Thyroid Antibodies Explained (TPO, TG, TRAb)
What TPO, TG and TRAb antibody results mean, how they point to Hashimoto’s or Graves disease, and what your GP does next.
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· · Reviewed against the sources listed at the end of this page.
The Quick Answer
There are three thyroid antibody tests, and they answer different questions. TPO antibodies (thyroid peroxidase antibodies) are the marker of Hashimoto’s thyroiditis, the autoimmune cause of an underactive thyroid. TG antibodies (thyroglobulin antibodies) are less specific on their own, but become critical after thyroid cancer treatment because they interfere with the thyroglobulin tumour marker used to watch for recurrence. TRAb (TSH receptor antibodies, including the stimulating subtype TSI) is the marker of Graves disease, the autoimmune cause of an overactive thyroid, and the test that separates Graves disease from a toxic nodule or a passing thyroiditis.
What a positive result actually establishes: increased risk, rather than a diagnosis on the day of the test. A positive TPO antibody with a normal TSH means an increased chance of developing an underactive thyroid over the following years, while your thyroid function today is still normal. Around one in ten healthy Australians carries a low-titre positive TPO antibody result with normal thyroid function for life.
The Three Thyroid Antibody Tests at a Glance
Each test targets a different part of the thyroid and answers a different clinical question. Your GP chooses which one (or which combination) to order based on whether TSH is high, low, or normal, and on the reason for testing. Reference ranges below are typical of Australian labs, but thyroid antibody assays vary between platforms, so the range printed on your own report always takes precedence.
Anti-TPO Antibodies
Targets: Hashimoto’s thyroiditis (underactive thyroid)
If positive: The main diagnostic marker for autoimmune underactive thyroid disease. Positive in roughly 90 to 95 percent of people with Hashimoto’s. A positive result with a normal TSH means increased future risk, not a current diagnosis.
Repeat testing: Not needed once positive. Titre does not track disease activity. Monitor TSH instead.
Anti-TG Antibodies
Targets: Less specific autoimmune marker; critical in thyroid cancer follow-up
If positive: Positive in Hashimoto’s, Graves disease, and around five to ten percent of people with no thyroid disease. Rarely used alone for diagnosis. After thyroid cancer treatment, a positive TG antibody makes the thyroglobulin tumour marker unreliable, so both are measured together.
Repeat testing: In cancer follow-up, retested at each surveillance visit alongside thyroglobulin. Otherwise not routinely repeated.
TRAb (including TSI)
Targets: Graves disease (overactive thyroid)
If positive: The test that distinguishes Graves disease from a toxic nodule or a transient thyroiditis as the cause of an overactive thyroid. Positive in over 95 percent of Graves disease. TSI specifically measures the stimulating subtype that drives hormone overproduction.
Repeat testing: Retested to help predict remission after antithyroid drug treatment, and specifically retested in pregnancy because it crosses the placenta.
Check Which Thyroid Antibody Was Tested and What Your Number Means
Upload your pathology PDF and SmarterBlood will identify every thyroid marker on the report, including TPO, TG and TRAb, and explain your specific result in plain English against the range your lab actually used.
Upload Your Thyroid ResultsAnti-TPO Antibodies: The Hashimoto’s Marker
Thyroid peroxidase is an enzyme inside thyroid cells that is essential for making thyroid hormone. In Hashimoto’s thyroiditis, the immune system mistakenly produces antibodies against this enzyme, gradually damaging the gland’s ability to produce hormone. Anti-TPO antibodies are positive in roughly 90 to 95 percent of people with confirmed Hashimoto’s, which is why it is the first-line blood test for identifying the autoimmune cause of an underactive thyroid.
A positive TPO antibody with a normal TSH is not a diagnosis of Hashimoto’s disease, and it is not a reason to start treatment. It means the autoimmune process is present and the thyroid is under attack, so the future risk of the gland eventually failing and TSH rising is higher than in someone with negative antibodies. Approximately ten percent of healthy adults, more often women and more common with increasing age, have a mildly positive TPO antibody result with normal thyroid function that never progresses to thyroid disease.
