Summary

• Your thyroid is a small gland at the base of your throat.  It makes hormones that regulate many of your body’s most important functions including how fast you burn calories, your heart rate, blood pressure and body temperature.
• 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.
• Depending on your symptoms and your TSH test result, you may need further thyroid hormone tests such as free T4 and free T3.
• If it is thought that you could have an autoimmune condition you may go on to have thyroid antibody tests.
• There are also tests that are used to monitor thyroid cancer following treatment such as thyroglobulin and calcitonin tests.

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 stronger 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 chemical 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 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.

Why get tested?

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

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.

What thyroid function tests will you have?

Your doctor will usually start by requesting a TSH test. This is the only thyroid test covered by Medicare if you do not have a history of thyroid problems. It is used to test for hypothyroidism and hyperthyroidism.

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.

If your TSH level is high or low, often the laboratory performing your testing will automatically go on to perform a FT4 or FT3 test on your original blood sample. If the TSH is high, FT4 is measured to see the degree of hypothyroidism, and if the TSH is low, FT4 and FT3 can be measured to see the degree of hyperthyroidism.

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.

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.

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 and to differentiate between thyroid and pituitary gland problems.

For example, a low FT4 level could mean that your thyroid is not 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 FT4 is low, a high TSH level would confirm that it is your thyroid gland and not your pituitary gland that is the problem.

If the FT4 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, your pituitary would need to be further investigated. The pituitary gland is involved in managing many of your hormones including your normal steroid, reproductive and growth hormones.

Tests for autoimmune conditions

Further tests may need to be performed if your medical team suspects that you have an underlying autoimmune condition. 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 (underactive thyroid) and Graves’ disease (overactive thyroid).

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 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 a malignant cancer 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.

Any more to know?

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. Any supplements containing biotin should not be taken for two days before you have a blood test. 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.

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.