Summary

• If you have symptoms that suggest you may have a thyroid problem your doctor will usually start by requesting a TSH (Thyroid Stimulating Hormone) blood test. This is the only thyroid test covered by Medicare if you do not have a history of thyroid problems.
• If your TSH level is too high or low, you may need to have a free T4 test to learn more.
• T4 is one of two thyroid hormones – the other is T3.  
• Because T4 is converted into T3, it is more important to measure T4. Any changes in your thyroid hormone levels show up in T4 first.
• The tests are termed free T3 and free T4. This is because they measure the active form of thyroid hormones which are circulating in your blood ready to be used when needed. The other type is called bound (bound T3 or bound T4) because they are attached to proteins.  

What does your thyroid do?

Your thyroid is a small gland at the base of your throat. It uses iodine from your diet to make hormones that regulate many of your body’s metabolic processes – your body’s processes for breaking down food and converting it into energy.  

How your thyroid is working affects how fast you burn calories, your heart rate, blood pressure and body temperature. If you don't have enough thyroid hormones in your blood, many of your body functions will slow down. But if you have too much, they will speed up.

Thyroid hormones

There are two main hormones produced by the thyroid. These are T4 and T3. There is much more T4 hormone in your blood, however T3 is the more active hormone. Both hormones circulate in your blood, and it is important that levels stay constant to keep your metabolism running and in balance. Hormones are the body’s chemical messengers. They are made by various glands and travel through the bloodstream to regulate body functions.

There is a feedback mechanism to make sure thyroid hormone levels do not get too high or too low.

• A part of the brain called the hypothalamus and a gland at the base of the brain called the pituitary control your thyroid gland. The hypothalamus and the pituitary are connected by a stalk of blood vessels and nerves.
  The hypothalamus senses how much thyroid hormone is in your blood and if levels are too low it sends a signal to your pituitary to make TSH – thyroid stimulating hormone.
• TSH stimulates your thyroid to make thyroid hormones T3 and T4.
• If hormone levels are too high, it tells the thyroid to make less. 
• This means that when thyroid hormone levels are low, TSH levels rise and conversely, if thyroid hormones become too high, TSH levels fall.

Why get tested?

If you have symptoms that suggest you could have a thyroid problem your doctor will usually start by requesting a TSH test. This is the only thyroid test covered by Medicare if you do not have a history of thyroid problems. It is used to test for an underactive or overactive thyroid.

If the results of the TSH are too high or low, a free T4 and/or free T3 test will be ordered.

By testing levels of TSH, free T4 and free T3 it’s possible to work out if you have a thyroid, pituitary or hypothalamus problem. Often the laboratory performing your testing will automatically go on to perform a free T4 test or free T3 test on your original blood sample.

No single pathology test can accurately diagnose all types of thyroid conditions. A combination of tests is usually needed to detect disorders and to differentiate between thyroid and pituitary gland problems.

Because T4 is converted into T3, a free T4 is more important to measure. Any changes often show up in free T4 levels first.

Free T4 levels are also measured if you are being treated with thyroid hormone replacement therapy to make sure that it is working properly.

What does free T4 and T3 mean?
Free thyroid hormone tests measure the active form of thyroid hormones which are circulating in your blood ready to be used when needed. The other type is called bound thyroid hormone. It is attached to proteins and cannot enter cells to be used.

For more on thyroid testing go to Thyroid Function Tests.

Having the test

Sample

Blood.

Any preparation?

None.

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. In hypothyroidism due to a hypothalamic disorder (an issue with the hypothalamus), free T4 is decreased. Measuring free T4 will not identify the cause of the hypothyroidism as hypothalamic without other tests being done, a clinical evaluation and scans.

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.

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.

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 them everything 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?

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.

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