When TSH does become abnormal in someone with positive TPO antibodies, that combination confirms Hashimoto’s as the cause, and thyroid hormone replacement is guided by the TSH result rather than by how high the antibody titre is. A very high TPO antibody titre does not mean faster progression or worse disease than a mildly positive result.
Anti-Thyroglobulin (TG) Antibodies and Thyroid Cancer Follow-Up
Thyroglobulin is the protein the thyroid uses to store thyroid hormone before release. Antibodies against it are less useful as a standalone diagnostic test than TPO antibodies, because they appear in Hashimoto’s thyroiditis, in Graves disease, and in around five to ten percent of people with no thyroid disease at all. For this reason, TG antibodies are rarely ordered on their own to diagnose autoimmune thyroid disease, and TPO antibodies are usually the preferred first-line test.
TG antibodies matter most in a different setting: monitoring after treatment for thyroid cancer. Once the thyroid has been surgically removed for cancer, thyroglobulin itself becomes a tumour marker, a rising level can signal cancer recurrence. The problem is that TG antibodies interfere with the thyroglobulin assay, causing it to read falsely low and mask a recurrence, or occasionally falsely high. Because of this interference, every thyroglobulin test in cancer follow-up is reported alongside a TG antibody result, and a positive antibody means the thyroglobulin number cannot be trusted at face value.
TSH Receptor Antibodies (TRAb) and TSI: The Graves Disease Marker
TRAb targets the receptor on thyroid cells that normally responds to thyroid stimulating hormone (TSH) from the pituitary gland. A specific subtype of TRAb, called TSI (thyroid stimulating immunoglobulin), actually switches this receptor on, driving the thyroid to produce hormone independently of normal pituitary control. This is the mechanism behind Graves disease, the most common cause of an overactive thyroid in Australia. TRAb is positive in over 95 percent of people with Graves disease.
TRAb answers one specific question. When someone presents with a low TSH and an overactive thyroid, there are several possible causes, Graves disease, a toxic nodule (an overactive lump within the gland), or a transient thyroiditis (a temporary inflammation that leaks stored hormone). These conditions can look identical on symptoms alone, but they are managed very differently. TRAb is the test that tells them apart, because only Graves disease produces these receptor-stimulating antibodies. A positive TRAb points firmly to Graves disease and away from a nodule or thyroiditis, and often avoids the need for a nuclear medicine thyroid uptake scan.
TRAb titre is also used practically during treatment: after roughly twelve to eighteen months of antithyroid medication for Graves disease, a falling or negative TRAb helps predict a higher chance of remaining in remission once medication is stopped, while a persistently high titre suggests a higher chance of relapse and may prompt a discussion about radioactive iodine or surgery instead of a further medication trial.
See the full thyroid panel alongside your antibody result: TSH, free T4 and free T3.
See the Thyroid Markers Hub (TSH, Free T4, Free T3 and more)How to Interpret a Positive Result: The Essential Points
Positive TPO antibodies with a normal TSH is a risk marker, not a diagnosis
It means the thyroid is under autoimmune attack and the chance of developing an underactive thyroid over the following years is higher than average. It is not, by itself, a reason to start thyroid hormone treatment. Treatment decisions are driven by TSH, symptoms, and (in pregnancy) specific guidelines, not by the antibody result alone.
Around one in ten healthy Australians has a low-titre positive TPO antibody result
Population studies consistently find TPO antibodies in about ten percent of people with entirely normal thyroid function, more often in women and with increasing age. A mildly positive result in someone who feels well and has a normal TSH is a common, largely benign finding that still deserves periodic TSH monitoring rather than alarm.
Antibody titres generally do not need to be repeated once confirmed positive
Unlike TSH, which is checked repeatedly to track thyroid function over time, TPO and TG antibody titres do not reliably rise and fall with disease activity or treatment response. Once positive, they usually stay positive for life. Ongoing monitoring is done with TSH and free T4, not by re-testing the antibody.
