TPO Antibodies: Normal Range, High or Low

What is TPO antibodies?

Your thyroid makes hormone by building iodine into a protein, and the enzyme that does that is called thyroid peroxidase, TPO for short. In autoimmune thyroid disease your immune system mistakes that enzyme for an intruder and makes antibodies against it. Those antibodies are not the attack itself; they are the trace of an immune system that has entered the thyroid. The attack itself, by immune cells that slowly break down the thyroid tissue, is what makes the thyroid underactive over years. That is Hashimoto's disease, the most common reason for an underactive thyroid. That is why TPO antibodies say something different from TSH and free T4. Those two show how your thyroid is doing right now; the antibodies show whether an autoimmune process is under way. With a normal thyroid function, antibodies mean a raised chance that the thyroid becomes underactive in the years after, roughly a few percent a year, and with a slightly raised TSH they make it more likely that it progresses. With a clearly underactive thyroid they confirm the cause, but the treatment is the same, with or without antibodies. Your report shows TPO antibodies in kU/L, and the limit differs per lab because the tests are not on one scale: from 20 kU/L at the strictest lab to 35 kU/L at the widest. Below the limit on your own report no antibodies were detectable, and that is the healthy outcome.

Why is TPO antibodies relevant?

TPO antibodies answer the question why, and that is a question that changes something less often than you would think. With an underactive thyroid with a high TSH and a low free T4 the treatment is thyroid hormone, whether the antibodies are detectable or not; that is the reason the test is usually skipped in general practice. Where they do add something is with doubt: a slightly raised TSH with a normal free T4 corrects by itself in half of people, and antibodies make the chance that it progresses a good deal larger. That difference decides whether you wait or treat earlier. There is a second situation in which the antibodies count: a wish for a child or a pregnancy. Women with TPO antibodies and a normal thyroid function have miscarriages somewhat more often and more often a thyroid inflammation in the year after the birth, and their thyroid has less reserve in pregnancy. That is a reason to follow TSH in pregnancy, not to treat the antibodies. And there is a pitfall. One in ten healthy adults, especially women and older people, has detectable TPO antibodies without ever getting thyroid symptoms. Antibodies are a risk, not a diagnosis, and a raised value with a normal TSH is a reason to have TSH measured once a year, not to feel ill.

TPO antibodies reference ranges

What counts beside the number:

TSH and free T4 from the same draw, in that order: first how the thyroid is doing, then why
The limit and the unit of your own lab, because the tests are not on one scale and the numbers cannot be compared
Whether you are pregnant, wish for a child or gave birth in the past year
Whether you take iodine supplements or kelp, because an excess of iodine drives an autoimmune thyroid
Whether thyroid disease runs in your family, and whether you have other autoimmune diseases
With a normal TSH: a yearly TSH measurement, not a repeat of the antibodies

TPO antibodies reference ranges
Group	Reference range
AdultsThe lowest of the four published limits. The labs do not report on one scale, so this is not a conversion but the strictest limit; above this figure the verdict depends on which lab processed your test	0–20 kU/L

The limit on your own report counts more heavily here than with almost any other value, because the labs do not use the same limit and do not report in the same unit; above the limit the presence counts, not the height. In general practice the test is usually skipped, because the outcome rarely changes what happens: an underactive thyroid is treated with or without antibodies. In a pregnancy with a known thyroid condition they are not measured.

What Dutch labs actually use

With TPO antibodies there is no range, only a limit, and the first question is whether the four numbers are on the same scale at all. They are not: the labs use different tests with their own units, which cannot be traced to one international standard. Within that limitation Certe draws its line at 20, Unilabs and Clinical Diagnostics at 25, and Star-shl at 35. The same sample can fall just inside one lab's limit and just outside another's, without either being wrong. Certe publishes no reference value below the age of 18.

Adults · every lab calls this normal

0 – 20kU/L

Between 20 and 35 it depends on the lab, because each sets its limits on its own method and population. Above 35 every lab calls it abnormal.

