Thyroid
Free T3 (Triiodothyronine)
Thyroid
Active thyroid hormone. More sensitive than TSH for detecting thyroid imbalances.
VAT-exempt medical service
Triiodothyronine (T3) is the most potent thyroid hormone, primarily produced when the body converts T4 into T3. Most circulating T3 is bound to transport proteins; the small unbound fraction, known as Free T3 (FT3), is the metabolically active form that enters cells to regulate metabolism, heart rate, and body temperature. Measuring FT3 provides insight into how effectively your body converts and utilises thyroid hormone.
Testing Free T3 is essential for a complete thyroid evaluation. TSH and Free T4 alone can miss cases where conversion of T4 to T3 is impaired. FT3 testing is particularly useful if you have persistent thyroid-related symptoms despite normal TSH and T4, are on thyroid medication and need to confirm adequate conversion, or have risk factors such as chronic inflammation, calorie restriction, or nutrient deficiencies that impair T4-to-T3 conversion.
What a high value can mean
Elevated Free T3 strongly suggests overproduction of thyroid hormone (hyperthyroidism). Graves' disease is the most common autoimmune cause. Other contributing factors include excessive iodine intake, thyroid nodules, pregnancy, and certain medications. In hyperthyroidism, TSH is typically low while T3 and T4 are normal to high.
What a low value can mean
Low Free T3 may indicate hypothyroidism or impaired conversion of T4 to T3. Causes include Hashimoto's thyroiditis, chronic inflammation, severe calorie restriction, low-carbohydrate diets, hormonal imbalances (low oestrogen or testosterone), and deficiencies in iodine, selenium, zinc, iron, and B vitamins. In hypothyroidism, TSH is usually elevated while T3 and T4 are normal to low.
No special preparation is needed. Do not take thyroid medication until after the blood draw. Bring valid identification. If you take a supplement containing biotin (often sold for hair and nails), pause it for two days before the test, as high doses can distort the result.
Order your test
Choose your test and pay online. Your lab referral is created straight away.
Give your sample
Visit any of our partner sampling locations. No appointment needed at most of them.
The lab analyses it
Your sample is analysed at an accredited Swedish laboratory.
Get your results
Your values appear in your account with reference ranges and an explanation of each marker.
Total T3 measures both protein-bound and unbound T3 in the blood, while Free T3 measures only the unbound fraction. Free T3 is the biologically active form and generally provides a more accurate picture of thyroid activity at the cellular level.
Yes. Very low-calorie and very low-carbohydrate diets can lower Free T3, because the conversion of T4 to T3 is partly insulin-dependent. Adequate intake of selenium, zinc, iron, and B vitamins also supports healthy conversion.
This can happen when the thyroid produces enough T4 but the body struggles to convert it to the active T3 form. Chronic stress, inflammation, and specific nutrient deficiencies are common culprits.