Free T3 Blood Test: Measuring Active Thyroid Hormone
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.
Why Test Free T3?
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 Do High Free T3 Levels 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 Do Low Free T3 Levels 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.
Symptoms to Watch For
- Persistent fatigue and weakness
- Unexplained weight gain or difficulty losing weight
- Cold hands, feet, and sensitivity to cold
- Dry skin and thinning hair
- Constipation
- Depression and poor memory
- Rapid heartbeat or palpitations (if high)
- Nervousness, anxiety, and tremors (if high)
- Heat intolerance and excessive sweating (if high)
Who Should Get Tested?
- People with symptoms of thyroid imbalance despite normal TSH
- Patients on thyroid hormone replacement therapy
- Individuals on very low-calorie or low-carbohydrate diets
- Those with nutrient deficiencies affecting thyroid conversion
- Anyone with a family history of thyroid disorders
How to Prepare for Your Test
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 Blood Test
Test your Free T3 levels with an accredited Swedish laboratory. Results within days, reviewed by Wellth.
Frequently Asked Questions
What is the difference between Free T3 and Total T3?
Can diet affect Free T3 levels?
Why might Free T3 be low even if TSH is normal?
Reviewed by
Wellth Team
Wellth
Our content is written and reviewed by Wellth, and fact-checked against peer-reviewed research and current Swedish clinical guidelines.
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The information on this page is provided for informational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Wellth provides health information and testing services but is not a substitute for clinical care. Always consult a qualified healthcare provider if you have questions about a medical condition. If you are experiencing severe or urgent symptoms, contact your doctor or call 1177 Vårdguiden. Blood test results should be interpreted by a qualified healthcare professional in the context of your complete medical history.