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Also known as
Thyroid stimulating hormone (TSH), Free Thyroxine (FT4), Free Triidothyronine (FT3), thyroid antibodies.

Thyroid function tests are tests that can give information about how your thyroid is working. The most common of these are: Thyroid stimulating hormone (TSH), Free Thyroxine (FT4) and Free Triidothyronine (FT3). Usually, Thyroid Stimulating Hormone (TSH) is tested first and the other tests are only performed if the TSH results indicate that more information is needed. If the other tests indicate a possible autoimmune thyroid disease, thyroid antibodies may also be tested.
 
When to get tested
If you have signs or symptoms that suggest the thyroid is not producing the right amount of thyroid hormones. These may include weight loss or gain, dry skin, heat or cold intolerance, increased heart rate, anxiety or a swelling of the thyroid gland (goitre).

The thyroid is a small butterfly-shaped gland that lies flat across the windpipe at the base of the throat. Its job is to make thyroid hormones and release them into the bloodstream. T3 and T4 are the two major hormones produced by the thyroid gland. Together, they help control the rate at which the body uses energy. They regulate things like your heart rate and body temperature.

It is important that thyroid hormone levels are in balance – not too high or too low – and there is a feedback mechanism to ensure levels remain stable. The thyroid gland works together with the pituitary gland to do this.

When thyroid hormone (T4 and T3) levels in the blood start to fall below normal, the pituitary gland steps in and releases more TSH. The increase in TSH signals the thyroid gland to pump out more T4 and T3. When the system is functioning normally, thyroid production turns on and off to maintain constant blood thyroid hormone levels.
 
Sample required
A blood sample drawn from a vein in your arm
 
Test preparation needed
None
 
What results can show
If your T4 levels are low, andr TSH is high, you may feel tired or experience weight gain. We call this hypothyroidism. If your T4 levels are too high, and TSH is low, everything runs at a faster rate and you may experience heat intolerance or weight loss. This is hyperthyroidism. The feedback control is very sensitive and TSH often changes before T4 becomes abnormal or symptoms appear – this is why TSH is often the first test performed.

If the feedback system involving the thyroid gland is not functioning properly due to one of a range of disorders, increased or decreased amounts of thyroid hormones may result. Here we have some examples of typical test results and their potential meaning.
TSH FT4 FT3 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; rare pituitary (secondary) hypothyroidism


FOR MORE DETAILED INFORMATION, GO TO THE SPECIFIC TEST PAGES:

Thyroid stimulating hormone (TSH), Free Thyroxine (FT4), Free Triidothyronine (FT3), Thyroid antibodies