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Thyroid function tests are currently the most accurate way to diagnose and manage thyroid disorders. Your doctor will interpret the blood test results, together with your symptoms and how you feel, to reach a diagnosis to manage your treatment.

The most common thyroid function tests

  • TSH - Thyroid Stimulating Hormone
  • FT4 - Free T4 (the active part of thyroxine)
  • FT3 - Free T3 (the active part of triiodothyronine)

Other blood tests

  • Thyroid antibodies - to check the cause and severity of the thyroid disorder
  • Thyroglobulin and calcitonin - to monitor people with thyroid cancer

Typical reference ranges for normal thyroids

Test From To Units
TSH 0.4 4.5 mU/L (milliunits per litre)
FT4 9.0 25.0 pmol/L (picomoles per litre)
FT3 3.5 7.8 pmol/L (picomoles per litre)

These ranges are only a guide. The reference range for FT4 in particular does currently vary between methods and so any 'typical' reference range quoted will be subject to method and local interpretation

Test results outside the reference range

A high TSH level with a low FT4 level: Hypothyroidism (under-active thyroid)

A low TSH level with a high FT4 level and a high FT3 level: Hyperthyroidism (over-active thyroid)

Abnormal TSH levels together with normal FT4 levels indicate you may be at risk of developing a thyroid disorder

A low TSH levels together with a low FT4 levels can indicate a disorder of the pituitary gland

Management of thyroid disorders

If you have a diagnosed thyroid disorder, thyroid function tests will

  • check that your treatment is working
  • help to fine-tune your treatment

Other points

  • Treatment aims to get your TSH levels back within the reference range
  • Ask your doctor for a blood test if you have symptoms of a thyroid disorder
  • You should have an annual blood test if you have had previous treatment for an overactive thyroid
  • If you have a diagnosed thyroid disorder you should have a blood test
    • once a year, or more often if your doctor advises
    • in early pregnancy or if you are planning a pregnancy

It is well recognised that thyroid problems often run in families and if family members are unwell they should be encouraged to discuss with their own GP whether thyroid testing is warranted