If you have symptoms that suggest a thyroid disorder your doctor will usually start by requesting a TSH test which measures the amount of Thyroid Stimulating Hormone in your blood. If your TSH level is high or low, you may need to have a Free T4 test to identify the problem. A free triiodothyronine (FT3) test may sometimes also be requested. The Free T4 test measures the amount of free thyroxine in your blood.

Why get tested?

Your thyroid makes hormones that are important for many of your body's functions. How your thyroid is working affects your metabolism, heart rate, blood pressure and body temperature.   Metabolism refers to all the physical and chemical processes in the body that convert or use energy for breathing, blood circulation, body temperature control, brain and nerve function and more.

There are two main hormones produced by the thyroid. These are T4 and T3.  They circulate in your blood and it is important that levels stay constant to keep your metabolism running and in balance. There is a feedback loop to make sure they don't get too high or too low.

  • Your thyroid is under the control of your hypothalamus, which is part of your brain, and your pituitary gland which sits at the base of your brain.
  • The pituitary gland tells your thyroid gland how much thyroid hormone to make and release and it does this by controlling the blood levels of TSH (Thyroid Stimulating Hormone).   
  • If thyroid hormone production falls, TSH rises and conversely, 
  • If thyroid hormones become too high, TSH levels fall.


If you have symptoms that suggest you have a thyroid problem your doctor will start by ordering a Thyroid Stimulating Hormone (TSH) test. 

If your TSH level is high or low, you may need to have a Free T4 test to identify the problem and sometimes a free triiodothyronine (FT3) test may also be requested. 

T4 makes up most of the thyroid hormones produced by the thyroid gland and T3 makes up less than 10 per cent.

A ‘free’ T4 or T3 test refers to the fact that hormones are circulating freely and available to be absorbed by body tissues.

Having the test



Any preparation?


Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form.  You will see separate columns or lines for each of these tests.  

High free T4 results may indicate an overactive thyroid gland (hyperthyroidism) or recent consumption of thyroxine medication

Low free T4 results may indicate an underactive thyroid gland (hypothyroidism).

Both decreased and increased free T4 results are associated with a range of temporary and chronic thyroid conditions. 

A low free T4 result in conjunction with a low TSH level, or a high free T4 result along with a high TSH may indicate a pituitary gland condition.

Patterns of thyroid function test results and their most common causes.








Normal thyroid function




Mild (sub-clinical) underactive thyroid (hypothyroidism) or acute illness



Low or normal

Underactive thyroid - too little thyroid hormone being produced resulting form a problem with thyroid (hypothyroidism).




Mild (sub-clinical) overactive thyroid (hyperthyroidism)


High or normal

High or normal

Overactive thyroid - too much thyroid hormone being produced (hyperthyroidism)




HypothyroidismDecreased activity of the thyroid gland. Most common causes are Hashimoto’s thyroiditis and overtreated hyperthyroidism. resulting from a problem with pituitary or hypothalamus signalling that control the thyroid gland


Reference intervals

Your results will be compared to a reference interval (sometimes called a normal range or reference range).

  • Reference intervals are the range of results expected in healthy people 
  • When compared against them your results may be flagged high or low if they sit outside this range
  • Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms.   It is important that you tell themeverything you think might help. 
You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely. 
Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

  • Why does this test need to be done?
  • Do I need to prepare (such as fast or avoid medications) for the sample collection?
  • Will an abnormal result mean I need further tests?
  • How could it change the course of my care?
  • What will happen next, after the test?

Any more to know?

It is important to remember that thyroid tests are a snapshot of what is occurring at the time the test is taken.  Your free T4 results may vary and be affected by temporary changes.

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

Get further trustworthy health information and advice from healthdirect.

Last Updated: Thursday, 1st June 2023

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