost or extra blood draw. Free T3 is added less consistently: many labs reflex to free T4 alone when TSH is high, and add free T3 mainly when TSH is low.
There are two practical reasons for this. First, in someone with an underactive thyroid, the body itself converts free T4 into free T3 as needed, so free T3 rarely adds information once TSH and free T4 are known, and it rarely changes the thyroxine dose your GP prescribes. Second, once you are established on thyroxine replacement, free T3 is a poor marker to monitor treatment by: taking T4 by mouth tends to leave free T4 sitting relatively higher and free T3 relatively lower than in a naturally working thyroid, so free T3 does not reliably reflect whether the dose is right. TSH, with free T4 alongside it, is what dose adjustments are based on.
Free T3 is worth ordering when hyperthyroidism is suspected, particularly when TSH is low but free T4 looks normal. In that scenario, some people are producing disproportionately more T3 than T4, a pattern called isolated T3 toxicosis, and free T3 is the only test that will pick it up. If your GP has a specific reason to look for this, they can request free T3 directly on the pathology form, and it uses the same blood sample as TSH and free T4.
Non-Thyroidal Illness (Sick Euthyroid) Syndrome
Any significant acute illness, major surgery, severe infection, or admission to hospital, especially intensive care, can temporarily change TSH, free T4 and free T3 without any true underlying thyroid disease. This is called non-thyroidal illness, or more traditionally sick euthyroid syndrome. The word euthyroid means the thyroid gland is actually working normally.
The typical pattern is a fall in free T3 first, because the body reduces conversion of T4 to T3 during acute illness as a protective, energy-conserving response. Free T4 may stay normal, fall in more severe illness, or occasionally rise. TSH is usually mildly low during the illness itself and can rebound to being mildly and transiently high during recovery, a pattern that can look confusingly like both hyper- and hypothyroidism at different points of the same admission.
Because of this, thyroid function testing is best avoided during an acute illness unless there is a specific clinical reason to test, and an unexpected result taken while unwell is usually repeated four to six weeks after full recovery rather than acted on straight away. If you were tested while in hospital or shortly after a significant illness and the result surprised you, this is the first thing worth discussing with your GP.
Two Things That Can Distort Your Result: Biotin and Dose Timing
High-dose biotin supplements
Most Australian laboratories measure TSH, free T4 and free T3 using immunoassays that rely on a biotin and streptavidin binding step in the machine itself. Biotin (vitamin B7) supplements taken for hair, skin and nail growth are often sold at doses of 5 to 10 mg or more per day, far above the roughly 30 microgram daily requirement from food, and this excess biotin can interfere directly with the assay. Depending on how a particular assay is built, the typical effect is a falsely high free T4 and free T3 with a falsely low TSH, mimicking hyperthyroidism in someone whose thyroid is completely normal. Stop high-dose biotin supplements for at least two to three days before a thyroid blood test, and up to a week for very high doses, and always tell the collection centre or your GP that you take biotin so they can flag it if a result looks inconsistent with how you feel.
Timing your blood test around a levothyroxine dose
If you take levothyroxine, absorbing that morning's tablet causes a small, temporary rise in free T4 for a few hours afterwards. Having blood drawn soon after swallowing the dose can overestimate your true, steady trough level, which matters if your GP is fine-tuning your dose based on the number. The straightforward fix is to have your blood taken before that day's tablet, for example first thing in the morning on an empty stomach, and take the tablet once the sample has been collected. TSH itself changes far more slowly, over weeks rather than hours, so same-day dose timing has much less effect on TSH than it does on free T4.
Symptoms, and When to Seek Urgent Care
Symptoms track whether the thyroid is under- or overactive rather than the free T4 or free T3 numbers themselves. Many people with a subclinical pattern feel entirely well.
Fatigue, weight gain, feeling cold
Classic symptoms of an underactive thyroid (low free T4). Often develops gradually over months, which can make it easy to dismiss as normal ageing or a busy life.
Constipation and dry skin
Reduced thyroid hormone slows gut motility and skin cell turnover. Often improves within weeks of starting or adjusting thyroxine replacement.
