Hashimoto’s Thyroid Antibody (TPO & TgAb) Interpreter

What your antibody results actually mean

If you have just typed in your numbers and you are looking at the result, here is the plain-English version. These two blood tests check for antibodies — proteins made by your immune system — that are aimed at your own thyroid gland. The TPO antibody test is the one doctors rely on most. Thyroid peroxidase is an enzyme your thyroid needs in order to make its hormones. When antibodies against it turn up in your blood, that is a strong sign your immune system is involved in your thyroid problem — most often Hashimoto’s thyroiditis, the most common cause of an underactive thyroid.

The second test, TgAb, looks for antibodies against thyroglobulin, a protein your thyroid uses as a building block for its hormones. TgAb is also positive in many people with Hashimoto’s, but it is a little less specific and can appear in other conditions too. That is why doctors read the two results together, alongside your TSH and free T4, instead of treating either number as a diagnosis on its own.

A result below your lab’s cut-off point — often around 35 IU/mL for TPO antibodies, although every lab sets its own range — is generally read as negative. A result just over the line still counts as positive: it tells your doctor the autoimmune process is present, even if your hormone levels are still normal for now. Very high numbers, in the hundreds or even thousands, are common in Hashimoto’s and do not automatically mean your case is more severe. Antibody levels and symptoms do not track each other closely, and the size of the number does not reliably predict how quickly your thyroid will slow down. What a positive result does change is follow-up: if your TPO antibodies are positive but your hormone levels are normal, your doctor will usually want to recheck your TSH more often, because the chance of developing hypothyroidism over the coming years is higher than average.

When to see a doctor

If this is the first time you have seen a positive antibody result, make an appointment with your doctor or an endocrinologist and take your TSH and free T4 results with you. Do the same, sooner rather than later, if you are pregnant or planning a pregnancy — thyroid needs change during pregnancy, and doctors generally monitor women with positive TPO antibodies more closely — or if you have ongoing symptoms such as persistent tiredness, unexplained weight change, feeling cold all the time, thinning hair, or a feeling of fullness in the front of your neck. And if your result was negative but your symptoms are not going away, it is still worth following up: a small number of people with Hashimoto’s test negative for both antibodies, so a negative result does not rule thyroid problems out.

This tool is here to help you understand your lab report — it is not a diagnosis. Bring your results to your appointment. Walking in with the actual numbers in hand makes for a much more useful conversation.