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Thyroid function tests

The thyroid gland is an endocrine gland in the lower front of the neck. This butterfly-shaped gland produces thyroid hormones which are secreted into the blood and transported to every tissue in the body.

Thyroid hormones help keep the body warm and use energy. It also supports the heart, brain and muscle function, including other organs in the body.

How does the thyroid gland function?

The thyroid gland mostly secretes thyroxine, or T4, because it has four iodine atoms. T4 gets converted into triiodothyronine (T3) to become useful to cells. T3 contains three iodine atoms, and the conversion mainly occurs in the liver and some tissues where T3 functions, like in the brain.

T4 production by the thyroid gland is controlled by a hormone produced in the pituitary gland called thyroid stimulating hormone (TSH). The pituitary gland is found at the base of the brain. The release of TSH from the pituitary gland into the bloodstream depends on the amount of T4 the pituitary gland detects.

If the pituitary gland detects low T4 in the blood, it produces more TSH, which triggers the thyroid gland to produce more T4. The pituitary gland stops TSH production if the T4 in the bloodstream goes above a certain level.

The mechanism of the thyroid and pituitary gland is similar to that of the heater and thermostat. When your heater is off and your room becomes cold, the thermostat picks up this temperature and turns on the heater. When the heat rises to the appropriate levels, the thermostat notices this and turns off the heater. In the case of the pituitary and thyroid glands, the pituitary gland is the thermostat.

T3 and T4 in circulation usually bind to a specific transport protein. A change in the level of these transport proteins will change the amount of T3 and T4 measured. This often happens while using birth control pills or during pregnancy. The free T3 and T4, meaning unbound thyroid hormones, can enter and cause changes in body tissues.

Thyroid function blood tests

Different blood tests are available to measure thyroid hormones, but some aren’t useful in all cases. The tests for evaluating thyroid function include:

Other thyroid function test

Since T4 contains iodine, the thyroid gland needs to get a large amount of iodine from the blood to produce the required amount of T4. The thyroid gland is a very active mechanism for taking iodine from the bloodstream, so healthcare professionals can measure the activity of the thyroid gland by giving you a small amount of radioactive iodine to swallow. The iodine’s radioactivity allows your doctor to track the route of the iodine. By measuring the amount of radioactivity that the thyroid gland takes (radioactive iodine intake or RAIU), the doctor can determine the function of the thyroid gland.

High levels of RAIU are present in people with an overactive thyroid gland (hyperthyroidism), and low RAIU values indicate an underactive thyroid gland (hypothyroidism).

You can also have a thyroid scan to show a picture of the thyroid gland and check for parts of the thyroid that took up the iodine.

Medications that affect thyroid function tests

The following are common medications that affect thyroid function tests.

Our healthcare professional will inform you of the preparations before your thyroid function testing. You can visit our clinic or call 02071830244 to book an appointment for your thyroid function tests.