BiomarkersFree T4
Thyroid
Free T4
The main thyroid hormone in circulation
On your report it may say: FT4, Free thyroxine
T4 is the main hormone the thyroid releases. 'Free' T4 is the active portion not bound to proteins. Read with TSH, it shows how well the thyroid is working.
Typical ranges · ng/dL
pmol/L = ng/dL × 12.87
Ranges vary between labs.
Ranges vary by lab, age and sex. The range printed on your report applies.
If it's low
Low thyroid hormone, usually with a high TSH.
Common causes
- Underactive thyroid
- Pituitary problems (rare)
What you might notice
- Tiredness
- Feeling cold
- Weight gain
If it's high
High thyroid hormone, usually with a low TSH.
Common causes
- Overactive thyroid
- Too much thyroid medicine
What you might notice
- Fast heartbeat
- Weight loss
- Anxiety
Testing
- Who should test
- When TSH is abnormal or thyroid symptoms are present.
- How to prepare
- Stop biotin 2–3 days before.
- When to retest
- With TSH, 6–8 weeks after changes.
What can help
SYNC doesn't recommend supplements from thyroid results. Thyroid problems need a doctor's diagnosis and treatment.
See a doctor about abnormal results.
Common questions
Free or total T4?
Free T4 is preferred because it isn't affected by binding proteins.
Do I need it if TSH is normal?
Often not, unless symptoms are strong.
Explore further
Sources
Free nutrition assessment
Not sure what your body needs?
Answer a few questions about your diet, lifestyle and goals, and SYNC builds a personalised supplement plan with the reason for every suggestion. Free, about 6–8 minutes.
Build My Nutrition PlanFor general education only, not medical advice. Ranges shown are typical for adults; the range printed on your lab report applies. Talk to a qualified doctor about your results.