Other names: T3

The test measures the level of triiodothyronine (also known as T3) in the blood. T3, along with thyroxine (T4), are hormones which are produced by the thyroid gland. T3 is much stronger than T4; despite T3 only making up 10 percent of thyroid hormones, it is believed that T3 produces most of the effects of thyroid hormones. Many cells in the body convert T4 into T3. Thyroid hormones are important for controlling metabolism in the body.

When is the test used?

The test is used to assess the function of the thyroid gland; in most cases, the test is used to help with the diagnosis of hyperthyroidism (overactive thyroid gland). The test is not normally useful for investigating hypothyroidism (underactive thyroid gland).

The test is usually ordered after other test results, including T4 or TSH (thyroid stimulating hormone), if found to be abnormal and the doctor suspects that the patient may have hyperthyroidism.

How is the test done?

The test is done by inserting a needle into a vein in the arm; the blood is then drawn out and collected in a syringe. The blood is then bottled, labelled with the patient’s name and sent off to the laboratory for analysis.

What do the test results show?

If the results of the free or total T3 test show a high concentration of T3 in the blood, this may indicate that the patient has hyperthyroidism (overactive thyroid gland).

If the results show a low level of T3, this may indicate that the patient has hypothyroidism (underactive thyroid gland); further tests will be carried out if this is the case.

Certain types of medication may affect the test result; drugs including aspirin, some forms of oral contraceptive and oestrogen can interfere with the results. It is important for patients to tell their doctor about any medications they are taking prior to the test.

When a patient is ill the levels of T3 will be lower; consequently, patients who are ill enough to be in hospital often have low T3 levels. T3 tests are not usually carried out on patients who have been hospitalised as the results may not be accurate.

Specific Blood Tests

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