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If you have symptoms that suggest a thyroid disorder your doctor will usually start by requesting a TSH test which measures the amount of Thyroid Stimulating Hormone in your blood. If your TSH level is high or low, you may need to have a Free T4 test to identify the problem. A free triiodothyronine (FT3) test may sometimes also be requested. The Free T4 test measures the amount of free thyroxine in your blood.

What is being tested?

This test measures the amount of free thyroxine, or FT4, in your blood.

T4 is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine, or T3).

T4 makes up nearly all of what we call thyroid hormone, while T3 makes up less than 10 per cent. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions).


Most T4 in blood is attached to a protein; less than 1 per cent is unattached. The blood test measures the amount of free (unattached) T4 hormone in your blood since this is the biologically relevant fraction.

There is a feedback system between the pituitary gland which produces TSH and the thyroid. Normally TSH produced in the pituitary drives thyroid T3 and T4 production. If thyroid hormone production falls, TSH rises and if T3 and T4 become too high, TSH levels fall.

How is it used?

Free thyroxine (FT4) can assist in determining whether the thyroid is performing properly. It aids in the diagnosis of hypothyroidism or hyperthyroidism. The test may also be used to help evaluate a patient with an enlarged thyroid gland, called a goitre. It may aid in the diagnosis of pituitary disorders and female infertility. FT4 is used in monitoring effectiveness of treatment in a person with known thyroid disorder or pituitary disease, it may also be used to monitor patients with thyroid cancer along with TSH measurement.

When is it requested?

A free thyroxine test is usually used in response to an abnormal TSH test result.

What does the result mean?

High free thyroxine results may indicate an overactive thyroid gland (hyperthyroidism).

Low free thyroxine results may indicate an underactive thyroid gland (hypothyroidism).

Is there anything else I should know?

Some medicines can affect thyroid function, so tell your doctor about any drugs you are taking.

Common questions

  • How is hyperthyroidism treated?

Hyperthyroidism can be controlled through treatment. This will normally involve either tablets which stop the thyroid gland producing thyroid hormones, radioiodine treatment which destroys thyroid tissue, or surgery to remove part or all of the thyroid gland.

  • How is hypothyroidism treated?

Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.

  • How does pregnancy affect thyroid hormone levels?

Pregnancy can increase total T4 levels. Normally, free T4 levels will not change significantly as a result of pregnancy, though due to differences in testing techniques between laboratories, some centres will have different 'normal' ranges for pregnancy.

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