The free T3 test measures the free portion of T3, the most active thyroid hormone; it is ordered especially when an overactive thyroid gland is suspected.
- Sample
- Blood
- Fasting
- Not required
- Turnaround time
- As soon as possible
- Test group
- Hormones
Preparation: You do not need to fast before the test. You may need to pause biotin supplements; tell your doctor about your thyroid medications.
Insurance: We work with private health insurers. For this test, we obtain the pre-approval and collect your sample once approval is received; we do not charge a co-payment. Insurance and payment
Contents
What is free T3?
T3 (triiodothyronine) is the most active thyroid hormone, regulating metabolic rate, heart rate and the body's energy use. The thyroid gland produces only a small part of T3 directly; most of it is formed by conversion of the T4 hormone in tissues such as the liver and kidneys. For this reason, the T3 level reflects not only how the thyroid gland is working but also the rate of conversion in the tissues.
Almost all T3 in the blood circulates bound to carrier proteins, and only the small free portion can enter cells. The free T3 test measures this free portion using an immunoassay. While TSH shows the signal coming from the brain and free T4 shows the gland's main product, free T3 reflects the final hormone acting in the tissues; that is why it provides additional information, especially in evaluating hyperthyroidism. A slight decrease in free T3 levels may be seen with age.
When is a free T3 test ordered?
Free T3 is usually not a first-line test; it is mostly ordered when TSH is low or when there are symptoms of hyperthyroidism. In some patients, only T3 rises while T4 is normal, and this can be detected only by measuring T3. In diagnosing hypothyroidism, on the other hand, it is generally not preferred, because free T3 can remain normal for a long time. Since TSH can stay suppressed for a long time after hyperthyroidism treatment begins, free T3 and free T4 are evaluated together to monitor the effect of treatment.
- Low TSH with normal free T4
- Palpitations, weight loss, sweating and hand tremor
- Suspected Graves' disease or toxic nodule
- Determining the severity of hyperthyroidism
- Monitoring during hyperthyroidism treatment
How do I prepare for a free T3 test?
No fasting is required for free T3, and the test can be done at any time of day; choosing similar times for follow-up measurements makes comparison easier. If you take thyroid medication, especially one containing T3, follow your doctor's instructions, as the time you take the medication can affect the result. Unless your doctor has instructed otherwise, you do not need to pause the other medications you take regularly.
Supplements containing biotin (vitamin B7) can make free T3 falsely high, depending on the immunoassay used; if you take them, you may need to pause them for a while before the test after consulting your doctor. A period of severe illness or a very low-calorie diet can also temporarily lower the value. The sample can be taken from a vein in your arm by a nurse at your home or workplace.
What does a free T3 result mean?
High free T3 usually indicates that the thyroid gland is overactive. When only T3 is high while TSH is low and free T4 is normal, the condition is called T3 thyrotoxicosis; it can be seen in some cases of nodular goiter or early Graves' disease. Taking medication that contains T3 can also raise the value. A fall in free T3 during hyperthyroidism treatment is one of the findings that show the treatment is working.
Low free T3 does not always mean thyroid disease; serious illnesses, prolonged fasting and some medications can reduce the conversion of T4 to T3. It may also be low in advanced hypothyroidism. Because reference ranges can vary with the method, laboratory, age and sex, this page shows the range of the laboratory performing the test. The result should be evaluated by your doctor together with TSH and free T4, taking your symptoms into account.
Reference range
Unit: pg/mL- 2 - 4.4
This is the value used by our laboratory; it may vary depending on age, sex, the method used and the laboratory. Review your result together with your doctor.
How is Free T3 (FT3) done at home?
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01
Book your appointment
Fill in the online form or call us, and let's plan a day and time that suit you together.
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02
Your blood is drawn at home
Our nurse comes to your address and collects your blood sample using sterile, single-use supplies.
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03
You receive your result
Your sample is transported to the laboratory under appropriate conditions; your result is sent to you when it is ready.
Frequently asked questions
The information on this page is for general information only; it does not replace diagnosis or treatment. Consult your doctor for decisions about your health.