• 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 test result comes back abnormal, it usually means that your thyroid gland isn’t making the right amount of thyroid hormones.
• The main thyroid hormones are T4 and T3.
• Doctors don’t always order a Free T3 test as part of routine thyroid testing. The next step is usually Free T4.
• Free T3 is most useful if your symptoms suggest hyperthyroidism (you are making too much thyroid hormone) but your Free T4 is not raised.
• If TSH is low but T4 is normal, measuring T3 can help show early hyperthyroidism.
• A free T3 test is not very useful in diagnosing hypothyroidism (when you are not making enough thyroid hormones), since it often stays normal until your symptoms are severe.
• 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.

Your thyroid is a small gland at the base of your throat. It uses iodine from your diet to make hormones that control many of your body’s metabolic processes. 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.

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 thyroxine (T4) and triiodothyronine (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 control 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.

If you have symptoms that suggest you have a thyroid problem your doctor will start by ordering a 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 T3 test may also be requested.

T3 makes up less than 10 percent of thyroid hormones produced but it is about four times as strong as T4.

A ‘free’ T4 or T3 test refers to the fact that hormones are moving around freely and are available to be absorbed by body tissues. T4 and T3 can also be in the body in “bound” forms.

Sample

Blood.

Any preparation?

None.

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.

FT3 is used mainly to help diagnose hyperthyroidism when your thyroid produces too much hormone and you have an overactive thyroid. Hyperthyroidism is controlled through treatment. This will normally involve either tablets that stop the thyroid gland producing thyroid hormones, radioiodine treatment which destroys thyroid tissue, or surgery to remove part or all of the thyroid gland.

Testing for free T3 is not usually helpful if your doctor thinks you have hypothyroidism, a condition where your thyroid produces too little hormone.

Testing for T3 can be useful because T3 levels can become abnormal earlier than T4 and return to normal later than T4. This test may also be used for monitoring of patients on T3 therapy.

Reference intervals - comparing your results to the healthy population

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

• Reference intervals are the range of results expected in healthy people.
• They are used to provide a benchmark for interpreting a patient's test results.
• When compared against them, your results may be flagged high or low if they sit outside this range.
• Some reference intervals are harmonised or standardised, which means all labs in Australia use them.
• Others are not because for these tests, labs are using different instruments and chemical processes to analyse samples.
• Always compare your lab results to the reference interval provided on the same report.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation.

Reference intervals for FT3 vary between labs so you will need to go through your results with your doctor. Your doctor is the best person to interpret your results.

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?

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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