Thyroid Function Tests

Thyroid conditions are common and can affect people of all ages from babies to the elderly. They can be caused by a range of underlying conditions. Your doctor will usually start by requesting a TSH test. This measures the amount of Thyroid Stimulating Hormone (TSH) in your blood. TSH is the hormone that stimulates your thyroid gland to make thyroid hormone. This is the only thyroid test covered by Medicare if you do not have a history of thyroid problems. Depending on your symptoms and your TSH result, you may need further testing of your thyroid hormones with measurement of FreeT4 and Free T3. If your doctor thinks you could have an autoimmune condition that is affecting your thyroid gland you may go on to have thyroid antibody tests. There are also tests that are used to monitor thyroid cancer following treatment. They include thyroglobulin and calcitonin tests which are used to detect and monitor some rare forms of thyroid cancer.

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..   

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 mechanism to make sure they don’t get too high or too low.

  • The pituitary gland at the base of your brain controls your thyroid.
  • It produces Thyroid Stimulating Hormone (TSH) that interacts with the thyroid gland to increase or decrease production of T4 and T3 when levels are too low or too high.
  • If thyroid hormone levels are too low, TSH rises and conversely if thyroid hormones become too high, TSH levels fall.


Underactive and overactive thyroid conditions

Because the thyroid is important to so many bodily processes, too much or too little thyroid hormone can make you feel very unwell.  Your doctor will be looking for:

  • Hypothyroidism: when thyroid hormone levels are too low, and you have an underactive thyroid. Your symptoms may include fatigue, weakness, weight gain, muscle aches, constipation, slow heart rate and cold intolerance.
  • Hyperthyroidism is when thyroid hormone levels are too high, and you have an overactive thyroid. Your symptoms may include heat intolerance, weight loss, rapid heartbeat, nervousness, sweating and increased frequency of stools.

Thyroid function tests 

If you have symptoms that suggest you have a thyroid problem your doctor will start by ordering:

  • A Thyroid Stimulating Hormone (TSH) test which is used to test for hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), to screen newborn babies for hypothyroidism, and to monitor treatment with thyroid hormone tablets.  

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

  • FT4 – to test for hypothyroidism and hyperthyroidism.
  • FT3 – to test for hyperthyroidism.


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.  





Any preparation?


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.

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 from a problem with the 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.


Is it your pituitary?

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

For example, a low T4 level could mean that your thyroid isn’t producing enough T4 or that your pituitary gland is not stimulating the thyroid to produce T4.  Since the pituitary gland would normally release TSH if the T4 is low, a high TSH level would confirm that it is your thyroid gland and not your pituitary gland that’s the problem.

If the T4 level and TSH are both low or if they are both high, the pituitary gland is more likely to be the cause. If this is the case, it would change the course of the investigation.  The pituitary gland is involved in managing many of you hormones including your normal steroid, reproductive and  growth hormones.

Tests for autoimmune conditions

Further tests may need to be performed if your doctor suspects that you have an underlying autoimmune condition – you have an enlarged thyroid or symptoms suggesting thyroid dysfunction. In this case you may need to have antibody tests.

Autoimmune conditions are when your immune system, designed to defend you, mistakenly makes antibodies that attack healthy tissue. There are two main autoimmune conditions affecting the thyroid: 

  • Hashimoto's thyroiditis  – the immune system attacks the thyroid gland, causing inflammation which slows the production of thyroid hormones.
  • Graves’ disease – the immune system makes an antibody that acts like TSH and causes the thyroid gland to produce too much thyroid hormone.  


Some of the antibody tests used are:

  • Thyroid peroxidase antibodies (TPO) 
  • Thyroid-stimulating hormone (TSH) receptor 
  • Thyroid-stimulating immunoglobulin (TSI)


Tests for cancer diagnosis and monitoring

The thyroglobulin test is used as a tumour marker to assess the effectiveness of thyroid cancer treatment and to monitor for recurrence after the thyroid has been completely removed. Since thyroglobulin normally is made only in the thyroid, it should drop to very low or undetectable levels in these patients.

A calcitonin test is sometimes ordered to diagnose and monitor two rare conditions, C-cell hyperplasia and medullary thyroid cancer (MTC). In these conditions, large amounts of calcitonin are produced. C-cell hyperplasia is a benign condition that may or may not progress to become MTC. MTC is malignant which means it can spread beyond the thyroid and can be difficult to treat if it is not discovered early.


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.

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?

Many multivitamins, supplements – especially biotin found in hair and nail supplements– and over-the-counter and prescription medications can affect thyroid test results, so tell your doctor about any drugs you are taking. Your thyroid hormone medication also affects results. When your doctor adjusts your dose of thyroid hormone, it is important that you wait at least one to two months before checking your TSH again, so that your new dose can have its full effect.

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.