fT4 test

FT4 – what is this test? 

FT4 (free thyroxine) is a hormone produced by the thyroid gland that plays a key role in regulating the body's metabolism. The FT4 test measures the level of free thyroxine in the blood to assess thyroid function, diagnose hormonal disorders, and detect thyroid diseases. 

The service can be redeemed within 3 months from the date indicated in the order confirmation.

When is it worth doing an FT4 test? 

The FT4 laboratory test is recommended if: 

• you have symptoms of hypothyroidism (e.g., fatigue, weight gain, dry skin); 
• you have symptoms of hyperthyroidism (e.g., palpitations, weight loss, excessive sweating); 
• you are monitoring thyroid treatment (e.g., Hashimoto’s disease, Graves–Basedow disease); 
• a doctor has detected abnormal TSH levels and wants to assess thyroid function more precisely; 
• you are pregnant – monitoring thyroid hormones is especially important for the baby’s health; 
• a thyroid cancer suspicion has been detected; 
• you are a woman experiencing infertility.

It is recommended to measure FT4 together with TSH, which enables more accurate interpretation of the results. TSH is a hormone secreted by the pituitary gland that regulates thyroid hormone production. 

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What does the test diagnose?

Elevated FT4 – when does it occur? 

Causes of elevated FT4 levels include: 

• iodine-induced hyperthyroidism (earlier than fT3), 
• early stage of Hashimoto’s disease, 
• Graves–Basedow disease, 
• nodular goiter, 
• subacute thyroiditis, 
• treatment with high doses of thyroxine. 

Symptoms associated with high FT4 levels include:  

• excessive sweating, 
• hot flashes, 
• weight loss despite normal appetite, 
• chronic diarrhea, 
• muscle tremors, 
• heart palpitations, 
• menstrual disorders in women, 
• sleep problems. 
• increased irritability, mood swings,
• anxiety, 
• concentration problems, 
• eye symptoms – bulging eyes, double vision, swelling and redness of the eyelids or conjunctiva (typical of Graves–Basedow disease). 

Low FT4 – symptoms and possible causes 

Causes of low FT4 levels: 

• hypothyroidism, 
• thyroid inflammation, 
• post-thyroidectomy state (after thyroid removal), 
• post-radiotherapy state, 
• treatment with antithyroid medications. 

Symptoms associated with low FT4 levels overlap with those of hypothyroidism, including: 

• fatigue, 
• weight gain, 
• low mood, 
• cold intolerance, 
• dry skin,  
• concentration problems,  
• depressive symptoms, 
• menstrual irregularities. 

In children and infants, hypothyroidism can lead to delayed growth and psychomotor development. 

Learn more: 

• Hypothyroidism – causes, symptoms and treatment
• Hyperthyroidism – symptoms, causes and treatment
• Hashimoto’s disease – symptoms, diagnosis and treatment
• Thyroid-stimulating hormone (TSH) – what it is, when to test, how to interpret results

FT4 Test – key information before the examination

1. Sample type: The test is performed by collecting blood from a vein, which is then sent to the laboratory for analysis.
2. Blood collection time: A few minutes – a standard blood draw procedure.
3. Do I need to be fasting? YES 
4. Recommended time for collection: It is recommended to perform the test in the morning, between 7:00 and 10:00. 
5. Do I need to stop taking medications before the test? NO – unless your doctor advises otherwise. Inform the staff about all medications you are taking.
6. Additional information: Avoid intense physical activity and stressful situations on the day of the test, as they may affect hormone levels. 

FT4 test results 

Waiting time for the result: 1–3 business days. 

You can find more information about collecting your results at the selected facility HERE.

Interpretation of results should always be performed in the context of other hormonal tests, such as TSH (thyroid-stimulating hormone) and FT3 (free triiodothyronine). A high TSH level with low FT4 may indicate primary hypothyroidism, while low TSH and high FT4 suggest hyperthyroidism. If results are abnormal, it is recommended to consult a doctor for detailed evaluation.

A consultation with a specialist is recommended. 

FT4 reference ranges – what are normal results? 

The reference range for FT4 is 10–35 pmol/l (8–28 ng/l), although values may vary depending on the laboratory. 

These values may also change depending on the patient's age and health condition — for example during pregnancy, where reference values differ between trimesters. 

Factors that may affect the test result

• pregnancy,
• taking certain medications.

Who should you consult about abnormal results?

• Endocrinologist
• Internist

Frequently Asked Questions

1. FT4 – what is this test?

The FT4 test measures the level of free thyroxine — a thyroid hormone that regulates the body's metabolism.

2. How much does the FT4 test cost?

The price of the FT4 test ranges from 35 to 90 PLN, depending on the laboratory.

3. How long does it take to get FT4 results?

Results are available within 1–3 business days.

4. Does the FT4 test require preparation?

Yes, you should come for the test in the morning, on an empty stomach.

5. FT4 — when should you test?

The test is performed when thyroid problems are suspected, when TSH is abnormal, or during pregnancy.

6. Can the FT4 test be done together with TSH?

Yes, FT4 and TSH are often performed together to more accurately assess thyroid function.