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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 and/or a Free T3 test to identify the problem. The Free T3 test measures the amount of free triiodothyronine in your blood.

What is being tested?

The test measures the amount of free triiodothyronine, or FT3, in your blood.

T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). T3 makes up less than 10 per cent of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.

Thyroid hormones help regulate the body’s metabolism (how the body functions).

About 99.7 per cent of the T3 in blood is attached to a protein, and the rest is unattached. The blood test measures the free (unattached) T3 hormone in the blood since this is the biologically relevant fraction.

There is a feedback system between the pituitary gland, which produce 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 triiodothyroinine (FT3) can assist in determining whether the thyroid is performing properly, and is used mainly to help diagnose hyperthyroidism, since T3 can become abnormal earlier than T4 and return to normal later than T4. This test may also be used for monitoring of patients on T3 therapy.

FT3 is not usually helpful if your doctor thinks you have hypothyroidism.

When is it requested?

A free T3 test may be performed if you get an abnormal TSH or FT4 test result.

What does the result mean?

A high free T3 result may indicate an overactive thyroid gland (hyperthyroidism).

Low free T3 results may indicate an underactive thyroid gland (hypothyroidism).

Is there anything else I should know?

When you are sick, your body decreases production of T3 from T4. Most people who are sick enough to be in the hospital will have a low FT3 level. For this reason, doctors do not usually use FT3 as a routine thyroid test for patients in hospitals.

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 T3 (and total T4) levels. Normally, free T3 levels will not change significantly as a result of pregnancy, though due to the difference in testing techniques between laboratories, some centres will have different 'normal' ranges for pregnancy.

Last Updated: Thursday, 1st June 2023

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