Thyroid Function Tests

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Related tests: TSH, total T4, total T3, free T4, free T3, Thyroid Antibodies, Calcitonin, Thyroglobulin

At a Glance

Why Get Tested?

To help evaluate thyroid gland function, to help diagnose thyroid disorders and to monitor the response to treatment.

When to Get Tested?

As part of a health checkup or when symptoms suggest hypo- or hyperthyroidism due to a condition affecting the thyroid; following commencement of treatment for disorders of the thyroid gland.

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

None needed; however, certain medications can interfere with the tests included in the panel, so tell your doctor about any drugs that you are taking.

The Test Sample

What is being tested?

Thyroid function tests (TFTs) are a group of tests that are requested together to help evaluate thyroid gland function, to help diagnose thyroid gland disorders and to monitor the response to treatment of thyroid disorders. TFTs include a measure of the amount of thyroid hormones, Thyroxine (T4) or Tri-iodothyronine (T3) in your blood. These hormones are chemical substances that travel through the bloodstream and control or regulate your body’s metabolism—how it functions and uses energy. Thyroid hormones are present in the blood in either protein bound forms (the majority) or the free and active form of the hormone. Currently, the majority of UK laboratories measure the free form of the hormones – Free T4 (FT4) or Free T3 (FT3).

Thyroid Stimulating Hormone (TSH) is produced by the pituitary gland and is part of the body’s feedback system to maintain stable amounts of thyroid hormones in the blood. When thyroid hormone concentrations decrease in the blood, the pituitary gland is stimulated to release TSH, which in turn stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, TSH production turns on and off to maintain constant blood thyroid hormone concentrations.

Thyroid Function Tests usually include some combination of:

How is the sample collected for testing?

A blood sample taken from a vein in your arm

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

None needed; however, certain medications can interfere with the tests included in the panel, so tell your doctor about any drugs that you are taking.

The Test

Common Questions

Ask a Laboratory Scientist

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.