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Free thyroxine (T4-Free) or tetraiodothyronine is a thyroid hormone

Thyroxine T4

Thyroxine is one of the two important thyroid hormones. The thyroid regulates the entire body’s metabolism.

The thyroid is a gland weighing about 20 grams and it is located in the neck. Its overactivity or insufficiency may cause various symptoms in the body.

What is free thyroxine T4?

Thyroxine (or tetraiodothyronine) is one of the important thyroid hormones in addition to triiodothyronine. Thyroxine amounts to more than 80 per cent of the thyroid hormones in blood.

Free thyroxine is not bound to transporter proteins. Thyroxine is a derivative of the amino acid tyrosine. It contains four iodine atoms.

Most of the thyroxine circulating in the body is bound to transporter proteins. The total amount of T4 depends on the amount of transporter proteins.

In hypothyroidism, the amount of thyroxine in the blood is lower than normal and, conversely, in hyperthyroidism it is higher than normal.

In addition to the T4 study, the amount of thyroid stimulating hormone (TSH) secreted by the pituitary gland can be analysed.

What are the reference values for free thyroxine P-T4?

The reference values for free thyroxine P-T4 for all people are 12–22 pmol/l (picomoles per litre).

The reference values vary from laboratory to laboratory and according to the analysis method.

The analysis can be misrepresentative due to a rheumatoid factor, albumin binding abnormalities, heparin therapy and T4 antibodies.

High T4, free thyroxine

Free thyroxine may also be high due to reasons other than actual thyroid dysfunction. All of the following conditions and situations may cause free T4 to rise:

• Hyperthyroidism is diagnosed if other tests also suggest it and the patient develops symptoms indicative of it.
• The postpartum period may cause the values to rise. In this case, the test should be repeated after some time so that the body has time to recover. The value may decrease on its own over time or the condition may develop into hyperthyroidism.
• Certain medications may increase the values.
• Diseases that involve an increase in free fatty acids may also increase the amount of free T4.

Low T4, free thyroxine

Hypothyroidism decreases the amount of free T4.

Why is free thyroxine T4 studied?

When examining hyperthyroidism or hypothyroidism, the amount of free thyroxine should be studied as its concentration is less affected by the amount of transporter proteins. This provides a more accurate picture of the function of the thyroid gland.

The overall level of thyroxine is more influenced by the amount of transporter proteins, which can be analysed with a separate test.

Free thyroxine should be studied if symptoms of thyroid dysfunction occur. Typical symptoms of hyperthyroidism include weight loss, rapid heartbeat and sweating. Typical symptoms of hypothyroidism are weight gain, slow pulse, fatigue and general poor condition.

Potential insufficiency in newborns is automatically checked as a standard procedure as failure to treat it can lead to serious health problems.

Determining the amount of free T4 is important, especially if other concentrations that may indicate thyroid problems, such as TSH, are within the reference values or there are inconsistencies in the interpretation of their results.

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