Free T4 Blood Test: What Your Main Thyroid Hormone Level Means

What is free T4?

Free T4 (free thyroxine) is the share of thyroxine in your blood that isn't bound to carrier proteins. Thyroxine is the main hormone your thyroid makes, and most of it is held by proteins, with less than 0.1% circulating free. That free share is what your tissues can take up and turn into the active hormone, free T3.

Your thyroid makes T4 when the pituitary gland in your brain sends thyroid-stimulating hormone (TSH). Each T4 molecule carries four iodine atoms, which is why your thyroid needs iodine.

T4 is best thought of as a reserve. It lasts about a week in the blood, so it changes slowly, which is why doctors wait around six weeks after a dose change before rechecking.

What does a free T4 test show?

A free T4 test shows how much hormone your thyroid is putting out. Read with TSH, it tells your doctor whether the thyroid is underactive, overactive or working normally.

TSH, the thyroid marker in your Baseline, is very sensitive, because the pituitary turns it up or down in response to small changes in thyroid hormone. Free T4 adds the detail, and the pair together show whether a problem starts in the thyroid or the pituitary.

Thyroid hormone sets the pace of your metabolism, so free T4 is linked to energy, body temperature, heart rate, bowel habit, periods, mood and cholesterol. An underactive thyroid is one of the reasons doctors check thyroid function when LDL cholesterol rises.

Normal does not always mean optimal. A free T4 at the bottom of the range with a TSH drifting upwards is worth tracking, even when both are technically normal.

Free T4 normal range

For adults, free T4 is usually 10 to 25 pmol/L.

Free T4 ranges depend on the lab's method, and some Australian labs use an upper limit closer to 20 pmol/L, so check the range on your report. Optimal ranges are what we aim for to support long-term health, not diagnostic cut-offs.

What does high free T4 mean?

A high free T4 with a low TSH usually means an overactive thyroid (hyperthyroidism) or an inflamed thyroid leaking stored hormone (thyroiditis). It needs a GP to find the cause. A high free T4 with a normal TSH is more often a testing issue.

Everyday reasons to rule out first:

• Biotin supplements. High-dose biotin, often in hair, skin and nail products, can make free T4 read falsely high and TSH falsely low on some lab methods.
• Taking levothyroxine shortly before your blood test, which can lift free T4 for a few hours.
• Too high a dose of thyroid medication.
• Heparin injections, which can raise free T4 in the test tube.

Medical causes a doctor will look for:

• Graves' disease, an autoimmune condition and the most common cause of an overactive thyroid. It can come with eye changes.
• Overactive thyroid nodules.
• Thyroiditis, for example after a viral illness or within a year of having a baby. Levels often settle on their own, sometimes after an underactive phase.
• Medicines and iodine loads, such as amiodarone or iodine-containing contrast for scans.
• Rarely, a pituitary or hormone-resistance condition, suggested by a high free T4 with a TSH that isn't low.

What does low free T4 mean?

A low free T4 with a high TSH usually means an underactive thyroid (hypothyroidism). In Australia, the most common cause is Hashimoto's thyroiditis, an autoimmune condition. A low free T4 with a low or normal TSH needs a GP to check the pituitary.

Other causes:

• Not enough thyroid medication, missed doses, or taking it with food, coffee, iron or calcium, which reduces absorption.
• After thyroid surgery or radioactive iodine treatment.
• Significant illness, which can lower thyroid hormones for a while (more on our free T3 page).
• Medicines such as carbimazole, lithium and amiodarone.
• Low iodine intake. Less common since Australian bread has used iodised salt, but possible in pregnancy or if you avoid dairy, seafood and iodised salt.

What can affect your free T4 result

Biotin, medication timing, pregnancy and recent illness are the biggest things to plan around.

• Biotin. Pause it for a couple of days before your test, after checking with your doctor.
• Levothyroxine timing. Keep taking your medication. Many doctors prefer you have your blood taken before that day's dose. Ask your GP what they want, and be consistent from test to test.
• Pregnancy. Thyroid hormone needs rise, and the hormone hCG affects thyroid results in early pregnancy, so results need pregnancy-specific ranges and a doctor's review.
• Recent illness, surgery or hospital stay. Wait until you've recovered unless your doctor needs results sooner.

Do you need to fast? No. If you're testing with the Full Body Health Check, you'll fast anyway for glucose and insulin. Testing at a similar time each round makes your results easier to compare.

How free T4 fits with your other results

Free T4 is always read with TSH first, then with free T3.

