Free Triiodothyronine
Free T3· also: Free T3, fT3
Clinical Overview
Free T3 is the most biologically active thyroid hormone — it exerts most of the metabolic effects at the cellular level. Most T3 is produced by conversion of T4 to T3 in peripheral tissues, not by direct thyroid secretion. Free T3 is measured when T3 toxicosis or sick euthyroid syndrome is suspected.
Why This Test Matters
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).
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 2.3 – 4.2 | pg/mL | — |
Also reported in: pmol/L.
What Causes Abnormal Results?
High Free T3 Causes
- Graves' disease (often FT3 rises before FT4)
- T3 toxicosis (toxic adenoma secreting primarily T3)
- Excessive tri-iodothyronine (liothyronine) supplementation
- Thyroid storm (extreme hyperthyroidism)
Low Free T3 Causes
- 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)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. Avoid biotin supplements for 24 hours.
Factors That Can Affect Results
- 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)
Related Topics
Frequently Asked Questions
What is T3 toxicosis?
T3 toxicosis is a form of hyperthyroidism where FT3 is elevated but FT4 is normal, with suppressed TSH. It occurs when a thyroid nodule or the entire gland selectively secretes T3 rather than T4. It is important to measure FT3 in hyperthyroid patients with low TSH and normal FT4, as standard TSH + FT4 testing alone would miss this diagnosis.