The exception is TRAb, which is repeated in two specific situations
TRAb titre is rechecked in Graves disease near the end of a course of antithyroid medication, because a falling titre helps predict whether the condition will go into remission when treatment is stopped. It is also specifically rechecked during pregnancy in anyone with current or past Graves disease, because the titre level affects the risk to the baby.
Titre level does not reliably predict how severe the disease will become
A very high TPO antibody titre does not necessarily mean a faster or more severe course of Hashimoto’s than a mildly positive result. The titre confirms the autoimmune process is present; it is TSH and free T4 that determine severity and guide treatment, not how high the antibody number is.
A negative antibody result does not completely exclude autoimmune thyroid disease
A small proportion of people with Hashimoto’s thyroiditis, and an even smaller proportion with Graves disease, test negative for the relevant antibody despite genuine autoimmune disease being present on biopsy or ultrasound. If the clinical picture strongly suggests autoimmune thyroid disease despite a negative antibody test, your GP may still pursue further assessment.
Thyroid Antibodies in Pregnancy and Pre-Conception
Thyroid antibodies matter well beyond a single blood test result when pregnancy is being planned or is underway. Positive TPO antibodies, even with normal thyroid function, are associated with a higher risk of miscarriage and with postpartum thyroiditis, a temporary inflammation of the thyroid that can occur in the months after birth and cause a phase of overactivity followed by a phase of underactivity. Because the body's thyroid hormone requirement rises in pregnancy, someone with positive TPO antibodies and a borderline TSH before conception is more likely to need thyroid hormone support once pregnant, and closer TSH monitoring throughout the pregnancy is reasonable.
TRAb has a distinct and more direct risk in pregnancy: it is an IgG antibody, and IgG is actively carried across the placenta. In a woman with current or past Graves disease, whether or not her own thyroid function is currently normal, well controlled, or even surgically or radioactively treated, a significantly elevated TRAb can cross into the baby’s circulation and overstimulate the baby’s own thyroid, a condition called foetal or neonatal Graves disease. This is precisely why TRAb, unlike TPO and TG antibodies, is specifically retested during pregnancy in anyone with a Graves disease history, usually early on and again around 18 to 22 weeks if it is still positive, so that the baby can be monitored when the titre is high.
If you are planning a pregnancy and know you have positive thyroid antibodies of any kind, or a personal history of Hashimoto’s or Graves disease, raise it with your GP or obstetrician early, ideally before conception, rather than waiting for the first antenatal blood tests.
Medicare Rebates and When Testing Is Actually Indicated
Thyroid antibody testing attracts a Medicare rebate under the MBS pathology schedule when it is ordered for a clinically appropriate reason. It is not intended as a routine screening test added automatically to every blood test panel in someone with no symptoms, no goitre, and a normal TSH. Reasonable indications your GP will typically consider include:
MBS rules also limit how many autoantibody tests are rebated on a single request and how soon the same test can be repeated, which fits the point made earlier: once positive, the titre rarely needs re-checking. A repeat request that falls outside those rules becomes a private-pay item, and the out-of-pocket cost differs between pathology providers, so ask for a quote before the blood is taken.
What Your GP Will Do Next
Interpret the antibody alongside TSH and free T4, never in isolation
The antibody result on its own rarely changes management. Your GP will look at whether TSH is normal, low, or high, and whether you have symptoms, before deciding what the antibody result actually means for you.
Confirm which antibody was tested and why
TPO antibodies investigate a suspected underactive thyroid, TG antibodies are mainly used in thyroid cancer follow-up (or occasionally alongside TPO), and TRAb investigates a suspected overactive thyroid, particularly Graves disease.
If TSH is normal and antibodies are positive: reassure and monitor
No treatment is started. Your GP will typically recheck TSH every six to twelve months, or sooner if symptoms develop, because the risk of eventually developing an underactive thyroid is higher than in someone with negative antibodies.
