What is free thyroxine?

Free thyroxine (FT4) is a thyroid hormone that contains iodine and is a derivative of the amino acid tyrosine, occurring naturally as l-thyroxine; its chemical name is tetraiodothyronine. It is formed and stored in the thyroid follicles as thyroglobulin and released from the gland by the action of a proteolytic enzyme. Free thyroxine and free triiodothyronine (FT3) regulate the metabolic processes in the body. The free thyroxine is the most abundant of the thyroid hormones in the body, and is regulated by the thyroid stimulating hormone (TSH), which is produced by the pituitary. The free form of thyroxine is not bound to plasma proteins. Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes, and water; vitamin requirements; reproduction; and resistance to infection. Pharmaceutical preparations of thyroxine, extracted from animals or made synthetically, are called levothyroxine.


Why measure the level of free thyroxine?

This tests allows us to evaluate the levels of FT4 and consequently the proper thyroid function in the body. This examination is often prescribed in combination of a TSH and the FT3 test. Thyroid function tests are used to help diagnose an underactive thyroid (hypothyroidism), and an overactive thyroid (hyperthyroidism). They are also used to evaluate thyroid gland activity and monitor responses to thyroid therapy. The FT4 examination is also used to help diagnose pituitary disorders, aid in the diagnosis of female infertility, monitor people with pituitary disease and to make sure that the thyroid is still working as well as monitor hormone treatment if it is not working. It is also used to monitor individuals with thyroid cancer, in which the tumors respond to TSH. TSH and T4 levels will be regularly checked to make sure that enough thyroid hormone is being given to keep TSH low without making the T4 too high.

A free thyroxine examination may be required when the doctor suspects the patient to have the following signs and symptoms of hyperthyroidism. Increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, diarrhea, light sensitivity, visual disturbance, and the when the eyes may be affected (hence the appearance of puffiness around the eyes). Other symptoms of hypothyroidism may include weight gain, dry skin, constipation, cold intolerance, hair loss, puffy skin, enlarged thyroid gland, and menstrual irregularity in women.

Normal TSH levels for adults range from 0.5 to 5.0 mU/L.


Standard of preparation

Sampling is usually done in the morning in the hospital. There are no special preparations needed for this test. The doctor will advise and recommend if you need to be fasting prior to the blood examination. You should inform your doctor of any medication you are taking prior to the exam, as some medical treatments may interfere with the blood results.


Is the examination painful or dangerous?

The examination is neither painful nor dangerous. The patient may feel a tingling sensation with the entrance of the needle in the arm when blood is being extracted for examination.


How is the exam performed?

The exam consists of a simple blood sample test.