Formal Name
Thyroid Function Tests
This article was last reviewed on
This article waslast modified on 16 April 2018.
At a Glance
Why Get Tested?

To help evaluate thyroid gland function, to help diagnose thyroid disorders and to monitor the response to treatment.

When To Get Tested?

As part of a health checkup or when symptoms suggest hypo- or hyperthyroidism due to a condition affecting the thyroid; following commencement of treatment for disorders of the thyroid gland.

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

None needed; however, certain medications can interfere with the tests included in the panel, so tell your doctor about any drugs that you are taking.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you will be able to access your results online.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, gender, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Thyroid function tests (TFTs) are a group of tests that are requested together to help evaluate thyroid gland function, to help diagnose thyroid gland disorders and to monitor the response to treatment of thyroid disorders. TFTs include a measure of the amount of thyroid hormones, Thyroxine (T4) or Tri-iodothyronine (T3) in your blood. These hormones are chemical substances that travel through the bloodstream and control or regulate your body’s metabolism—how it functions and 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).

Thyroid Stimulating Hormone (TSH) is produced by the pituitary gland and is part of the body’s feedback system to maintain stable amounts of thyroid hormones in the blood. When thyroid hormone concentrations decrease in the blood, the pituitary gland is stimulated to release TSH, which in turn stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, TSH production turns on and off to maintain constant blood thyroid hormone concentrations.

Thyroid Function Tests usually include some combination of:

How is the sample collected for testing?

A blood sample taken from a vein in your arm

Is any test preparation needed to ensure the quality of the sample?

None needed; however, certain medications can interfere with the tests included in the panel, so tell your doctor about any drugs that you are taking.

Accordion Title
Common Questions
  • How is it used?

    TFTs are used to help diagnose hypo- and hyperthyroidism which can be due to various thyroid diseases, or occasionally, disorders of the pituitary gland or 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 with a FT4 measurement (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, and this is known as reflex testing.  This saves your doctor time from having to wait for the results of the initial test and then requesting the additional testing to confirm or clarify a diagnosis. Follow-up tests are often performed on the original sample that was submitted when the initial test was requested.

  • When is it requested?

    TFTs may be requested as part of a health check-up or 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 may include weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women.

    Signs and symptoms of hyperthyroidism 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 fails and the concentrations of thyroid hormones fall, a feedback system will result in increased TSH release from the pituitary gland. Similarly, if the thyroid gland becomes overactive and increased amounts of thyroid hormones are released, TSH production will be suppressed. If the feedback mechanism is not functioning properly, as can occur with a variety of illnesses not directly affecting the thyroid, the release of TSH may be reduced and the concentation of thyroid hormones in the blood may fall as a result. Very rarely, TSH concentrations may be increased, due to a tumour of the pituitary, in which case the thyroid will make and release inappropriate amounts of T4 and T3, and the patient may experience symptoms associated with hyperthyroidism. If there is decreased production of thyroid hormones, the patient may experience symptoms of hypothyroidism.

    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 overreplacement and a careful check for clinical signs or symptoms of such should be made.

  • Is there anything else I should know?

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

  • What conditions are associated with hypo- and hyperthyroidism?

    The most 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 (thyroid inflammation), thyroid cancer, and excessive or deficient production of TSH.

  • 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