fT3

Do you have palpitations, restlessness, sweating and weight loss despite eating as before? The fT3 test measures free triiodothyronine, the active form of thyroid hormone, and helps detect an overactive thyroid. It is most informative when TSH is low because, in some people, only fT3 is initially elevated.

Who is this test for

• people with symptoms that may suggest an overactive thyroid: palpitations, restlessness, hand tremor, increased sweating, weight loss, insomnia or frequent bowel movements;
• when results show low TSH, particularly if fT4 is normal;
• people with a thyroid nodule or enlarged thyroid alongside low TSH;
• for monitoring treatment of an overactive thyroid, for example Graves’ disease, as agreed with the doctor;
• when a doctor wants a complete picture of thyroid function, together with TSH and fT4.

What the test reveals

The thyroid produces mainly thyroxine (T4) and less triiodothyronine (T3). Most T3 is formed later in the liver, kidneys and muscles through conversion from T4. T3 is what acts in cells: it increases energy use, heart rate and digestion, and affects body temperature and mood.

Almost all T3 in blood is bound to carrier proteins, with only around 0,3 % free. This free portion enters cells and is measured by the fT3 test. Unlike total T3, fT3 changes less when carrier protein levels change, for example during pregnancy or with contraceptive pills.

Save with a panel

Test FT3 is also included in these panels.

Thyroid panel with free hormone fractionsTSH, fT3, fT4Price of individual tests: 30.00€27.00€Details ›Complete thyroid panelTSH, T3, T4, fT3, fT4 and 2Price of individual tests: 102.00€92.00€Details ›Hormonal status panel – womenTSH, fT3, fT4, cortisol and oestrogen, progesterone and 3113.00€Details ›

All panels including FT3 (13) ›

The test and preparation

The sample is venous blood, and collection takes a few minutes. Fasting is not required for thyroid tests unless glucose or blood lipids are also sampled. It is best to come in the morning.

• If you take thyroid medication, take it after blood collection on the sampling day. Do not stop treatment yourself.
• If you take biotin (e.g. hair and nail supplements), mention this at blood collection because it can affect thyroid test results.

What your results mean

The reference interval depends on method and age and is printed on the report; different cut-offs apply in pregnancy. A doctor interprets the result alongside TSH, fT4, symptoms and the medicines you take.

• High fT3 with low TSH suggests an overactive thyroid (hyperthyroidism), most commonly due to Graves’ disease or nodules producing hormones independently. Sometimes only fT3 is high while fT4 remains normal. A doctor investigates the cause using further tests and ultrasound. High fT3 may also result from an excessive thyroid hormone dose.
• Low fT3 is not a good indicator of an underactive thyroid because it falls late in hypothyroidism. It is often temporarily low during severe illness, after surgery, with starvation or certain medicines despite normal thyroid function.
• Normal fT3 with low TSH and normal fT4 may mean mildly increased thyroid activity, but also an effect of medicines or early pregnancy. The doctor usually repeats the tests.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contactpage). Prices for all tests and panels are listed in the price list. Because fT3 is interpreted alongside TSH and fT4, the Thyroid Panel with Free Hormone Fractionsis more practical.

Frequently asked questions

What is the difference between T3 and fT3?

T3 is total triiodothyronine, bound and free together, while fT3 is only the free portion that acts in cells. Total T3 also depends on carrier proteins, so fT3 is more reliable when their levels change.

Do I need fT3 if I have Hashimoto’s thyroiditis?

TSH and fT4 are usually sufficient to assess an underactive thyroid and monitor levothyroxine treatment. A doctor will assess whether you also need fT3.

Can low fT3 cause fatigue or weight gain?

If TSH and fT4 are normal, low fT3 is rarely a sign of thyroid disease. It more often accompanies another illness, stress or a strict diet, so a doctor also investigates other causes of fatigue.

Can I drink coffee before thyroid blood tests?

Yes, if no tests requiring fasting, such as glucose or blood lipids, are sampled at the same time. Otherwise, have coffee after blood collection.