Thyroid
Free T3
LOINC 3051-0 · pg/mL
Ranges
Reference range: 2.3–4.2 pg/mL
What it is
Free T3 is the active, unbound form of triiodothyronine, the more potent thyroid hormone that drives metabolism.
Why it matters
It shows how much active thyroid hormone is available to your tissues, complementing TSH and Free T4.
What high values can mean
A higher value can occur with an overactive thyroid and may cause symptoms like a fast heartbeat or anxiety.
What low values can mean
A lower value can occur with an underactive thyroid or during illness.
How to improve it
Thyroid hormone levels are managed medically; consistent treatment and follow-up keep them in range.
Food & lifestyle
Adequate iodine and selenium support thyroid function, but hormone levels are guided by your clinician.
Preparing for the test
A blood test; fasting is not required — follow your lab's instructions.
When to talk to a doctor
Discuss abnormal Free T3 with a doctor, interpreted alongside TSH and Free T4.
Related markers
Sources
- MedlinePlus (NIH)
- Testing.com (AACC)
Content reviewed on June 26, 2026
Medical disclaimer
This content is for educational purposes only. It is not medical advice and does not diagnose, treat, or prevent any condition. Reference ranges vary between laboratories — always use the range printed on your report and discuss your results with a healthcare professional.
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