Thyroid
Free T4
LOINC 3026-2 · ng/dL
Ranges
Reference range: 0.8–1.8 ng/dL
What it is
Free T4 is the unbound, available form of thyroxine, the main hormone the thyroid releases. It is converted into the more active T3.
Why it matters
With TSH, Free T4 is central to assessing whether the thyroid is overactive or underactive.
What high values can mean
A higher value suggests an overactive thyroid (hyperthyroidism).
What low values can mean
A lower value suggests an underactive thyroid (hypothyroidism).
How to improve it
Thyroid hormone is managed with medication and monitoring; consistency keeps levels steady.
Food & lifestyle
Adequate iodine and selenium support the thyroid; medication dosing is 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 T4 with a doctor, interpreted with TSH.
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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