Other names: T4

Thyroxine (T4) is one of two main hormones produced by the thyroid gland; the other is T3. The thyroid gland is responsible for the regulation of metabolic processes in the body and changes in the level of thyroid hormones in the body can have implications for a patient’s health. Around 99 percent of the body’s T4 supplies are attached to protein; the remaining free hormone, which makes up around 1% of thyroxine supplies, is responsible for most of the effects of thyroid hormone.

When is the test used?

The test is used to assess the function of the thyroid gland; it can measure the levels of T4 in the blood and will give an idea of whether a patient has problems with their thyroid gland. In most cases, an alternative test, the FT4 test, will be used; this measures the concentration of free T4 hormone in the blood.

The test may be useful if a doctor suspects that a patient has either hyperthyroidism (overactive thyroid gland) or hypothyroidism (underactive thyroid gland); the test may also be useful for female patients who are experiencing problems with conceiving a baby. The test may also be used for patients with goitre (this occurs when the thyroid gland is enlarged).

How is the test performed?

The test is performed by collecting and analysing a sample of the patient’s blood; a needle is inserted into a vein in the arm and the blood is drawn out and collected in a syringe. Once the doctor or nurse has a sufficient sample, the blood will be placed in a bottle, labelled with the patient’s name and sent off for analysis.

What do the test results show?

If the results of the test show a high level of T4, this may indicate that a patient has hyperthyroidism (overactive thyroid gland), while low levels may indicate hypothyroidism.

Certain types of medication may also affect the test results so it is important that the patient tells their doctor about any medication they are taking before they have the test.

Specific Blood Tests

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