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.
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
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)
Hypothyroidism resulting from a problem with pituitary or hypothalamus signalling that control the thyroid gland
Your results will be compared to a reference interval (sometimes called a normal range or reference range).
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:
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.
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