BiomarkersFree T3
Thyroid
Free T3
The most active thyroid hormone
On your report it may say: FT3, Free triiodothyronine
T3 is the most active thyroid hormone, mostly made by converting T4 in body tissues. It's useful when an overactive thyroid is suspected.
Typical ranges · pg/mL
pmol/L = pg/mL × 1.536
T3 falls during illness, fasting or heavy calorie restriction.
Ranges vary by lab, age and sex. The range printed on your report applies.
If it's low
Can reflect an underactive thyroid or recent illness or dieting.
Common causes
- Underactive thyroid
- Illness or crash dieting
What you might notice
- Tiredness
- Feeling cold
If it's high
Often seen with an overactive thyroid.
Common causes
- Overactive thyroid
- Excess thyroid medicine
What you might notice
- Fast heartbeat
- Anxiety
- Weight loss
Testing
- Who should test
- When an overactive thyroid is suspected or results are unclear.
- How to prepare
- Stop biotin 2–3 days before.
- When to retest
- As your doctor advises.
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
Does selenium raise T3?
Selenium helps conversion, but supplements aren't a thyroid treatment.
Why is T3 low when I'm unwell?
The body lowers T3 temporarily to save energy.
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.