Free T3 (Free Triiodothyronine)
Free T3 measures the small fraction of triiodothyronine that is not bound to blood proteins and is therefore biologically active. Free T3 reflects thyroid activity more accurately than total T3 because it is not affected by changes in binding proteins (seen in pregnancy, oral contraceptive use, or liver disease). It is most useful when evaluating hyperthyroidism or unusual thyroid patterns.
Reference Ranges
Male
2.3 – 4.2
pg/mL
Female
2.3 – 4.2
pg/mL
Child
2.5 – 5
pg/mL
Ages 6–12
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Low Free T3 usually points to hypothyroidism, but it can also appear in non-thyroid illness (sick euthyroid syndrome), severe caloric restriction, or after major surgery. Free T3 is not a reliable screening test for hypothyroidism — TSH and Free T4 are preferred.
What High Levels Mean
High Free T3 indicates hyperthyroidism — most often Graves' disease or a toxic nodule. Symptoms include weight loss, heat intolerance, tremor, rapid heartbeat, and anxiety. T3-toxicosis is a pattern where Free T3 is high but Free T4 is normal, typical of early Graves' or toxic nodules.
Frequently Asked Questions
When is Free T3 ordered?
Free T3 is most useful when TSH is suppressed (suggesting hyperthyroidism) but Free T4 is normal — this identifies T3-toxicosis. It is also ordered to confirm the severity of hyperthyroidism. Free T3 is not recommended as a routine thyroid screening test.
What is the difference between T3 and Free T3?
Total T3 measures all T3 in blood, including the 99.7% bound to proteins like thyroxine-binding globulin. Free T3 measures only the 0.3% that is unbound and biologically active. Free T3 is more accurate when binding proteins are altered by pregnancy, oral contraceptives, or severe illness.
Can Free T3 be normal with hypothyroidism?
Yes. In mild or early hypothyroidism, Free T3 is often preserved while Free T4 drops — the body prioritizes T3 production. This is why Free T3 alone is a poor screening test; TSH is always the first-line thyroid test, followed by Free T4.
Related Thyroid tests
See all →TSH (Thyroid Stimulating Hormone)
Master switch that controls thyroid gland activity.
mIU/LThyroidT3 (Triiodothyronine)
Active thyroid hormone that drives metabolism.
ng/dLThyroidT4 (Thyroxine)
Main hormone produced by the thyroid gland.
µg/dLThyroidFree T4 (Free Thyroxine)
Unbound, active portion of T4 available to your body.
ng/dLThis information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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