What is the free triiodothyronine (FT3)?

The thyroid, a gland located in the neck, regulates metabolic processes in the body through the production of free triiodothyronine (FT3) and thyroxine (FT4). The free triiodothyronine is regulated by the thyroid hormone TSH, which is the thyroid-stimulating hormone produced by the pituitary gland. The free form means that it is not bound to plasma proteins.

The FT3 test is mainly ordered to help diagnose hyperthyroidism (overactive thyroid) and it may be ordered in cases of monitoring treatment of patients with a known thyroid disorder.

What is the purpose of measuring free triiodothyronine (FT3)?

This test allows accurate measuring of the levels of FT3 in the body and consequently it can help in the assessment of the thyroid function. The test is often ordered in combination with TSH and the FT4 (free thyroxine) tests.

Typically, the test is ordered if the doctor suspects a problem with the thyroid gland. Such potential problems may include:

  • Hyperthyroidism (overactive thyroid)
  • Primary or secondary hypothyroidism (underactive thyroid)
  • Hypopituitarism (underactive pituitary)
  • Thyrotoxic periodic paralysis (high levels of thyroid hormones)

The thyroid gland and the hormones it produces affect almost all bodily functions. Therefore, a thyroid disorder may cause a wide range of symptoms such as weakness and fatigue, anxiety, constipation, weight loss or gain, difficulty sleeping, menstrual irregularities, and increased heart rate.

Moreover, the FT3 test may be ordered after previous tests have indicated an abnormal TSH results.

It is important to understand that thyroid tests are a glance of what is happening in the human body. However, the FT3 levels as well as FT4 and TSH levels may vary from person to person or they may be affected by:

  • Pregnancy
  • Liver disease
  • Increases, decreases and alterations in the proteins that bind T3 and T4
  • Oestrogen and other medications
  • Resistance to thyroid hormones
  • Systemic illnesses

In addition, the test results may also be affected by certain medications such as oestrogen, birth control pills, and large doses of aspirin.

Standard of preparation

In general, the examination is performed in the morning. It may also be recommended that the patient fast for several hours before the exam. The patient should also disclose any information about on-going medications because they might affect the test results.

Is the examination dangerous and/or painful?

The examination is neither dangerous nor painful. The patient may experience a slight pinch at the entrance of the needle in the arm.

How is the exam performed?

The exam is performed through a simple blood sample test.


Whether the FT3 hormones are increased or decreased, such test results indicate an imbalance between the body’s supply and requirements. However, the test results cannot specifically determine the cause of the excess or deficiency.


High FT3 levels indicate the following thyroid disorders:


  • Hyperthyroidism (overactive thyroid)
  • Silent thyroiditis
  • Graves’ disease (autoimmune disease that stimulates the thyroid to produce too much hormones)
  • Toxic nodular goiter (an enlarged thyroid gland that has malfunctioning thyroid nodules)
  • Thyrotoxic periodic paralysis



Low FT3 levels indicate the following thyroid disorders:


  • Long-term illness
  • Starvation
  • Hypothyroidism (underactive thyroid)