Unilabs(Saltro, Medlon, SHO, Atalmedial)

0–25 kU/L

Star-shl

< 35 kU/L

Certe

< 20 kU/L

Clinical Diagnostics

< 25 kU/L

Every age, per lab

What Dutch labs actually use
Laboratory	Group	Reference range
Unilabs(Saltro, Medlon, SHO, Atalmedial)2026	All ages	0–25 kU/L
Star-shl2026	All ages	< 35 kU/L
Certe2026	From 18 yrs	< 20 kU/L
Clinical Diagnostics2026	All ages	< 25 kU/L

Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.

TPO antibodies high or low: what it means

A low value does not exist here as a finding. Below the limit on your own report no antibodies were detectable, and that is the healthy outcome. A negative result does not completely rule out Hashimoto's, by the way, because a small share of people with an underactive thyroid from autoimmune disease has no detectable TPO antibodies; then TSH and free T4 say enough. A raised value means your immune system is directed at the thyroid, and TSH and free T4 say what that means right now. With a high TSH and a low free T4 it is Hashimoto's, and the treatment is thyroid hormone. With a slightly raised TSH and a normal free T4 it is a thyroid that still keeps up but will probably become underactive, and then you follow TSH every six months to a year. With a normal TSH it is a raised chance of later, and then you measure TSH once a year. In the year after a birth, antibodies fit a thyroid inflammation that runs first overactive and then underactive and usually recovers by itself. How high the value is says little: above the limit the presence counts, not the number, and the numbers of different labs cannot be compared. What you do about it: nothing to the antibodies themselves, because there is no treatment that removes them and that is not needed either. What you do do is give the thyroid its raw materials and keep the axis calm: enough iodine from bread with iodised salt, dairy, fish and eggs, but no iodine supplements or kelp, because an excess of iodine drives an autoimmune thyroid; selenium from nuts, fish and eggs, which dampens the inflammation in the thyroid a little; not smoking; and a healthy weight. With a wish for a child you have TSH followed before and during the pregnancy. Set TPO antibodies beside TSH and free T4, in that order: first how the thyroid is doing, then why. Tg antibodies are the second thyroid antibody, which arises in the same process and rarely adds anything.

What raises it

Your immune system is directed at your thyroid, and TSH and free T4 say what that means right now.

What raises it: TPO antibodies
Cause	How often
Autoimmune thyroid diseaseHashimoto's, the most common reason for an underactive thyroid, in which the antibodies are detectable in most people; in Graves' disease too they are often present	Often
Thyroid inflammation after childbirthIn the year after a birth the thyroid becomes inflamed in one in twenty women, first overactive and then underactive; with antibodies the chance is larger and recovery more often incomplete	Sometimes
Antibodies without thyroid diseaseOne in ten healthy adults, especially women and older people, has detectable antibodies with a normal TSH; that is a raised chance of later and not a diagnosis	Sometimes

How does a TPO antibodies blood test work?

Referral: Not needed. You can have TPO antibodies measured yourself, though they rarely change anything about the treatment; they are most useful with a slightly raised TSH and with a wish for a child.
Fasting: Not needed; antibodies do not move with food, the time or the day.
When: At any moment, because the value is stable over weeks to months. Stop biotin three days beforehand for the TSH and free T4 you have measured alongside.
The draw: One tube of blood from a vein in your arm. TSH and free T4 come from the same draw, and without those two an antibody value cannot be placed.
Repeating: Usually not needed: a positive result stays positive, and a repeat changes nothing about the plan. What you do repeat is TSH, once a year with antibodies and a normal TSH.

Where to draw: You can draw at 250+ locations near you, no referral needed. See the locations

Frequently asked questions

What are TPO antibodies?

Antibodies your immune system makes against thyroid peroxidase, the enzyme your thyroid uses to build iodine into hormone. They are the trace of an immune system that has entered the thyroid, as in Hashimoto's disease, in which immune cells slowly break down the thyroid tissue over years. The antibodies themselves do not do the damage; they show that the process is under way.

