What Is Free T3 (Triiodothyronine)?
Free T3 (free triiodothyronine) is the active, unbound form of the thyroid hormone triiodothyronine. While the thyroid gland primarily produces T4 (thyroxine), most T3 is produced by the conversion of T4 to T3 in peripheral tissues such as the liver and kidneys. Only about 0.3% of total T3 circulates in its free, biologically active form.
Free T3 is the most metabolically potent thyroid hormone, approximately 3–5 times more active than T4. It regulates your metabolic rate, body temperature, heart rate, brain development, and energy production. Measuring Free T3 alongside TSH and Free T4 provides the most complete picture of thyroid function.
Why Is Free T3 Tested?
A Free T3 test may be requested to:
- Investigate symptoms of hyperthyroidism when TSH is suppressed but Free T4 is normal (T3 thyrotoxicosis)
- Evaluate the severity of hyperthyroidism - Free T3 often rises before Free T4
- Assess thyroid function in patients on T3-containing thyroid medication (liothyronine)
- Monitor treatment response in Graves' disease or toxic nodular goitre
- Investigate persistent hypothyroid symptoms despite normal TSH and Free T4 (possible poor T4-to-T3 conversion)
- Evaluate thyroid function in critically ill patients (euthyroid sick syndrome/non-thyroidal illness)
Normal Ranges
Free T3 is measured in pmol/L in the UK:
| Group | Normal Range (pmol/L) |
|---|---|
| Adults | 3.1 – 6.8 |
| Pregnancy (1st trimester) | 3.2 – 5.9 |
| Pregnancy (2nd/3rd trimester) | 3.0 – 5.2 |
| Elderly (over 70) | 2.8 – 5.5 |
Reference ranges may vary between laboratories. Always interpret Free T3 alongside TSH and Free T4.
Check Your Free T3 (Triiodothyronine) Levels at Home
The Hormone 7 Blood Test includes Free T3 (Triiodothyronine) testing along with other key biomarkers. Results in 2 working days with a free at-home phlebotomist visit.
View Hormone 7 Blood Test →Causes of High Free T3
Elevated Free T3 may indicate:
- Graves' disease: The most common cause of hyperthyroidism, with autoimmune stimulation of the thyroid
- T3 thyrotoxicosis: A form of hyperthyroidism where Free T3 is elevated but Free T4 remains normal
- Toxic multinodular goitre: Overactive thyroid nodules producing excess hormones
- Toxic adenoma: A single overactive thyroid nodule
- Thyroiditis: Inflammation of the thyroid causing stored hormones to leak into the bloodstream
- Excessive thyroid medication: Over-replacement with liothyronine (T3) or combination T4/T3 therapy
- Exogenous T3 intake: Weight loss supplements or bodybuilding products containing thyroid hormones
Causes of Low Free T3
Low Free T3 levels may be caused by:
- Hypothyroidism: Underactive thyroid producing insufficient hormones
- Poor T4-to-T3 conversion: Selenium deficiency, chronic illness, or stress can impair conversion
- Non-thyroidal illness syndrome (NTIS): Also called sick euthyroid syndrome - low T3 during acute illness, surgery, or fasting
- Caloric restriction: Severe dieting or eating disorders reduce T4-to-T3 conversion as a protective mechanism
- Medications: Amiodarone, beta-blockers, and corticosteroids can reduce T3 levels
- Selenium deficiency: Selenium is essential for the deiodinase enzymes that convert T4 to T3
- Ageing: Free T3 levels naturally decline with age
How to Support Healthy T3 Levels
- Ensure adequate selenium intake: Brazil nuts, seafood, eggs, and sunflower seeds support T4-to-T3 conversion
- Maintain sufficient zinc levels: Zinc is required for thyroid hormone synthesis and receptor function
- Get enough iodine: Iodine is a building block for thyroid hormones - dairy, fish, and iodised salt are good sources
- Avoid extreme dieting: Very low-calorie diets suppress T3 production as a survival mechanism
- Manage stress: Chronic stress and elevated cortisol can impair T4-to-T3 conversion
- Exercise moderately: Regular physical activity supports thyroid function, but overtraining can suppress T3
- Address iron deficiency: Iron is needed for the enzyme thyroid peroxidase (TPO) which produces thyroid hormones
- Discuss medication with your GP: If T3 remains low despite normal T4 and TSH, liothyronine supplementation may be considered
When Should You Get Tested?
Consider a Free T3 test if you:
- Have symptoms of hyperthyroidism (weight loss, tremor, rapid heartbeat, anxiety, heat intolerance)
- Have a suppressed TSH with normal Free T4 (possible T3 thyrotoxicosis)
- Are on thyroid medication and still experiencing symptoms
- Have persistent fatigue, brain fog, or cold intolerance despite normal TSH and T4
- Are being treated for Graves' disease and need monitoring
- Have unexplained weight changes or hair loss
Which Lola Health Tests Include Free T3?
Free T3 is included in several Lola Health panels:
- Peak Insights - our most comprehensive panel including full thyroid function (TSH, Free T4, Free T3) plus 70+ biomarkers
- Vital Check — includes complete thyroid panel with Free T3
- Hormone Check — thyroid and hormone markers including Free T3
Check Your Free T3 Levels
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This biomarker is included in our Free T3 Test and Thyroid & Hormonal Function — results in 2-3 working days with GP-reviewed insights.
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