Lab test glossary · Thyroid
Free T4: what it means
Also known as: FT4, free thyroxine
Free T4 is the active, unbound thyroid hormone available to your tissues, read alongside TSH to assess thyroid function.
What it measures
Free T4 (thyroxine) is the portion of thyroid hormone circulating freely — not bound to proteins — and therefore available to act on your body. Paired with TSH, it helps distinguish where a thyroid problem sits and how significant it is.
Typical range
Commonly around 0.8–1.8 ng/dL, varying by lab and assay.
Reference ranges differ between labs and are printed on your own report — always read your result against that range, not a general one.
What a high result can mean
- Overactive thyroid
- Too much thyroid-hormone replacement
A high value is a starting point for a conversation, not a diagnosis — many causes are benign or temporary.
What a low result can mean
- Underactive thyroid
- Pituitary problems
Why the trend matters
Free T4 is tracked with TSH to confirm the direction of a thyroid change and to guide treatment adjustments over time.
When to talk to your doctor
Interpret free T4 with your doctor and your TSH together — one without the other tells only half the story.
This page is general education about a common lab test — it isn’t medical advice and can’t interpret your specific result. What a value means depends on your history, symptoms, and the rest of your labs, which is a conversation for you and your clinician.