Thyroid Function Tests

Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.

What is a thyroid function test?

Thyroid function tests are blood tests that check the levels of the hormones (chemicals) made by the thyroid gland. Some thyroid function tests also check the level of a hormone made by the pituitary gland in the brain, which acts on the thyroid gland.

What is the thyroid?

The thyroid is a gland found in the neck. Its main function is to make hormones.

Hormones are chemicals which are released into the bloodstream. They act as messengers, affecting cells and tissues in distant parts of your body. Thyroid hormones affect the body’s metabolic rate and the levels of certain minerals in the blood.

What does the thyroid do?

The thyroid makes three hormones that it secretes into the bloodstream. Two of these hormones, called thyroxine (T4) and triiodothyronine (T3), increase your body’s metabolic rate. The other hormone helps to control the amount of calcium in the blood.

In order to make T3 and T4, the thyroid gland needs iodine, a substance found in the food we eat. T4 is called this because it contains four atoms of iodine. T3 contains three atoms of iodine. In the cells and tissues of the body most T4 is converted to T3. T3 is the more active hormone; it influences the activity of all the cells and tissues of your body.

How does the thyroid work?

The main job of the thyroid gland is to produce hormones T4 and T3. To do this the thyroid gland has to take a form of iodine from the bloodstream into the thyroid gland itself. This substance then undergoes a number of different chemical reactions which result in the production of T3 and T4.

The activity of the thyroid is controlled by hormones produced by two parts of the brain, the hypothalamus and the pituitary. The hypothalamus receives input from the body about the state of many different bodily functions. When the hypothalamus senses levels of T3 and T4 are low, or that the body’s metabolic rate is low, it releases a hormone called thyrotropin-releasing hormone (TRH). TRH travels to the pituitary via the connecting blood vessels. TRH stimulates the pituitary to secrete thyroid-stimulating hormone (TSH).

TSH is released from the pituitary into the bloodstream and travels to the thyroid gland. Here TSH causes cells within the thyroid to make more T3 and T4. T3 and T4 are then released into the bloodstream where they increase metabolic activity in the body’s cells.

High levels of T3 stop the hypothalamus and pituitary from secreting more of their hormones. In turn this stops the thyroid producing T3 and T4. This system ensures that T3 and T4 should only be made when their levels are too low.

For more information on how the thyroid gland works see separate leaflet called ‘Thyroid and Parathyroid Glands’.

How do thyroid function tests work?

There are several different types of thyroid function tests which may be carried out. Interpreting all the different tests is complicated as there are various conditions which can change the level of these hormones. A rough guide to the different types of tests and their interpretation is given below. However, your doctor or specialist doctor should explain individual test results.

Usually the first test to check thyroid function measures the levels of TSH in your blood. In people with hypothyroidism (underactive thyroid) the amount of TSH will usually be high. This is usually because the thyroid is not making enough T3 to stop the pituitary producing TSH. If the level of TSH is high, you will usually have further tests to check the levels of T3 and T4 in the blood.

In people with hyperthyroidism (overactive thyroid) the level of TSH will usually be low. This is usually because the thyroid gland is making too much of its hormones. When levels of T3 and T4 are high, the pituitary is ‘turned off’ and the amount of TSH produced is less. If you are found to have low levels of TSH you may have some more blood tests to check the levels of T3 and T4 in the blood. These tests may help doctors to find a specific cause of the low TSH.

What are thyroid function tests used for?

Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). Thyroid function tests can also be done to:

  • Monitor treatment with thyroid replacement medicine for people who have hypothyroidism.
  • Check thyroid gland function in people who are being treated for hyperthyroidism.
  • Screen newborn babies for inherited problems with the thyroid.

What happens during a thyroid function test?

A thyroid function test is a simple blood test. The blood sample is then sent to the laboratory for analysis, and the results are sent back to the doctor who asked for the tests.

What should I do to prepare for a thyroid function test?

Thyroid function tests usually require very little preparation. Tell your doctor if you are taking any medication as some drugs can alter the test results and how they are interpreted. It is also important to mention if you have had any X-ray tests that have used a special contrast dye as this may contain iodine which can affect the results. Levels of thyroid hormones also change in pregnancy, so tell your doctor if you are pregnant when the test is taken.

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