What is this test?
Free T4 is the main hormone produced by the thyroid gland that circulates unbound in the blood. Evaluated together with TSH, it clarifies whether the thyroid is underactive or overactive.
When is it requested?
Diagnosis and treatment monitoring of hyper- or hypothyroidism
What does a high free T4 result mean?
A high free T4 may suggest an overactive thyroid (hyperthyroidism) or too much thyroid medication; it is always interpreted together with TSH.
- Thyroid medication taken right before the test, or a dose that is too high
- High-dose biotin supplements (falsely high result)
- Graves' disease
- An overactive thyroid nodule
- Early phase of thyroid inflammation (thyroiditis)
- Certain medicines such as amiodarone
What does a low free T4 result mean?
A low free T4 usually indicates an underactive thyroid (hypothyroidism); this is more likely when TSH is also high.
- Thyroid medication dose too low or missed
- Hashimoto's thyroiditis
- After thyroid surgery or radioactive iodine treatment
- Later stages of pregnancy (method-related lower values)
- A period of severe illness or certain epilepsy medicines
- Rarely, an underactive pituitary gland
My result is outside the reference range. What should I do?
If you take thyroid medication, take it after your blood draw unless your doctor says otherwise. Pause biotin supplements before the test; your doctor will interpret the result together with TSH.
Reference ranges depend on the method, age and sex; always use the range printed on your report. This information is for general guidance only; consult your doctor for diagnosis and treatment.
Sample information
- Sample type: Serum
- Tube / container: Yellow-top tube
- Sample volume: 1 mL
Preparation
Fasting sample recommended.
Testing and results
- Method: ECLIA
- Run schedule: Daily
- Turnaround time: 4 hours
Price and appointment
Add the test to your list to receive a price quote by e-mail, or call +90 850 640 07 30 / message us on WhatsApp (+90 537 301 22 14). You can have the test done at our branches or with our home blood collection service. Results can be viewed online with your Turkish ID number (or passport number for foreign nationals).