Summary

• If you have symptoms that suggest you may have a thyroid problem 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 free T4 and free T3.
• A high TSH result may mean an underactive thyroid gland (hypothyroidism).
• A low TSH result may indicate an overactive thyroid gland (hyperthyroidism) or a problem with your pituitary, depending on your thyroid hormone results. 

What your thyroid does

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. These 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 chemicals 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 T4 and T3.
• 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?

By testing levels of TSH, free T4 and free T3 it’s possible to work out if you have a thyroid or pituitary problems.  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.  

Because thyroid hormones are so important to the function of your entire body, if you have too much or too little you can feel very unwell. 

• Hypothyroidism – when thyroid hormone levels are too low, and you have an underactive thyroid. Your symptoms may include tiredness, weakness, weight gain, muscle aches, constipation, slow heart rate and cold intolerance.
• Hyperthyroidism – when thyroid hormone levels are too high, you have an overactive thyroid. Your symptoms may include sweating, weight loss, rapid heartbeat, nervousness, sweating and needing to poo often.

Thyroid conditions include:

• Iodine imbalance – a lack of iodine in your diet can lead to hypothyroidism. Too much iodine leads to hyperthyroidism.
• Hashimoto’s thyroiditis – the immune system attacks the thyroid gland, causing inflammation which slows the production of thyroid hormones (underactive thyroid).
• Grave’s disease – the immune system makes an antibody that acts like TSH and causes the thyroid gland to produce too much thyroid hormone (overactive thyroid).
• Thyroid nodules – these are small non-cancerous growths that can stimulate your thyroid to make too much hormone.
• Thyroid cancer – there are several types of thyroid cancer with the most common being papillary thyroid cancer. Thyroid cancer is common in Australia and the most common types have very good long-term outcomes, especially if the cancer is found early.

The TSH test

If you have symptoms that suggest you have a thyroid problem your doctor will start by ordering a TSH test.  

If the results of this are too high or low, a free T4 (FT4) and/or free T3 (FT3) 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 FT4 test or FT3 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.

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 TSH result

• A high TSH result may mean an underactive thyroid gland (hypothyroidism).
• A high TSH result can also occur in people with underactive thyroid glands who have been receiving too little thyroid hormone medication.
• TSH can also be high when someone has an illness. If you are not well, it is important to check again when you have recovered.

Low TSH result

• A low TSH result may indicate an overactive thyroid gland (hyperthyroidism) or a problem with your pituitary, depending on your thyroid hormone results. 
• A low TSH result can also occur in people with an underactive thyroid gland who are receiving too much thyroid hormone medication.
• Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumour, in what is known as secondary hyperthyroidism, or a genetic condition where the pituitary gland does not respond to thyroid hormone levels as it should.

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.
• Thyroid testing reference intervals may be different between different labs.

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?

Results for TSH may be low during the first trimester of pregnancy. This is because one of the hormones of pregnancy (HCG) has a TSH-like affect so the pituitary is not required to produce as much TSH.

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. Get further trustworthy health information and advice from healthdirect.