Hypothyroidism Explained: What an Underactive Thyroid Is and How It Is Measured

What hypothyroidism is

Hypothyroidism means the thyroid gland does not make enough thyroid hormone for normal body function. The thyroid is a small gland at the front of the neck, and the hormones it produces regulate how the body uses energy, which is why the gland influences so many systems at once.

Two hormones do most of the work. Thyroxine, written as T4, is the main hormone the thyroid releases. Triiodothyronine, written as T3, is the more active form, and most of it is produced when the body converts T4 into T3 in other tissues. Sitting above both is thyroid stimulating hormone, or TSH, which is released by the pituitary gland in the brain rather than by the thyroid itself. TSH is the instruction to the thyroid to produce more hormone or less.

That feedback loop is what makes thyroid testing work the way it does. When thyroid hormone levels run low, the pituitary responds by releasing more TSH to push the gland harder, so a raised TSH is the pattern most often seen when the thyroid is underactive. Australian Prescriber notes that the relationship is sensitive in both directions, and that small changes in free T4 cause large inverse changes in TSH concentrations. A hormone made in the brain therefore ends up being the most useful single measure of how the thyroid itself is performing.

Hypothyroidism develops over months or years rather than appearing suddenly, and it is usually permanent once established.

How common it is in Australia

Hypothyroidism is the most common thyroid disorder in Australia, affecting around 1 in 33 Australians according to both healthdirect and Hormones Australia, the patient resource of the Endocrine Society of Australia. It occurs more often in women than in men, and more often in people aged over 60.

The total splits into two groups that matter for how results are read. The RACGP puts the prevalence of overt hypothyroidism, where TSH is clearly raised, at 0.3 to 0.5 per cent of the population. Subclinical hypothyroidism, where TSH is raised but free T4 still sits inside the reference interval, is considerably more common at 2 to 2.5 per cent. The practical consequence is that a mildly raised TSH is a much more frequent finding than a clearly abnormal one, and the two are handled very differently.

A family history of thyroid or other autoimmune conditions is associated with a higher likelihood, because the most common cause in Australia has a genetic component. Previous thyroid surgery or radioactive iodine treatment also changes the picture, as does anyone already taking thyroid hormone replacement, whose levels are monitored on an ongoing basis.

What causes hypothyroidism

In Australia, the leading cause is autoimmune. The RACGP states that in iodine-replete countries, autoimmune chronic lymphocytic thyroiditis is the most common aetiology, and healthdirect and Hormones Australia both name Hashimoto's disease as the most prevalent cause here.

Other recognised causes include:

• Previous thyroid surgery, where part or all of the gland has been removed
• Radioactive iodine treatment or radiation therapy to the head and neck
• Certain medicines, including lithium and amiodarone
• Some cancer immunotherapies, a more recently recognised cause noted by Hormones Australia
• Congenital causes, where the gland has not developed or functioned normally from birth
• Genetic disorders affecting thyroid hormone production
• Iodine deficiency, which is the leading cause worldwide but an uncommon one in Australia

The iodine point is worth separating out, because it is often carried across from international content without the local context. Iodine is the raw material the thyroid uses to build its hormones, and severe deficiency limits production. Australia, however, is considered iodine-replete, and mandatory fortification has required iodised salt in most commercially produced bread since 2009. Iodine deficiency remains a real mechanism, and it is simply not the usual explanation in an Australian population.

For some people no single cause is identified, which is part of why testing is the starting point rather than working backwards from an assumed explanation.

Hashimoto's thyroiditis and thyroid antibodies

Hashimoto's thyroiditis is an autoimmune condition in which the immune system produces antibodies that act against the thyroid gland, gradually reducing its ability to produce hormone. Hashimoto's and hypothyroidism are related but not the same thing. Hashimoto's is a cause, and hypothyroidism is the state of underproduction that can result from it. Someone can carry thyroid antibodies while thyroid hormone production still sits inside the reference interval.

Antibody testing is what separates an autoimmune cause from the others. Pathology Tests Explained describes the main markers as thyroid peroxidase antibodies, thyroglobulin antibodies and TSH receptor antibodies, and notes that significantly high levels usually indicate a thyroid autoimmune disorder such as Hashimoto's thyroiditis or Graves' disease. It also makes two qualifications worth carrying over. Rising levels may be more significant than stable levels, because they indicate increasing autoimmune activity. And some people test positive for thyroid antibodies without ever developing thyroid dysfunction, while others develop it later, which is why antibody results are interpreted alongside TSH and free T4 rather than on their own.