If TSH is abnormal and antibodies are positive: confirm the autoimmune cause and treat the thyroid function
A high TSH with positive TPO antibodies confirms Hashimoto’s as the cause of an underactive thyroid, and thyroid hormone replacement is started based on the TSH result. A low TSH with positive TRAb confirms Graves disease, and antithyroid medication, radioactive iodine, or surgery are discussed depending on severity and preference.
Consider an ultrasound if there is a goitre or nodule
A thyroid ultrasound is added when the gland feels enlarged or irregular on examination, to look for nodules that need separate assessment, alongside the antibody and hormone results.
Refer to an endocrinologist for complex, pregnancy-related, or resistant cases
Pregnancy planning with positive TRAb, treatment-resistant Graves disease, unclear TRAb results, or unusual antibody patterns are typically referred to a specialist for ongoing management rather than managed solely in general practice.
When to Seek Urgent Care
Known Graves disease with fever, racing heart, agitation, or confusion
These can be signs of thyroid storm, a rare but life-threatening surge in thyroid hormone. This needs same-day emergency assessment, not a routine GP appointment.
Known underactive thyroid with severe cold intolerance, confusion, or collapse
Severe, longstanding untreated hypothyroidism can rarely progress to myxoedema coma, a medical emergency. Seek urgent care if these symptoms appear suddenly or worsen quickly.
Bulging or painful eyes with double vision, in someone with Graves disease
This can indicate Graves ophthalmopathy affecting the eye muscles and optic nerve. Prompt review by an eye specialist alongside your endocrinologist is warranted, particularly if vision is changing.
A rapidly enlarging or hard, fixed thyroid lump
This pattern is not typical of straightforward autoimmune thyroid disease and should be assessed promptly to exclude a thyroid malignancy, separate from the antibody result.
Pregnant with known Graves disease and a very high TRAb result
This combination carries a risk of the antibody affecting the baby’s thyroid, even when the mother’s own thyroid is well controlled. This needs timely specialist obstetric and endocrine review, not routine-interval follow-up.
Sources and reference ranges
Adult reference ranges on this page follow the AACB and RCPA harmonised reference intervals unless stated otherwise; the range printed on your own report always takes precedence because laboratories differ.
- healthdirect Australia: thyroid function tests
- Pathology Tests Explained (Australasian Association for Clinical Biochemistry and Laboratory Medicine)
- Australasian Association for Clinical Biochemistry and Laboratory Medicine, publisher of the harmonised reference intervals
- healthdirect Australia: full blood count
Thyroid Antibodies (TPO, TG, TRAb): Frequently Asked Questions
What does a positive TPO antibody test mean?
A positive anti-thyroid peroxidase (TPO) antibody result means your immune system is producing antibodies against an enzyme used to make thyroid hormone. It is the laboratory hallmark of Hashimoto’s thyroiditis, the most common cause of underactive thyroid in Australia. On its own, a positive TPO antibody with a normal TSH is not a diagnosis of thyroid disease. It means your future risk of developing an underactive thyroid is higher than average, and your GP may recheck your TSH periodically. Around one in ten healthy adults has a low-titre positive result and normal thyroid function that never goes on to become thyroid disease.
What is the difference between TPO antibodies and thyroglobulin antibodies?
TPO antibodies target thyroid peroxidase, the enzyme that makes thyroid hormone, and are the main marker used to diagnose Hashimoto’s thyroiditis. Thyroglobulin (TG) antibodies target thyroglobulin, the storage protein for thyroid hormone, and are less specific: they appear in Hashimoto’s, Graves disease, and in some people with no thyroid disease at all, so they are rarely used alone to diagnose autoimmune thyroid disease. TG antibodies matter most in a different setting: after thyroid cancer surgery, because their presence interferes with the thyroglobulin blood test used to monitor for cancer recurrence, so both are measured together in that follow-up program.
What is TRAb and how is it different from TPO and TG antibodies?