Low mood and slowed thinking
Underactive thyroid can mimic or worsen depression and brain fog. Worth checking thyroid function before assuming symptoms are purely psychological.
Weight loss despite a normal or increased appetite
A hallmark of an overactive thyroid (high free T4 or free T3), because metabolism is running faster than usual.
Palpitations, tremor, heat intolerance
Excess thyroid hormone drives the heart and nervous system harder than normal. Palpitations that come with sweating and feeling too warm in a cool room should prompt thyroid testing.
Anxiety, irritability, difficulty sleeping
Common in hyperthyroidism and sometimes mistaken for a primary anxiety disorder, particularly in younger patients.
Neck swelling or a visible lump
Can occur with either an underactive or overactive thyroid, or with a normally functioning nodular goitre. Always worth a GP examination regardless of the blood test result.
No symptoms at all
Subclinical patterns, and even some overt cases caught early on a routine test, can be entirely symptom-free. This is why thyroid function is often tested opportunistically rather than only when symptoms appear.
What Your GP Will Do Next
Confirm the pattern and check the assay
Your GP first checks whether the TSH, free T4 and free T3 results form one of the recognised patterns above, and whether anything could have interfered with the test, such as biotin supplements, a recent thyroxine dose, or an acute illness at the time of the blood draw.
Repeat the test if the picture is unclear
Discordant results, borderline subclinical patterns, and any result taken during an acute illness are usually repeated 2 to 12 weeks later, ideally under standard conditions (fasting morning sample, before that day’s thyroxine dose if you take it).
Check thyroid antibodies if hypothyroidism is likely
Thyroid peroxidase antibodies (anti-TPO) support a diagnosis of Hashimoto thyroiditis and can help decide whether a subclinical result is likely to progress and needs closer monitoring.
Check TSH-receptor or thyroid-stimulating antibodies if hyperthyroidism is likely
These support a diagnosis of Graves disease when TSH is low and free T4 or free T3 is high, and help guide the choice between antithyroid medication, radioactive iodine, and surgery.
Consider thyroid ultrasound or nuclear scan for a lump or nodular pattern
If there is a palpable nodule, a goitre, or a hyperthyroid pattern without positive Graves antibodies, imaging helps distinguish a toxic nodule or multinodular goitre from Graves disease.
Start or adjust treatment and set a review date
Thyroxine replacement for hypothyroidism, or antithyroid medication such as carbimazole for hyperthyroidism, is started or adjusted based on the pattern, your symptoms, age, and any pregnancy plans. A follow-up TSH and free T4 is typically arranged for 6 to 8 weeks later, since thyroid hormone levels take that long to fully settle after a dose change.
Refer to an endocrinologist for complex or persistent cases
Central hypothyroidism, suspected pituitary disease, discordant results that do not resolve with repeat testing, pregnancy with abnormal thyroid function, and Graves disease being considered for radioactive iodine or surgery are typically referred to an endocrinologist.
Sources and reference ranges
Adult reference ranges on this page follow the AACB and RCPA harmonised reference intervals unless stated otherwise; the range printed on your own report always takes precedence because laboratories differ.
- healthdirect Australia: thyroid function tests
- Pathology Tests Explained (Australasian Association for Clinical Biochemistry and Laboratory Medicine)
- Australasian Association for Clinical Biochemistry and Laboratory Medicine, publisher of the harmonised reference intervals
- healthdirect Australia: full blood count
Free T4 and Free T3 Explained: Frequently Asked Questions
What is the difference between free T4 and total T4?
Most of the thyroxine (T4) circulating in your blood is bound to carrier proteins, mainly thyroxine binding globulin, and is biologically inactive while attached. Only the small unbound fraction, free T4, is available to enter cells and act on tissue. Total T4 measures bound plus free hormone together, so it rises and falls with the level of carrier protein itself, for example during pregnancy, on the oral contraceptive pill, or with liver disease, even when thyroid function has not changed. Australian labs report the free fraction by default because it reflects true thyroid status regardless of how much binding protein you have.
What is a normal free T4 and free T3 range in Australia?
