Thyroid Function Tests

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.

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.

Subclinical hyperthyroidism is a condition in which you have low levels of TSH but normal levels of T3 and T4.

If the thyroid gland fails the concentrations of T4 and T3 fall resulting in increased TSH release from the pituitary gland.

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.

* 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.

The actual thyroid tests offered may vary between hospital laboratories. For some, a strategy is employed to perform one initial test followed by others only if needed, and in other laboratories, the initial measurement includes both FT4 and TSH, with the addition of FT3 in selected cases.

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).