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Thyroid function test  

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What is a Thyroid function test?

The thyroid function test consists of a battery of blood tests that provide vital information about the state of your thyroid’s functioning. It includes testing for the following three hormones.

  • TSH – (Thyroid Stimulating Hormone)-This is actually released by the pituitary gland which causes the thyroid to produce and release thyroxine and triiodothyronine.
  • FT4 - Free T4 - Thyroxine is the main hormone secreted by the thyroid which is present in two states within the body. It is the free state that does not bind with a protein that is active. Known as FT4, it is mostly converted to T3 which is far more potent than T4.
  • FT3 - Free T3 – Triiodothyronine has an effect on almost all physiological processes within the body, thereby maintaining the body’s temperature, affecting growth and normal development as well as heart rate.

In some situations, the following two tests are also ordered at the time of a thyroid function test, but these are not diagnostic tests of thyroid function. Thyroid antibodies – this helps to determine the reason and the extent of the disorder. Thyroglobulin- this is ordered to observe people suffering from thyroid cancer.

Why is it conducted?

The hormones secreted by the thyroid have a wide influence on the normal functioning of the body and it is essential that it works properly. The levels of hormones found in thyroid function tests help doctors to understand the underlying thyroid disease and its possible cause. These tests also help to guide the treatment to be followed. It is also sometimes conducted on newborns if a congenital thyroid disorder is suspected.

Who should go for it?

  • Persons who exhibit symptoms of thyroid disease such as disturbances in metabolism and heart rate, weight gain or loss, fatigue, unable to tolerate cold, and so on.
  • Persons who observe neck thickening or swelling in the neck area.
  • Women who have abnormalities in their menstrual cycles or have fertility problems.
  • Women who are pregnant and have a previous history of thyroid disorder.
  • People who have diabetes may be advised thyroid function tests as there is a close link between diabetes and thyroid disorders.
  • Persons who have irregular heartbeats which may be caused by an overactive thyroid.
  • People who are suffering from osteoporosis are often advised this test.


This is a simple blood test that does not require any particular preparation. Blood will be drawn from a vein in the arm by a technician.

Interpretation of results

Normal range is considered to be:

  • TSH - (Thyroid Stimulating Hormone) –from 0.3 to 4 mIU/L.
  • FT4 - (Free T4 or thyroxine) - 0.8–2.0 ng/dL
  • FT3 – (Free T3 or triiodothyronine) – 2.3–4.2 pg/dL

High levels of TSH in the blood are a sign of hypothyroidism or less than normal functioning of the thyroid. On the other hand, low levels point towards hyperthyroidism or an overactive thyroid. However, it can also indicate pituitary disease and a diagnosis of thyroid disease can only me made when FT4 and FT3 results are taken into consideration. Generally speaking, a low TSH along with a low FT4 is indicative of pituitary disease whereas a low TSH with high FT4 or FT3 indicates hyperthyroidism.

Results are likely to be affected if a contrast dye with iodine is used for x-ray and during pregnancy. Some medicines like birth control pills, medicines for Parkinsons, amiodarone for abnormal heart rhythms, NSAIDs, rifampicin, proton pump inhibitors, antacids with aluminium and so on also influence results.

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Written by: Nandita tripati

Date last updated: January 10, 2015

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