Thyroid Function Tests

What is a Thyroid Function Test?

The function of the thyroid gland found in the neck is to regulate the body's metabolism. It does this mainly by two hormones, thyroxine (also called T4) and tri-iodothyronine (T3), which are released into the bloodstream. The thyroid mainly produces T4, while T3 is mainly produced outside the thyroid by the conversion of T4 to T3. In the blood, most T4 and T3 bind to protein; it is the unbound or 'free' form that is biologically active.

The thyroid is controlled by a hormone produced by the pituitary gland called thyroid-stimulating hormone (TSH). The pituitary, which is underneath the brain, monitors the thyroid hormone levels in the blood. If thyroid hormone levels fall below normal, then the pituitary releases TSH and the thyroid produces more T4 and T3; if levels rise above normal, then the pituitary does not release TSH and the thyroid produces less T4 and T3.

Thyroid function tests determine how well the thyroid and pituitary glands are working. The tests measure the levels of hormones produced by the thyroid and pituitary glands, certain enzymes, proteins that carry the hormones in the blood, and antibodies. If the thyroid (or pituitary) is not functioning normally because of abnormality or disease, then the levels of these substances will not be within the normal ranges associated with healthy people.

What are the main Thyroid Function Tests?

A thyroid function test usually measures the following:

• Total T4, the level of bound and free T4 in blood plasma
• Free T4 (FT4), the level of unbound T4 in blood plasma
• Free T3 (FT3), the level of unbound T3 in blood plasma
• TSH, the level of pituitary hormone controlling the production of T4 and T3
• Thyroglobulin, a protein made by thyroid follicular cells that is broken down to release T4 and T3
• Thyroglobulin antibodies, immune system proteins that attack thyroglobulin
• Thyroid peroxidase, an enzyme produced by the thyroid, needed to make T4 and T3
• Thyroid peroxidase antibodies, immune system proteins that attack thyroid peroxidase

It is important to inform the healthcare professional requesting the test of your medicines and illnesses, as these can affect your test results.

The Online Clinic offers the following tests:

Thypoid Function (Basic)

The blood sample is taken at home using a specially designed finger prick kit. Everything is sent back to our lab in a prepaid envelope and the results are then passed to the doctor for interpretation. Results in 4 hours. The test covers FT4 and TSH.

Thyroid Function (Advanced)

This test covers FT4 and TSH (as per the basic test) but also includes: Free T3, Total T4, Thyroglobulin Abs and Thyroid Peroxidase. Again, the sample is collected using a specially designed kit for use by patients at home. Results take 2 days and you receive an interpretation from a doctor along with your lab report.

Why is a Thyroid Function Test important?

By investigating the thyroid's functioning, thyroid function tests can reveal diseases, such as hypothyroidism, whereby the thyroid gland is underactive and secretes reduced thyroid hormone levels; hyperthyroidism, whereby the thyroid gland is overactive and produces increased thyroid hormone levels; and autoimmune thyroid disease, where your immune system produces antibodies against your thyroid such as with Hashimoto's disease (causes hypothyroidism) and Graves' disease (causes hyperthyroidism).

The signs and symptoms of hypo- and hyperthyroidism are opposed. Thus, those of hypothyroidism include weight gain, depression, dry skin, cold sensitivity, slow heart rate, high cholesterol levels, cardiovascular disease, and hearing loss; those of hyperthyroidism include weight loss, anxiety, heat sensitivity, rapid heart rate, heart irregularities, eye problems, and osteoporosis. Both hypo- and hyperthyroidism cause tiredness, goitre (swelling of the thyroid gland), and pregnancy complications.

Who should monitor their Thyroid Function?

Thyroid function tests are carried out when signs and symptoms indicate a thyroid abnormality. They can be used to monitor people with diagnosed thyroid disease, an autoimmune disease, or conditions that increase the risk of problems with the thyroid gland, such as type 1 diabetes, coeliac disease, and Down's syndrome. The tests may be used to screen for genetic diseases of the thyroid gland or if there is a family history of autoimmune disorders. People with osteoporosis or who are planning pregnancy or pregnant should have their thyroid function checked.

These tests are also used to monitor thyroid hormone replacement therapy in people with hypothyroidism, and thyroid function in people who are receiving or have previously received treatment for hyperthyroidism, or have had irradiation therapy to the head and neck due to cancer.

How to test for Thyroid Function?

Thyroid function tests can be done on a blood sample taken either by a healthcare professional or by yourself using an easy-to-use self-collection test kit. The test kit contains all you need to collect a finger-prick sample into a blood collection tube (sterile swabs, moist wipes, lancet, labels, collection tube), and to label and pack the sample in a protective packing wallet. There are detailed instructions on how to collect a 600 mcl blood sample from your little finger into the collection tube, which is then sealed with the cap and carefully inverted to mix. Your healthcare professional will inform you of the laboratory address to send your labelled blood sample for analysis. Results are reported to the healthcare professional requesting the tests.

What do Thyroid Function Test results indicate?

Screening for one factor only such as TSH can lead to misdiagnosis or a missed condition. T4 levels can also be high/low due to medicines, other health problems or if you are pregnant. A combination of factors is usually tested; often, FT4 and TSH. Test results indicating thyroid problems include:

• TSH high + T4/T3 low/normal: hypothyroidism, Hashimoto's disease
• TSH high + T4/T3 high: hyperthyroidism due to pituitary disease
• TSH low + T4/T3 high/normal: hyperthyroidism, Graves' disease, too high thyroxine replacement therapy dose, thyroid nodules, thyroid inflammation (thyroiditis)
• TSH low + T4/T3 low/normal: non-thyroid illness, hypothyroidism due to pituitary disease
• Thyroid antibodies; autoimmune thyroiditis e.g., Graves' disease and Hashimoto's disease.

What is a normal Thyroid Function Test result?

Thyroid hormone levels measured in blood samples are compared against the levels found in the general healthy population, the so called 'reference ranges', although each local laboratory has its own reference ranges. Your doctor will let you know your results. As a guide, in healthy people, FT4 is in the range 9-24 pmol/L, FT3 is 4-7.8 pmol/L, and TSH is 0.4-4.0 µIU/mL.

How to maintain or achieve good Thyroid Function

Annual thyroid function tests are recommended if you are taking thyroxine replacement therapy or antithyroid medicines. Eating a balanced diet, taking regular exercise, and drinking alcohol responsibly is key to keeping healthy. However, other dietary considerations may improve your well-being:

Foods to include in your diet are dairy products, eggs, seafood, and nuts, because they contain:

• Iodine, essential for T4 and T3 production (T4 has four iodine molecules, T3 has three)
• Selenium, enzymes required to convert T4 to T3 are dependent on selenium
• Zinc, required for the production of thyroid hormones.

Foods to use with caution include:

• Foods containing 'goitrogens', which block iodine uptake, e.g., raw brassica vegetables, such as broccoli, kale, and cauliflower (cook them to avoid any negative effect) and soy (a small amount is fine)
• Gluten: a gluten free diet may reduce inflammation of the thyroid gland, and may reduce thyroid antibody levels in autoimmune thyroid disease, but evidence is limited
• Supplements such as iodine, iron and calcium can interfere with your treatment and worsen your condition, so take advice before using them.