T3

Do you have palpitations, sweating, restlessness or weight loss without dieting? These symptoms may mean your thyroid is working too hard. The T3 test shows the amount of triiodothyronine, the most active thyroid hormone, in blood and is usually performed alongside TSH and T4.

Who is this test for

• people with symptoms that may suggest an overactive thyroid: palpitations, restlessness, sweating, hand tremor, weight loss or poor tolerance of heat;
• when TSH is low and T4 normal or only mildly elevated;
• people treated for an overactive thyroid, to monitor therapy as agreed with a doctor;
• for an enlarged thyroid or neck nodule, as part of doctor-led investigations;
• people taking a medicine containing T3 hormone.

When an underactive thyroid is suspected, T3 is less useful because it remains normal for a long time. The first tests considered are TSH and fT4.

What the test reveals

The thyroid produces mostly thyroxine (T4) and less triiodothyronine (T3). Most T3 forms when T4 is converted to the more active form in the liver, kidneys and other tissues. T3 acts on almost all cells, affecting metabolic rate, heart function, body temperature and energy use.

This test measures total T3, combining hormone bound to carrier proteins and the small free portion. The result is therefore also influenced by the quantity of these proteins, which increases in pregnancy and with oestrogen-containing medicines. If you are interested only in the free portion, a separate test is available, fT3. In an overactive thyroid, T3 often rises earlier and more markedly than T4, and sometimes it alone is elevated. Read about the sequence of thyroid tests in the article on laboratory thyroid investigations.

Save with a panel

Test T3 is also included in these panels.

Thyroid health panelTSH, T3, T4Price 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 ›

All panels including T3 (5) ›

The test and preparation

Blood is taken from a vein in the arm, taking a few minutes. Fasting is not required unless glucose or blood lipids are tested from the same collection. It is best to come in the morning and have follow-up samples taken at a similar time of day.

• If you take thyroid tablets, take them after blood collection on the sampling day unless your doctor advises otherwise. Do not stop treatment yourself.
• If you take biotin (e.g. hair and nail supplements), mention this at blood collection.
• Also mention at blood collection if you are pregnant or take contraceptive pills or hormone replacement therapy.

What your results mean

The reference range depends on age, sex and method and is printed on your report. T3 is always read together with TSH and T4; a doctor interprets it alongside symptoms and the medicines you take.

• High T3 with low TSH suggests an overactive thyroid (hyperthyroidism), for example due to Graves’ disease or nodules producing hormones independently. High total T3 with normal TSH more often results from increased carrier proteins, for example in pregnancy or with the pill.
• Low T3 may occur in an underactive thyroid, but usually only when the disorder is pronounced. It is more often temporarily low during severe illness, after surgery, with starvation or certain medicines despite a healthy thyroid.
• Normal T3 with abnormal TSH does not exclude thyroid disease. A doctor decides on further tests.

If palpitations are accompanied by chest pain, difficulty breathing or feeling faint, do not wait for results; call emergency services on 194 or 112.

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 T3 is interpreted alongside TSH and T4, the Thyroid Health Panel, containing all three tests, is more cost-effective.

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 acting in cells. Total T3 depends on carrier proteins, so fT3 is more reliable in pregnancy or with contraceptive pills.

Why is my TSH high but T3 normal?

In an underactive thyroid, T3 remains within range for a long time because the body increases conversion of T4 to T3. Reduced thyroid function is therefore assessed using TSH and T4 or fT4, not T3.

Do I need to fast for T3?

No, unless glucose or blood lipids are also tested from the same collection. In that case, have coffee after sampling.

Can T3 be low with normal thyroid function?

Yes. During severe illness, after surgery or prolonged starvation, the body reduces conversion of T4 to T3. This reduction usually resolves when the condition improves.