Free T4 and Free T3 (Thyroid Hormones)
Also known as: FT4, Free thyroxine, Thyroid profile, thyroid test, T3 T4 test
Measures the free (active) thyroid hormones in the blood: free T4 (thyroxine) and sometimes free T3. They are read together with TSH.
Why it's ordered
When TSH is abnormal, to confirm an underactive or overactive thyroid, to monitor thyroid treatment, and in pregnancy or pituitary problems.
Preparation
No fasting needed. Stop biotin supplements (often in hair and nail products) for at least 2 days before the test if your doctor agrees, as biotin can give false results. If you take thyroxine, ask your doctor whether to take it before or after the test.
Typical adult values
| Free T4 | about 0.8–1.8 ng/dL (about 10–23 pmol/L) |
|---|---|
| Free T3 | about 2.3–4.2 pg/mL (about 3.5–6.5 pmol/L) |
Ranges vary between labs and methods. Always use the range printed on your report.
If it's high
High free T4 or T3 with a low TSH suggests an overactive thyroid (hyperthyroidism), for example Graves' disease or thyroiditis. Taking too much thyroxine has the same pattern.
If it's low
Low free T4 with a high TSH means an underactive thyroid (hypothyroidism). Low free T4 with a low or normal TSH is uncommon and can point to a pituitary problem.
Good to know
A high TSH with a normal free T4 is called subclinical hypothyroidism; treatment depends on the TSH level, symptoms and pregnancy plans. Ranges differ in pregnancy and between labs. Severe illness can change thyroid results temporarily.
Common questions
What is a normal free T4?
Typically about 0.8–1.8 ng/dL (about 10–23 pmol/L). Always use the range printed on your report.
What does high TSH with low free T4 mean?
It means an underactive thyroid (hypothyroidism).
Can supplements affect thyroid tests?
Yes. Biotin can give false results; stop it for at least 2 days before the test if your doctor agrees.
What is subclinical hypothyroidism?
A high TSH with a normal free T4. Treatment depends on the TSH level, symptoms and pregnancy plans.