Types of thyroid function tests
What are thyroid function tests used for?
Thyroid function tests are used to assist in the diagnosis of two conditions:
- Hypothyroidism — a condition in which the thyroid gland does not produce enough thyroid hormone.
- Hyperthyroidism — a condition in which the thyroid gland produces and secretes excessive amounts of the free thyroid hormones: triiodothyronine (T3) and/or thyroxine (T4).
What tests are available?
Numerous tests1 are available to assess thyroid function.
- Thyroid stimulating hormone (TSH): the initial test for suspected thyroid function disorder.
- Free thyroxine (fT4): indicates if the thyroid is underactive (hypothyroidism) or overactive (hyperthyroidism).
- Free triiodothyronine (fT3): can be helpful to assess if a person has thyrotoxicosis from overproduction of fT3.
- Thyroid antibody test: may establish the cause of the dysfunction and determine whether an autoimmune condition is present.
- Thyroid peroxidase antibody assay: required to confirm the diagnosis of autoimmune thyroiditis.
TSH is a sensitive marker of thyroid function because it is influenced by small changes in fT4 concentrations. A low TSH usually indicates hyperthyroidism whereas raised TSH usually means hypothyroidism.
The combined measurements of TSH and fT4 can detect two types of hypothyroidism: overt and subclinical. A raised concentration of TSH with a low concentration of fT4 suggests hypothyroidism. A low concentration of TSH with a high concentration of fT4 suggests hyperthyroidism. fT4 measurement is highly cost-effective compared with previously used measurements of total T4 or fT3.1,2 Thyroid ultrasonography is only indicated if there is concern about structural thyroid abnormalities.3
TSH test is a very sensitive test
Over the years the lowest concentration of TSH that can be detected has progressively fallen, allowing better separation of normal and hyperthyroid states.1,2 The high sensitivity and specificity of the test allows detection of subclinical disease states, but the value of identifying this is not determined.
People with subclinical thyroid disease have few or no symptoms or signs of thyroid dysfunction. It is a laboratory diagnosis defined as a raised serum concentration of TSH with a normal serum concentration of fT4.
When used to confirm suspected disease in patients, the TSH test has a sensitivity above 98% and specificity greater than 92% for clinical and functional diagnosis.4 However, the accuracy of a TSH when used to test healthy asymptomatic people is difficult to evaluate.5
For more information
- Mortimer RH. Thyroid function tests. Australian Prescriber 2011;34:12–5. http://www.australianprescriber.com/magazine/34/1/12/5
- So M, MacIsaac RJ, Grossmann M. Hypothyroidism. Aust Fam Physician 2012;41:556–62.[PubMed]
- Cooper DS, Doherty GM, Haugen BR, et al. Revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancer. Thyroid 2009;19:1167–214. [PubMed]
- Helfand M, Crapo LM. Screening for thyroid disease. Ann Intern Med 1990;112:840–9. [PubMed]
- The Royal College of Pathologists of Australasia. RCPA Manual 2004. www.rcpamanual.edu.au (accessed 2 February 2013).