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Summary

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 and/or a Free T3 test to identify the problem. The Free T3 test measures the amount of free triiodothyronine 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. 

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 circulating freely and available to be absorbed by body tissues.

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.  

FT3 is used mainly to help diagnose hyperthyroidism when your thyroid produces too much hormone and you have an overactive thyroid. 

FT3 is not usually helpful if your doctor thinks you have hypothyroidism when your thyroid produces too little hormone.

The T3 level 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.

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

TSH

FT4

FT3

Interpretation

Normal

Normal

Normal

Normal thyroid function

High

Normal

Normal

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

Low

Low

Lor or Normal

Underactive thyroid too little thyroid hormone being produced resulting from a problem with the thyroid (HypothyroidismDecreased activity of the thyroid gland. Most common causes are Hashimoto’s thyroiditis and overtreated hyperthyroidism.)
 

Low

Normal

Normal

Mild (sub-clinical) overactive thyroid (hyperthyroidism)
 

Low

High or normal

High or normal

Overactive thyroid  too much thyroid hormone being produced (hyperthyroidism)
 

Low

Low

Low

Hypothyroidism resulting from a problem with pituitary or hypothalamus signalling that control the thyroid gland
 

 

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal 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?

HyperthyroidismOveractivity of the thyroid gland most commonly due to Graves disease. 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.

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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