What are thyroid function tests?

Content overview

The most common thyroid tests

Other blood tests

Test results outside the reference range

Management of thyroid disorders

Who should be offered a thyroid function test?

Symptoms of thyroid disease are non-specific and it is difficult to make a diagnosis based on symptoms alone. Thyroid function tests are simple blood tests and are currently the most accurate way to diagnose and manage over- or under-activity of the thyroid. 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 form of thyroxine)

• FT3 - Free T3 (the active form of triiodothyronine)

In most cases, your TSH level will be the first thyroid hormone to be tested. If this is normal, your thyroid is likely functioning normally.

You may be offered additional tests in the following circumstances:

• When a pituitary problem is suspected, in which case FT4 should be measured as well

• If you are child where it is good practice to measure FT4 as well. This is because FT4 is essential to brain development

If the TSH is abnormal, this will normally trigger the following tests:

•   FT4 blood level if the TSH level is above the reference range
  
  
• T4 and T3 blood levels if the TSH level is below the reference range 

Other blood tests

• Thyroid antibodies - to check whether the underlying cause of the abnormal tests is autoimmune thyroid disease

• Thyroglobulin and calcitonin - to monitor people with thyroid cancer

Typical reference ranges for healthy adults are:

Test	From	To	Units
TSH	0.4	4.0	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 and will vary according to the 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 indicates hypothyroidism (underactive thyroid)

• A low TSH level with a high FT4 level and a high FT3 level indicates hyperthyroidism (overactive thyroid)

• Abnormal TSH levels together with normal FT4 levels indicate you may be at risk of developing a thyroid disorder (subclinical thyroid disease)

• 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 you and your doctor set individual targets that are right for your particular circumstances.

Who should be offered a thyroid function test?

You should make an appointment with your GP and ask for a blood test if you have:

•  Symptoms of an over- or underactive thyroid
•   Swelling or thickening in the neck
•   An irregular or fast heart rat
•   High cholesterol (which causes atherosclerosis – a build-up of fat in the arteries)
•   Osteoporosis (fragile or thinning bones)
•   Fertility problems, abnormal menstrual cycles, recurrent miscarriage, low libido
•   Family history of autoimmune disorders, e.g., type 1 diabetes, vitiligo, etc

Or if you are

• Feeling unwell after having a baby
• planning pregnancy or in early pregnancy (and you have a family history or personal history of thyroid disorders, a past history of postpartum thyroiditis, or type 1 diabetes)

You should have a blood test once a year, or more frequently if your doctor advises, if:

•   You have a diagnosed thyroid disorder
•   You have had previous treatment for an overactive thyroid (radioactive iodine, thyroid surgery, medication)
•   You have had irradiation to the head and neck after surgery for head and neck cancer
•   Before you have treatment with amiodarone, lithium or alemtuzumab then 6-12 months during treatment and 12 months after treatment
• People with Down’s syndrome, Turner syndrome, type 1 diabetes, Addison’s disease or other autoimmune diseases should also be tested regularly.

Read our guide to thyroid function tests

Watch our webinar on understanding thyroid function tests

Read our article about thyroid antibodies