Typical Australian adult reference intervals are approximately 10 to 20 pmol/L for free T4 and 3.5 to 6.0 pmol/L for free T3, alongside a TSH of roughly 0.4 to 4.0 mIU/L. These numbers vary by pathology provider and by assay platform, so always read the range printed on your own report rather than these typical figures. Pregnancy has its own trimester-specific ranges, and children have age-specific ranges that differ from adults.
Why did my GP test TSH but not free T3?
In Australia, TSH is the first-line thyroid test, with free T4 added automatically by the laboratory (a reflex test) if the TSH is outside range. Free T3 is not added as consistently: many laboratories reflex to free T4 alone when TSH is high, and add free T3 mainly when TSH is low, because that is where it changes management. Free T3 rarely alters the management of an underactive thyroid and is not well suited to monitoring people already on thyroxine replacement. If your GP has a specific reason to check free T3, such as suspected isolated T3 toxicosis, they can request it directly on the pathology form.
Can biotin supplements affect my thyroid test results?
Yes. Most Australian labs measure TSH, free T4 and free T3 using immunoassays built on a biotin and streptavidin binding step. High-dose biotin supplements, commonly taken for hair, skin and nail growth at doses of 5 to 10 mg or more, can interfere with this step and produce falsely high free T4 and free T3 with a falsely low or suppressed TSH, mimicking hyperthyroidism in someone whose thyroid is actually normal. Stop high-dose biotin supplements for at least two to three days, and up to a week for very high doses, before having thyroid blood tests, and tell your GP or the collection centre that you take biotin.
Why is my TSH high but my free T4 normal?
This pattern is called subclinical hypothyroidism. The pituitary gland has detected that the thyroid is starting to struggle and is releasing extra TSH to push it harder, and this extra push is currently succeeding in keeping free T4 within the normal range. It usually reflects early or mild underactivity of the thyroid, most often from early Hashimoto thyroiditis. Whether it needs treatment depends on how high the TSH is, whether thyroid antibodies are positive, your symptoms, and whether you are trying to conceive or pregnant, so this is a decision your GP makes individually with you.
Why is my TSH low but my free T4 and free T3 normal?
This pattern is called subclinical hyperthyroidism. The pituitary has sensed slightly more thyroid hormone activity than it wants and has reduced TSH output in response, but free T4 and free T3 are still within the normal range. Common causes include early Graves disease, a nodular goitre, or, in someone already taking thyroxine, a replacement dose that is slightly too high for their needs. A repeat test in 6 to 12 weeks is often the first step, since a mildly low TSH can also be transient.
Does being unwell affect my thyroid test results?
Yes. Any significant acute illness, recent surgery, or admission to hospital can temporarily alter TSH, free T4 and free T3 without any underlying thyroid disease, a pattern called non-thyroidal illness or sick euthyroid syndrome. TSH is often mildly low or mildly high during the illness and free T3 typically falls, while free T4 can go either way. These changes usually correct themselves within weeks of recovery. Where possible, thyroid function is best tested when you are well, and an abnormal result during an acute illness is usually repeated after recovery rather than acted on immediately.
When should I have my blood taken if I am on thyroxine (levothyroxine)?
Ideally, have your blood drawn before that day’s levothyroxine dose, for example first thing in the morning, and take the tablet after the blood test rather than before it. Levothyroxine absorption causes a temporary rise in free T4 for a few hours after the tablet is swallowed, and testing soon after a dose can overestimate your true, steady trough level. TSH is far less affected by same-day dosing because it changes slowly over weeks, but free T4 timing still matters if your GP is fine-tuning your dose.
What does it mean if both TSH and free T4 are high at the same time?
A high TSH together with a high free T4 is an unusual, discordant combination and is not the typical pattern of either an underactive or overactive thyroid. The most common explanations are a laboratory assay problem such as heterophile antibody or biotin interference, or blood drawn too soon after a recent thyroxine dose (including a missed run of doses followed by catching up with several tablets at once). Rare causes include a TSH-secreting pituitary adenoma or inherited resistance to thyroid hormone. Your GP will usually repeat the test, check timing and supplement use, and consider a different assay method or specialist referral if the result persists.
Related Reading
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