Thyroid Function Tests
To help check how the thyroid gland is working, to diagnose thyroid disorders and to monitor the response to treatment.
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.
What is being tested?
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 thyroud 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 or Free T3
The release of thyroid hormones into the blood stream is controlled by Thyroid stimulating hormones (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 or hyperthyroidism and to monitor thyroid replacement therapy
- T4 or FT4– to test for hypothyroidism and hyperthyroidism
- T3 or FT3– to test for hyperthyroidism
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 hypothyroidism (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 / T4
What does the result mean?
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.
| 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 |
Page created: 12 June 2025