What is a normal value for TPO antibodies?

Below your own lab's limit, between 20 and 35 kU/L depending on the lab, because the tests are not on one scale. Below that limit no antibodies were detectable, and that is the healthy outcome. Above the limit the presence counts and not the height, and a number from one lab cannot be compared with another lab's limit.

Do TPO antibodies mean I have Hashimoto's disease?

Not on their own. In most people with Hashimoto's they are detectable, but one in ten healthy adults has them too without ever getting thyroid symptoms. Antibodies are a risk, not a diagnosis. It is Hashimoto's if your TSH is high and your free T4 low, with or without antibodies; with a normal TSH antibodies mean a raised chance that your thyroid becomes underactive later, roughly a few percent a year.

What does a low value mean?

Nothing, and that is good news: below the limit no antibodies were detectable. A negative result does not completely rule out Hashimoto's, because a small share of people with an underactive thyroid from autoimmune disease has no detectable TPO antibodies. Then TSH and free T4 say enough, and the result changes nothing about the treatment.

Why is the test usually skipped in general practice?

Because the outcome rarely changes what happens. An underactive thyroid with a high TSH and a low free T4 is treated with thyroid hormone, whether the antibodies are detectable or not, and a normal thyroid function is not treated, not even with antibodies. Where they do add something is with a slightly raised TSH, because antibodies make the chance of progression larger, and with a wish for a child.

My TPO antibodies are raised. What now?

Look first at your TSH and your free T4. If those are normal, nothing is wrong now and you have TSH measured once a year. If TSH is slightly raised, you follow it every six months to a year, because the antibodies increase the chance of an underactive thyroid. If TSH is high and free T4 low, it is Hashimoto's and thyroid hormone is the treatment. The antibodies themselves you need not measure again.

Can I do something about TPO antibodies myself?

Not about the antibodies, but about the thyroid. Enough iodine from bread with iodised salt, dairy, fish and eggs, but no iodine supplements or kelp, because an excess of iodine drives an autoimmune thyroid. Selenium from nuts, fish and eggs dampens the inflammation in the thyroid a little. Not smoking and a healthy weight keep the axis between brain and thyroid calm. And with a wish for a child you have TSH followed before and during the pregnancy.

What do TPO antibodies mean in a pregnancy?

That your thyroid has less reserve in pregnancy and that your TSH should be followed, because the thyroid has to work harder in pregnancy. Women with antibodies have miscarriages somewhat more often and more often a thyroid inflammation in the year after the birth, which runs first overactive and then underactive and usually recovers. The antibodies themselves are not treated; TSH is kept up in pregnancy.

Which tests include TPO antibodies?

TPO antibodies is part of Advanced, the broader panel. It is not orderable yet.

Related biomarkers

TSHThe first value to read: with a normal TSH antibodies say something about later, with a high TSH they confirm the cause.Free T4Says whether an underactive thyroid is still keeping up; with a low free T4 and antibodies it is Hashimoto's.Tg antibodiesThe second thyroid antibody, which arises in the same autoimmune process and rarely adds anything.SeleniumSelenium is a trace element that sits in the enzymes that neutralise harmful by-products of your metabolism and that convert thyroid hormone into its active form. How much you take in depends on the soil your food grew on, and a few Brazil nuts a week is enough.

Read on

Guide: HormonesHormones are your body's chemical conductors — tiny signalling molecules that regulate metabolism, energy, mood, libido, fertility and recovery.

Sources

1.Unilabs, test catalogue (Dutch): TPO antibodies. 2026. bepalingenklapper.nl
2.Star-shl, test catalogue (Dutch): TPO antibodies. 2026. star-shl.nl
3.Certe, test catalogue (Dutch): anti-TPO. 2026. bepalingenwijzer.certe.nl
4.Clinical Diagnostics, lab guide (Dutch): anti-TPO. 2026. clinicaldiagnostics.nl
5.NHG guideline on thyroid disorders (M31), Dutch College of General Practitioners. 2025. richtlijnen.nhg.org

Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.

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