T3 (triiodothyronine) is the most active form of thyroid hormone.
Even though the thyroid makes mostly T4, your body converts much of that T4 into T3.
T4 = storage hormone
T3 = active, working hormone
Free T3 is the part that is:
Unattached
Ready to be used
Directly affecting your cells
T3 is the hormone that actually tells your body to:
Use energy
Control temperature
Regulate heart rate
Affect mood and focus
Keep metabolism moving
So Free T3 shows what your body can actively use right now.
Free T3 is not always checked first.
Most thyroid testing starts with:
Free T3 is usually added when:
Symptoms don’t match other test results
Doctors need more detail
Most labs list a normal Free T3 range around:
➡ About 2.3 to 4.2 pg/mL
Exact ranges vary by lab.
Your thyroid is overactive
➡ Often points to hyperthyroidism
Especially if:
TSH is low
Free T4 is also high
Low thyroid function
Poor conversion of T4 to T3
Stress or illness
Medication effects
Low Free T3 alone does not always mean true hypothyroidism.
Low TSH + High Free T3
Strong sign of hyperthyroidism
Especially common in early Graves’ disease
Normal TSH + Low Free T3
This can happen because of:
Illness
Stress
Medications
Conversion problems
It doesn’t always mean a thyroid disease by itself.
Most thyroid medications (like levothyroxine) provide T4, not T3.
Your body is expected to convert T4 into T3 naturally.
In some cases, doctors may consider:
T3 medication
Combination T4/T3 therapy
But this depends on individual situations.
Free T3 results can be influenced by:
Recent illness
Starvation or extreme dieting
Certain medications
Stress
Pregnancy
Chronic health conditions
That’s why doctors rarely rely on Free T3 alone.
If your Free T3 is abnormal, doctors may:
Recheck levels
Compare with TSH and Free T4
Look for causes of hyperthyroidism
Adjust medication if you’re already on treatment
Free T3 is the most active thyroid hormone
It is especially useful for evaluating high thyroid
It’s usually checked after TSH and Free T4
Results must be interpreted in context
Free T3 alone rarely diagnoses a problem
The next page will explain:
➡ Thyroid Antibody Tests Explained
These tests help find autoimmune thyroid conditions like Hashimoto’s and Graves’ disease.