TRAb (TSH receptor antibodies) target the receptor for thyroid stimulating hormone on the thyroid gland itself, rather than an enzyme or storage protein inside the cell. A specific stimulating subtype, called TSI, switches the receptor on and drives the thyroid to overproduce hormone, which is what causes Graves disease. TRAb is the test that distinguishes Graves disease from other causes of an overactive thyroid, such as a toxic nodule or a transient thyroiditis, because only Graves disease produces these receptor-stimulating antibodies. TPO and TG antibodies are not useful for making this distinction.
Can you have a positive thyroid antibody test with normal thyroid function?
Yes, this is common. Roughly ten percent of healthy Australian adults, more often women, carry a low-titre positive TPO antibody result with entirely normal TSH and thyroid hormone levels, sometimes for their whole life. A positive antibody result identifies increased future risk, not current disease. Your GP will usually interpret it alongside your TSH: if TSH is normal, no treatment is needed, though periodic TSH monitoring (often yearly) is reasonable because the risk of developing an underactive thyroid over time is higher than in someone with negative antibodies.
Do thyroid antibody levels need to be retested regularly?
Generally no. Once TPO or TG antibodies are confirmed positive, the titre (the strength of the result) does not reliably track how active the disease is or how well it is controlled, so repeating the antibody test itself adds little useful information. The exception is TRAb in Graves disease, where the titre can help predict whether the condition will go into remission after a course of antithyroid medication, and where it is specifically retested during pregnancy because it crosses the placenta and can affect the baby’s thyroid function. Ongoing monitoring in most people with positive TPO or TG antibodies is done with TSH, not repeat antibody titres.
Why are thyroid antibodies important before and during pregnancy?
Positive TPO antibodies before conception are linked to a higher risk of miscarriage, and to postpartum thyroiditis, an inflammation of the thyroid that can occur in the months after birth and cause temporary overactivity followed by underactivity. Positive TRAb in a pregnant woman with current or past Graves disease is checked because the antibody crosses the placenta and can overstimulate the baby’s thyroid, even if the mother’s own thyroid function is currently well controlled or she has previously had her thyroid removed or treated with radioactive iodine. This is why TRAb is specifically retested during pregnancy when there is a history of Graves disease, unlike TPO and TG.
Is the thyroid antibody test covered by Medicare in Australia?
TPO antibody testing attracts a Medicare rebate under the MBS when ordered for a clinically appropriate reason, such as an abnormal TSH, a goitre, a personal or family history of autoimmune thyroid disease, or before conception or early in pregnancy in someone with risk factors. It is not intended as a routine screening test in someone with normal thyroid function and no risk factors, and Medicare rebate rules restrict repeat testing within short intervals. TRAb testing is rebated when Graves disease is suspected or being monitored. Ask your GP whether your specific situation meets the rebate criteria before requesting the test.
What will my GP do if my thyroid antibodies are positive?
Your GP will look at the antibody result together with your TSH and free T4, not in isolation. If TSH is normal, the usual plan is reassurance plus periodic TSH monitoring, often yearly, since a positive TPO antibody increases the future risk of an underactive thyroid. If TSH is already abnormal, the antibody result confirms an autoimmune cause and guides treatment, most commonly thyroid hormone replacement for Hashimoto’s or antithyroid medication for Graves disease. A GP will not usually repeat the antibody test itself at every review, and will refer to an endocrinologist for complex, pregnancy-related, or treatment-resistant cases.
Related Reading
Understand Your Thyroid Antibody Result in Plain English
Upload your pathology PDF and SmarterBlood’s AI will explain TPO, TG and TRAb alongside TSH, free T4 and free T3, flag whether your antibody result fits a Hashimoto’s or Graves pattern, and track every thyroid marker over time.
This page provides general educational information about thyroid antibody testing, including TPO, TG and TRAb antibodies. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss your own thyroid antibody result, TSH, and symptoms with your GP or endocrinologist, who can interpret them in the context of your full medical history. SmarterBlood does not provide medical care.