Thyroid Function Tests
As part of a health check-up when symptoms suggest the thyroid gland has either reduced or increased activity. Alternatively during a course of treatment for a disorder of the thyroid gland.
A blood sample taken from a vein in your arm
None needed; however, certain medications can interfere with the tests, so tell your doctor about any drugs that you are taking.
The thyroid gland is situated in the neck and produces two hormones Thyroxine or T4 and Tri-iodothyronine or T3. Thyroid function tests (TFTs) are a group of tests requested to help check how the thyroid gland is working, to diagnose thyroid gland disorders and to monitor the response of thyroid disorders to treatment. TFTs include measuring the amount of the thyroid hormones, Thyroxine (T4) or Tri-iodothyronine (T3) and/or the pituitary hormone, Thyroid Stimulating Hormone (TSH) in your blood. These hormones are chemical substances that travel through the bloodstream and control or regulate how fast your body uses energy. Thyroid hormones are present in the blood in either protein bound forms (the majority) or the free and active form of the hormone. Currently, the majority of UK laboratories measure the free form of the hormones – Free T4 (FT4) or Free T3 (FT3).
The release of thyroid hormones into the blood stream is controlled by Thyroid Stimulating Hormone (TSH) produced by the pituitary gland, situated at the base of the brain behind the bridge of your nose. When the blood thyroid hormone concentrations are lower than normal the pituitary gland releases TSH, which in turn increases the release of T4 and T3 into the blood stream. Under normal circumstances the amount of T4 and T3 present in the blood then returns to normal and TSH production is turned off, to maintain constant blood thyroid hormone concentrations.
Thyroid Function Tests usually include some combination of:
- TSH (thyroid stimulating hormone or thyrotropin)– to test for hypothyroidism and hyperthyroidism and to monitor thyroid replacement therapy
- T4 or FT4– to test for hypothyroidism and hyperthyroidism
- T3 or FT3– to test for hyperthyroidism
How is it used?
TFTs are used to help diagnose decreased and increased thyroid function (hypothyroidism and hyperthyroidism respectively), which, can be due to a variety of thyroid diseases, or occasionally, disorders of the pituitary gland. TFTs are also used to monitor the response to treatment of these conditions.
In some laboratories, the initial blood test for thyroid disorders is a TSH test. If your TSH concentration is abnormal, it will usually be followed up by measuring FT4 (or occasionally total T4). In some cases FT3 (or total T3) will also be performed. Often, the laboratory will do this follow-up testing automatically. This saves your doctor time having to wait for the initial hormone levels and then requesting additional tests to confirm or clarify a diagnosis.
When is it requested?
TFTs may be requested as part of a health check-up when symptoms suggest hypo- or hyperthyroidism due to a condition affecting the thyroid. They may also be requested following commencement of treatment for any detected thyroid condition.
Signs and symptoms of hypothyroidism (decreased function of the thyroid) may include weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women.
Subclinical hypothyroidism is an early, mild form of hypothyroidism. It is called subclinical because only the blood level of TSH is a little bit above normal.
Signs and symptoms of hyperthyroidism (increased function of the thyroid) may include increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and sometimes diarrhoea. There may be puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.
What does the test result mean?
If the thyroid gland becomes overactive and increased amounts of thyroid hormones are released, TSH production will be suppressed.
In a variety of illnesses not directly affecting the thyroid, the mechanism of TSH release may not function properly, so the release of TSH may be reduced and blood T4 and T3 may fall as a result. In this situation, because the pituitary does not respond, TSH is not elevated.
In some cases of pituitary failure TSH production does not occur normally (often associated with decreased or increased production of other hormones). Very rarely, TSH concentrations may be increased, due to a tumour of the pituitary, in which case the thyroid will make and release increased amounts of T4 and T3, and the patient may experience symptoms associated with hyperthyroidism.
The following table summarises test results and their potential meaning.
TSH T4 T3 Interpretation High Normal Normal Mild (subclinical) hypothyroidism High Low Low or normal Hypothyroidism Low Normal Normal Mild (subclinical) hyperthyroidism* Low High or normal High or normal Hyperthyroidism* Low Low or normal Low or normal Nonthyroidal illness; Rarely hypothyroidism
due to pituitary disease
* For patients on thyroxine replacement, this pattern of results might indicate over replacement and a careful check for clinical signs or symptoms of such should be made.
Is there anything else I should know?
What conditions are associated with hypo- and hyperthyroidism?
Common causes of thyroid dysfunction are autoimmune related; Graves’ disease causes hyperthyroidism, and Hashimoto’s thyroiditis causes hypothyroidism. Both hyper- and hypothyroidism can also be caused by thyroiditis (or thyroid inflammation including after childbirth), thyroid cancer, and excessive or deficient production of TSH. Another common cause of an over-active thyroid is a thyroid nodule or nodules (multi-nodular goitre).
What other tests may be requested in addition to TFTs?
Tests that may be performed in addition to TFTs may include:
- Thyroid antibodies - to help differentiate different types of thyroiditis and identify autoimmune thyroid conditions
- Calcitonin - to help detect the presence of excessive calcitonin production, which occurs in a particular cancer of the thyroid known as Medullary Carcinoma of the Thyroid.
- Thyroglobulin - to monitor treatment of thyroid cancer