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Hormonal
A free T3 test measures the triiodothyronine (T3) circulating unattached to carrier proteins in your blood, the fraction available for your cells to use directly. T3 is a thyroid hormone that affects how fast your body uses energy, influencing heart rate, body temperature, weight, and mood. Doctors order it mainly to help diagnose hyperthyroidism, often alongside TSH and T4. One thing worth knowing: for T3 specifically, many labs and doctors consider a total T3 test more reliable than free T3, so your doctor may choose between the two based on your situation rather than defaulting to free T3.
The test measures free T3, the small unbound fraction of triiodothyronine in your blood, separate from the larger, protein-bound total pool that circulates alongside it. A high result often points to hyperthyroidism, an overactive thyroid making more hormone than your body needs. Because free T3 assays can be technically less consistent than total T3 testing, your doctor typically reads this result together with TSH and T4, rather than relying on FT3 by itself.
Triiodothyronine (T3) is one of two main hormones your thyroid gland produces, alongside T4. Most T3 in your blood travels attached to carrier proteins; the small portion that isn't attached is called free T3, and it's the part your body's cells can actually take up and use. Gram for gram, T3 has a bigger effect than T4 on how fast your body burns energy.
- Symptoms of an overactive thyroid, such as rapid heartbeat or weight loss - Confirming hyperthyroidism when TSH results are unclear - Monitoring treatment for a known thyroid condition - Investigating a suspected pituitary problem affecting thyroid hormones
Unlike free T4, which most labs prefer over total T4 for accuracy, free T3 testing is technically harder to standardise, and some doctors consider a total T3 test more dependable. Your doctor may order either, or both, depending on what they're trying to rule in or out. Because thyroid hormones interact, with T3, T4, and TSH all moving together in patterns, a single T3 result rarely stands alone. Your doctor will read it alongside your other thyroid tests and your symptoms before reaching a conclusion.
A high free T3 usually points to hyperthyroidism, meaning your thyroid is producing more hormone than your body needs. Your doctor will confirm this alongside your TSH and T4 results and your symptoms.
It depends on the situation. Free T3 measures only the active, unbound hormone, while total T3 includes both bound and unbound forms and is considered more reliable by some labs. Your doctor will choose based on what they're investigating.
Fasting is generally not required for this test. Confirm with your doctor or the centre when you book, since instructions can vary.
Thyroid hormones work as a system, so reading FT3 alone can be misleading. Testing them together gives your doctor a clearer picture of whether the problem lies in the thyroid gland itself or elsewhere.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Normal ranges vary by lab and by the assay method used, so your report will list its own reference range. Ask your doctor to explain your specific number rather than comparing it to ranges you find online.
Yes, certain medicines, including some heart and psychiatric drugs, and biotin supplements, can interfere with thyroid test results. Tell your doctor about everything you're taking before the test.
A low FT3 can occur with hypothyroidism, but T3 changes are less specific than TSH or T4 for confirming an underactive thyroid. Your doctor will rely more on TSH and T4 for that diagnosis.