Revised 2018

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 of the thyroid disorder
  • Thyroglobulin and calcitonin - to monitor people with thyroid cancer

Typical reference ranges for normal thyroids

Test    From    To    Units  
0.4 4.0 mU/L (milliunits per litre)
9.0 25.0   pmol/L (picomoles per litre)
3.5 7.8 pmol/L (picomoles per litre)

These ranges are only a guide and will vary according to laboratory. There are different reference ranges for pregnant women and for testing babies and young children.

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 level together with a low FT4 level can indicate a disorder of the pituitary gland
  • Thyroid function tests can be influenced by medications and illnesses. Let the person taking your blood test know of anything that might affect the readings

Management of thyroid disorders

If you are receiving treatment for a thyroid disorder, thyroid function tests will

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

Within the limits set out above it is recommended that patients and their supervising doctors set individual targets that are right for their particular circumstances.

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 over-active 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

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.

This Quick Guide is one in a series about thyroid disorders. Other Quick Guides are available to read and download from the British Thyroid Foundation website.

A leaflet containing more detailed information about Thyroid Function Tests is also available.

The British Thyroid Foundation
tel: 01423 810093
The British Thyroid Foundation is a registered charity: England and Wales No 1006391, Scotland SC046037

Membership Information
Quick Guides

Endorsed by:

The British Thyroid Association - medical professionals encouraging the highest standards in patient care and research

The British Association of Endocrine and Thyroid Surgeons - the representative body of British surgeons who have a specialist interest in surgery of the endocrine glands (thyroid, parathyroid and adrenal)

First issued: 2008
Revised: 2010, 2011, 2015, 2018
Our literature is reviewed every two years and revised if necessary.

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