Antibody status also feeds directly into clinical decisions. In its guidance on mildly raised TSH, the RACGP advises testing for thyroid peroxidase antibodies as part of working out whether the finding is persistent, and recommends ongoing monitoring where antibodies are present.

How an underactive thyroid is measured

Australian clinical guidance is consistent and unusually simple on the starting point. The RACGP states that initial screening is by measuring the thyroid stimulating hormone level, and Australian Prescriber puts it the same way: measuring thyroid stimulating hormone is the first step, and if it is abnormal, free thyroxine should be measured.

TSH is first because of the feedback loop described above. It moves earlier and more dramatically than the thyroid hormones themselves, which makes it the most sensitive single indicator that something has shifted. Free T4 then establishes what the thyroid hormone level actually is, and the combination of the two is what distinguishes one pattern from another.

There is one exception worth knowing. The RACGP notes that a free T4 level should be ordered where there is a convincing clinical picture for hypothyroidism despite the absence of TSH elevation, to exclude the much less common possibility of central hypothyroidism, which originates in the pituitary rather than the thyroid. In that situation both TSH and free T4 run low, which is why a test that measures free T4 alongside TSH gives a doctor more to work with than TSH alone.

Free T3 sits differently. It is genuinely useful in some thyroid presentations, and Australian Prescriber describes it as helpful where TSH is low and free T4 is normal. For an underactive thyroid specifically, TSH and free T4 are the markers that carry the weight. Pathology Tests Explained summarises the general principle: a combination of tests is usually needed to detect disorders and to differentiate between thyroid and pituitary gland problems.

What each thyroid marker measures

A thyroid pathology report is a set of related measures rather than a single verdict. This is what each one is measuring, the units used in Australian laboratories, and what a pattern can point towards.

Units are those in general use in Australian pathology. Reference intervals are not listed here on purpose. As Pathology Tests Explained notes, many reference intervals vary between laboratories, so the interval printed beside your own result is the one that applies to it.

Read together, the combinations are what carry the meaning. Pathology Tests Explained sets out the pattern logic plainly: a raised TSH with a free T4 still inside the reference interval points towards mild hypothyroidism, a low TSH with raised free T4 and free T3 points towards an overactive thyroid, and a low TSH together with a low free T4 points towards a pituitary problem rather than a thyroid one. None of those patterns is self-interpreting, and none of them replaces a doctor reading the full report against your history.

Overt and subclinical hypothyroidism

The distinction between these two is where most of the nuance in thyroid testing sits, and it explains why two people with a raised TSH can be managed completely differently.

Overt hypothyroidism

Here TSH is clearly raised and free T4 has fallen below the reference interval, or TSH is above 10 mIU/L regardless of the free T4 result. The RACGP puts the prevalence at 0.3 to 0.5 per cent. This is the clearer of the two pictures, and treatment is standard.

Subclinical hypothyroidism

Here TSH is raised while free T4 remains inside the reference interval. The RACGP describes this as a mildly elevated TSH in the range of 4 to 10 mIU/L with a normal free T4, and estimates the prevalence at 2 to 2.5 per cent.

What happens next is genuinely contested, and it is worth saying so rather than implying otherwise. The RACGP describes the clinical significance, and therefore the benefit of treating subclinical hypothyroidism, as remaining controversial, and its own guidance is to avoid routine treatment where TSH sits between 4 and 10 mIU/L and free T4 is normal. Treatment may still be considered in particular circumstances, including where thyroid peroxidase antibodies are positive or where cardiovascular risk is elevated. Instead of treating by default, the recommended approach is to re-test in two to three months to establish whether the finding is persistent, and to test for TPO antibodies at the same time. Where antibodies are positive, the suggested pattern of monitoring is TSH and free T4 at three months, six months and annually after that.

Why one result is not the whole answer

A single blood test is the start of the process rather than the end of it. The RACGP is specific: where TSH is elevated, it should be repeated within two to eight weeks along with a free T4 measurement to confirm the picture. Confirming an underactive thyroid is a clinical decision that a doctor makes across more than one result, not a threshold a single number crosses.