How to support healthy free T4 levels

If your free T4 is low because of Hashimoto's or another thyroid condition, food and lifestyle won't fix it, and medication is the right tool. What you eat still matters for making thyroid hormone and for how well treatment works.

• Get enough iodine, without overdoing it. Use iodised salt in cooking and include dairy, eggs and seafood. Avoid kelp and seaweed supplements, as large doses of iodine can trigger thyroid problems.
• If you're pregnant, planning pregnancy or breastfeeding, the NHMRC recommends a daily 150 µg iodine supplement. Check with your doctor first if you have a thyroid condition.
• Look after your iron. Include red meat, legumes and leafy greens, and get your ferritin checked if you're tired or have heavy periods.
• Take levothyroxine the same way every day, on an empty stomach with water, and keep iron and calcium supplements about four hours away from it.

When to see your GP

See your GP if your free T4 is outside your lab's range, especially with an abnormal TSH, or if you have symptoms of an underactive thyroid (tiredness, feeling cold, weight gain, constipation, heavy periods) or an overactive thyroid (racing heart, weight loss, tremor, feeling hot, anxiety).

Book in promptly for a fast or irregular heartbeat, chest pain, breathlessness, bulging or sore eyes, or a new neck lump. If you're pregnant, have any abnormal free T4 reviewed soon.

Testing free T4 with Vively

Your Baseline Health Check screens TSH. Free T4 is added in our Full Body Health Check, together with free T3, and in our Weight Management Test, Female Hormone Health Test, and Perimenopause and Menopause Hormone Tests. The exact markers can depend on the referring doctor's review.

We arrange your referral through our GP network, and you have one blood draw at one of 4,000+ accredited partner collection centres. Results arrive in the app, usually within a few days. Your practitioner looks at free T4 next to your TSH and our optimal ranges, flags anything your GP should see, builds a personalised plan, and you retest to track change.

Check your free T4 with the Full Body Health Check.

Frequently asked questions

Do I need to fast before a free T4 test?

No, free T4 doesn't need fasting. If it's part of a test with fasting markers, such as Vively's Full Body Health Check, you'll fast overnight anyway. Stop biotin supplements for a couple of days beforehand after checking with your doctor, and ask your GP whether to take levothyroxine before or after your blood test.

What is the difference between free T4 and TSH?

TSH is the signal from your pituitary gland telling your thyroid to work. Free T4 is the hormone your thyroid makes in response. TSH is the more sensitive screening test, and free T4 shows how much hormone is reaching your blood. Reading both together shows where a problem starts.

Is a free T4 of 9 pmol/L low?

It is slightly below the typical range of 10 to 25 pmol/L, though some labs set their lower limit around 9. If your TSH is high, it suggests an underactive thyroid and needs GP review. If your TSH is normal or low, it may reflect recent illness, lab variation or, less often, a pituitary issue.

Does biotin affect thyroid test results?

Yes. High-dose biotin can interfere with some thyroid tests, making free T4 and free T3 look falsely high and TSH falsely low. This can mimic an overactive thyroid. Stop biotin for a couple of days before your test after checking with your doctor, and tell your practitioner you take it.

Can I get a free T4 test through Medicare?

Many blood tests can be Medicare-funded when a doctor orders them for a clinical reason. With Vively, we arrange the referral through our GP network, so you don't need a separate GP visit first. Your own GP can also order free T4 with TSH for a clinical reason.

Does Vively replace my GP?

No. Vively helps you understand and track your thyroid results, and our practitioners will tell you when something needs medical review. You should still see your GP for diagnosis, thyroid medication, pregnancy care and symptoms that need a doctor. If your results point that way, we'll tell you.

References

• Royal College of Pathologists of Australasia (RCPA). RCPA Manual: Thyroxine (free). https://www.rcpa.edu.au/Manuals/RCPA-Manual/Pathology-Tests/T/Thyroxine-Free
• Pathology Tests Explained. Thyroid function tests. https://www.pathologytestsexplained.org.au
• Healthdirect Australia. Hypothyroidism. https://www.healthdirect.gov.au/hypothyroidism
• Walsh JP. Managing thyroid disease in general practice. Medical Journal of Australia. 2016;205(4):179-184.
• National Health and Medical Research Council (NHMRC). Iodine supplementation for pregnant and breastfeeding women. Public statement. 2010.
• U.S. Food and Drug Administration. The FDA warns that biotin may interfere with lab tests: FDA safety communication. 2017.