Interpret Your Free Triiodothyronine Results
Enter your Free T3 value below and get an instant, free AI interpretation — a plain-English explanation checked against WHO/IFCC reference ranges.
Medical Disclaimer
This AI-generated interpretation is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Results may vary based on individual health factors not captured here.
Always consult a qualified healthcare professional before making any health decisions. Read full disclaimer →
Free Triiodothyronine Normal Ranges
Free T3 is the most biologically active thyroid hormone — it exerts most of the metabolic effects at the cellular level.
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 2.3 – 4.2 | pg/mL | — |
Also reported in: pmol/L.
Why Doctors Order the Free Triiodothyronine Test
Free T3 is not routinely needed for thyroid assessment but is important in specific situations: T3 toxicosis (hyperthyroidism with high FT3 and suppressed TSH but normal FT4), monitoring in patients on combination T3/T4 therapy, assessing the severity of hyperthyroidism (very high FT3 in thyroid storm), and investigating non-thyroidal illness (FT3 is preferentially reduced in sick euthyroid syndrome).
How to Enter Your Free T3 Result
Free Triiodothyronine can be reported in pg/mL or pmol/L. Choose the unit printed next to your result on the lab report so it is compared with the matching reference range.
What a Low or High Free T3 Can Be Associated With
Below the reference range
- Hypothyroidism (FT3 falls late, after FT4)
- Non-thyroidal illness (sick euthyroid) — most common cause of low FT3
- Amiodarone (blocks T4 to T3 conversion)
- Starvation and caloric restriction
- Liver and kidney disease (impaired T4-to-T3 conversion)
Above the reference range
- Graves' disease (often FT3 rises before FT4)
- T3 toxicosis (toxic adenoma secreting primarily T3)
- Excessive tri-iodothyronine (liothyronine) supplementation
- Thyroid storm (extreme hyperthyroidism)
These are common associations, not a diagnosis. A single result is read alongside your symptoms, history and other tests — discuss it with your doctor.
What Can Change Your Free T3 Result
Before you read too much into a result, consider whether any of these applied:
- Non-thyroidal illness (severe illness preferentially reduces FT3)
- Amiodarone (inhibits T4-to-T3 conversion — FT3 low despite normal FT4)
- Biotin supplements (interfere with FT3 immunoassays)
Preparing for the test: No fasting required. Avoid biotin supplements for 24 hours.
Symptoms That Often Prompt Free T3 Testing
- Similar to hyperthyroidism when elevated
- Similar to hypothyroidism when low
Want the full clinical guide to Free Triiodothyronine?
Reference ranges, causes, symptoms, preparation, and FAQs.