T3 (free triiodothyronine)

T3, or triiodothyronine, is an active thyroid hormone that regulates the body's metabolism, energy metabolism, temperature control, and affects the heart, muscles, and nervous system. T3 has a stronger biological effect than T4 but is found in lower concentrations in the blood.

Normal levels of free T3 (fT3) are usually between 3.5–6.5 pmol/L, but reference values ​​can vary slightly between laboratories. Total T3, which includes protein-bound T3, is often between 1.2–2.8 nmol/L.

T3 testing is performed when hyperthyroidism is suspected or when TSH and T4 do not explain the patient's symptoms. The test is particularly useful when T3-dominant hyperthyroidism is suspected or when monitoring treatment with T3 preparations.

A high T3 level may indicate hyperthyroidism, where the thyroid gland produces too much hormone. Symptoms include palpitations, weight loss, sweating, and nervousness. High T3 is often seen in Graves' disease or in overtreatment with thyroid hormone.

Low T3 may indicate hypothyroidism or a temporary downregulation of metabolism in the event of a serious illness (non-thyroidal illness syndrome). Symptoms of low T3 include fatigue, weight gain, feeling cold, and a slow metabolism.

T3 treatment involves administering the active thyroid hormone triiodothyronine in the form of the drug Liothyronine. The treatment is used for hypothyroidism, usually as a complement to T4 treatment (Levaxin), when the patient does not become symptom-free despite TSH and T4 being within normal limits.

Some people have difficulty converting T4 to T3 in the body, which can lead to persistent symptoms such as fatigue, chills and low metabolism, despite correct treatment with T4. In such cases, supplementing with T3 can help restore the balance.

Because T3 has a stronger and faster effect than T4, the treatment requires careful dosing and regular follow-up with blood tests. Excessively high levels of T3 can lead to side effects such as palpitations, anxiety and sleep problems. T3 treatment should always be done in consultation with a doctor with experience in thyroid treatment.