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.
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.
Different blood tests are available to measure thyroid hormones, but some aren’t useful in all cases. The tests for evaluating thyroid function include:
The most suitable initial test for thyroid function is the TSH levels blood test. Changes in TSH levels usually serve as an early warning system. This often occurs before thyroid hormones become too low or high.
High TSH levels indicate the thyroid gland isn’t producing enough thyroid hormone, a condition called primary hypothyroidism. On the other hand, when a TSH level is low, it often indicates the thyroid hormone produces excess thyroid hormone, a condition called hyperthyroidism.
Sometimes, a low TSH level may occur from a pituitary gland disorder which prevents the gland from producing enough TSH to stimulate the thyroid gland. This condition is called hypothyroidism. A normal TSH level in healthy individuals may mean the thyroid gland isn’t functioning properly.
T4 is the main thyroid hormone in the bloodstream. The total T4 test measures the unbound and bound T4 hormone. The total T4 level can change when the binding protein level changes. The free T4 test measures the thyroxine hormone that isn’t bound that can enter and affect the tissues in the body.
The tests that measure free T4, either the free T4 index (FTI) or free T4 (FT4), give a more accurate result of how the thyroid gland functions when performed with a TSH.
A high TSH and low FTI or FT4 indicate primary hypothyroidism resulting from a thyroid gland disease. A low TSH and low FTI or FT4 result show hypothyroidism resulting from a problem in the pituitary gland. A low TSH with increased FTI or FT4 indicates hyperthyroidism.
T3 tests are usually for determining the severity of hyperthyroidism or diagnosing hyperthyroidism. People with hyperthyroidism will have higher T3 levels. In some people with low TSH, FTI or FT4 level is normal, and T3 level is elevated.
T3 testing rarely helps in people with a hypothyroid gland because it is usually the last test to show abnormalities. Patients with severe hypothyroidism have high TSH and low FTI or FT4, but the T3 level will be normal.
Measuring free T3 levels is possible, but this test isn’t always reliable and may not be helpful.
Reverse T3 is a biologically inactive protein with a similar structure to T3, but the iodine atoms are in different positions, making it inactive. The body produces some reverse T3, but it rapidly degrades. In healthy people who aren’t hospitalised, reverse T3 measurement doesn’t determine the presence or absence of hypothyroidism and isn’t clinically useful.
The body’s immune system can protect it from foreign invaders like viruses and bacteria by destroying these foreign bodies with antibodies produced by blood cells called lymphocytes. In most people with hyperthyroidism or hypothyroidism, lymphocytes also attack the thyroid (thyroid autoimmunity) and produce antibodies against thyroid cell proteins.
The two common thyroid antibodies that the test measures as thyroglobulin and thyroid peroxidase antibodies. Measuring these antibodies will help determine the cause of thyroid problems.
For example, if a patient with hypothyroidism has a positive antithyroid peroxidase or anti-thyroglobulin antibody positive result, the diagnosis may be Hashimoto’s thyroiditis. While antibody detection helps in the initial diagnosis of hypothyroidism resulting from autoimmune thyroiditis, over time, their level changes don’t help detect the response to therapy or the development of hypothyroidism. The FT4 and FSH levels can better explain the exact thyroid levels and function.
Another antibody that can be detected in people with hyperthyroidism is TSI or stimulatory TSH receptor antibody. It makes the thyroid overactive, a condition caused by Graves’ disease.
The doctor may order an antibody test to detect the thyrotropin receptor (TRAb or TSHR) if you have Graves’ disease. This test detects blocking and stimulating antibodies. Monitoring the antibody levels in people with Graves’ disease may help assess hyperthyroidism treatment. It helps determine if you can discontinue the antithyroid medication or assess the risk of the antibody passing down to the foetus during pregnancy.
Thyroglobulin – Tg is a protein in the normal thyroid cells and thyroid cancer cells procedure. This test doesn’t measure thyroid function or diagnose thyroid cancer when the thyroid gland is still present. This test is usually for patients who had surgery for thyroid cancer to monitor their health after treatment.
Although doctors sometimes order the Tg test, it isn’t a primary test for measuring thyroid hormone function.
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.
The following are common medications that affect thyroid function tests.
Oestrogen, such as birth control pills, and during pregnancy can cause elevated total T3 and T4 levels. This elevation occurs because oestrogen increases the level of binding proteins. In these cases, free T4 and TSH are necessary for thyroid evaluation.
Biotin, a common over-the-counter supplement, can cause abnormal thyroid function tests when the thyroid hormone levels are normal. Avoid taking biotin for two days before your blood sample collection for 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.