There are sound reasons for the repeat. TSH varies within an individual over time, and a single reading can be pushed outside its usual range by factors unconnected to how the thyroid is performing. Australian Prescriber notes that starvation or severe illness can be associated with dysregulation of TSH secretion and reduced conversion of T4 to T3, a pattern described as sick euthyroid syndrome, where the numbers reflect the illness rather than a thyroid disorder. A repeat test taken at a different time tells you whether the first result was a persistent finding or a transient one.

This is also why testing the same way each time is useful. Using the same laboratory network where you can keeps the reference intervals and the assay consistent, which makes two results genuinely comparable rather than approximately so.

What can affect a thyroid blood test

A few practical points affect how a thyroid result should be read.

• Fasting is not required. You can eat and drink normally before a thyroid function test.
• Medicines can interfere. Australian Prescriber names heparin, frusemide and phenytoin as interfering with free hormone assays, and notes that amiodarone commonly causes abnormal results. Lithium and amiodarone can also affect thyroid function itself.
• Severe illness shifts the numbers. The sick euthyroid pattern above means a test taken during a significant acute illness may not represent your usual thyroid function.
• Supplements are worth mentioning to your doctor. Some supplements can affect thyroid results or thyroid function, so list what you are taking when you discuss a report.
• Consistency helps. Where you are tracking results over time, testing under similar conditions and through the same laboratory network makes the comparison more reliable.

If you already take thyroid hormone replacement, the timing of your dose relative to the blood draw matters to how the result reads, so follow the instructions your treating doctor has given you.

Checking your thyroid with MediTests

MediTests is a self-request service so you can order your own blood test without booking a GP appointment, waiting for a referral, or explaining why you want it. The process is the same for every thyroid test in the range.

1. Choose your testPick the thyroid test that covers the markers you want measured, then choose, pay and go.
2. Get your pathology requestYour request form is generated and emailed to you straight after checkout, with no referral needed.
3. Walk into any of our collection centresOver 3,300 collection centres across Australia through our pathology network. No appointment needed.
4. Get your resultsResults in 24 to 48 hours, delivered securely to you. All pathology results are reviewed by AHPRA-registered medical practitioners.

Which thyroid test covers what

The range is built in tiers, with each one adding markers to the one before it. These are the four core tests.

The Thyroid Function Test covers the two markers Australian guidance leads with, TSH and free T4, and includes free T3 as well. Where an autoimmune cause is the question, the Full Thyroid Check adds the two antibody markers most associated with Hashimoto's, which is the step that moves a report from what thyroid function looks like to what may be driving it.

More targeted tests sit alongside the tiers for people who need one specific part of the picture, including the Thyroid Antibodies Test, the Thyroid Autoimmune Test, the Thyroid Conversion Test and the Thyroid Nutrient Status Test. Iodine can be measured through the Iodine Status Test as an at-home collection kit. You can see the full range on the thyroid testing page, compare it against the biomarker guide, or select individual markers yourself with the Create Your Own Blood Test builder.

If a result sits outside the reference interval

Take the report to a doctor. A thyroid result is interpreted against your history, your medicines and usually a second test taken later, and that is a clinical judgement rather than a reading exercise. All MediTests pathology results are reviewed by AHPRA-registered medical practitioners, and anything outside the reference interval is flagged on your report.

Where hypothyroidism is confirmed, it is generally a long-term condition managed by a doctor rather than one that resolves on its own. The standard approach in Australia is thyroid hormone replacement with levothyroxine, which the RACGP describes at a replacement dose calculated on lean body weight, taken daily and monitored with follow-up blood tests. Hormones Australia notes that treatment is usually lifelong and that levothyroxine is taken on an empty stomach before breakfast. The right dose is not fixed for life either, and it can shift with weight change, pregnancy and age, which is what the ongoing monitoring is for.

The RACGP also notes that a meta-analysis of 11 randomised studies covering more than 1,000 patients showed no obvious benefit from combining levothyroxine with T3 therapy, and that desiccated thyroid products are not recommended because they lack quality control. If you are pregnant or planning a pregnancy, thyroid testing and any treatment decisions should be discussed with your doctor, as the considerations are different.

You can read more about thyroid blood testing, or see how the wider range works across our 500+ available blood tests.