Subclinical hypothyroidism is when a person’s thyroid tests show a raised level of TSH but a normal level of free T4. This pattern of thyroid tests can suggest that a person is more likely to develop an underactive thyroid. People with subclinical hypothyroidism and raised TPO antibodies are more likely to develop an underactive thyroid than those with normal levels of TPO antibodies. The term subclinical hypothyroidism only applies to people who have not been diagnosed with a thyroid condition. Subclinical hypothyroidism is also sometimes called mild hypothyroidism.

An autoimmune disease is when a person’s own immune system attacks part of their body. The most common cause of an underactive thyroid is the immune system attacking the thyroid gland. This autoimmune disease is called Hashimoto’s thyroiditis or autoimmune hypothyroidism.

TPO is a protein inside the thyroid gland. TPO antibodies are antibodies made by our immune system that bind to TPO. The majority of people have some TPO antibodies in their blood. High levels of TPO antibodies can indicate that someone may have an autoimmune disease affecting the thyroid.

TPO antibodies of 35 or above:

You have raised levels of TPO antibodies. If your thyroid tests show a subclinical hypothyroid pattern, this suggests that you may have a higher chance of developing an underactive thyroid in the future. If you have already been diagnosed with an underactive thyroid, this suggests that this condition is caused by an autoimmune disease affecting the thyroid.

TPO antibodies of 34 or below:

You have normal levels of TPO antibodies. If your thyroid tests show a subclinical hypothyroid pattern, this suggests that you have a lower chance of developing an underactive thyroid in the future.

If you have already been diagnosed with an underactive thyroid, this result does not indicate whether or not this condition is caused by an autoimmune disease affecting the thyroid. Up to 1 in 10 people with an autoimmune underactive thyroid condition have TPO antibodies within the normal range (i.e. 34 or below).

It is not clear what a high level of TPO antibodies means for someone with normal thyroid tests. There is some evidence to indicate that high levels of TPO antibodies could suggest a higher chance of developing an underactive thyroid in the future. However, many people with high levels of TPO antibodies will not go on to develop an underactive thyroid.

We do not report a numeric value below the cut-off of 35 imU/mL. The reason for this is that a large number of healthy individuals have detectable TPO in this range and it is just background ‘noise’. These low levels of TPO are not associated with thyroid disorders, can lead to confusion and in some cases the assumption that values in this range may be significant.

For all positive results we do report a numeric value.

TPO antibodies are present in the majority of people with hyperthyroidism but this is not helpful for determining the cause of hyperthyroidism. If your test results indicate that you have hyperthyroidism then we recommend discussing the result with your doctor. 

There is no clear evidence to suggest that changes in TPO antibody levels are linked to changes in disease or treatment. For this reason, we do not feel it be good value for our customer’s money to repeat this test.

Thyroglobulin antibodies also known as anti-thyroglobulin antibodies are another test related to the thyroid. Studies have demonstrated that TPO antibodies are linked to an underactive thyroid. The same studies found that thyroglobulin antibodies were only associated with an underactive thyroid in people who also had positive TPO antibodies. When thyroglobulin antibodies were found on their own they were not associated with an underactive thyroid. For this reason, we do not feel that measuring thyroglobulin antibodies adds any information above measuring TPO antibodies alone and so is not of benefit to our customers.

Thyroid peroxidase antibodies

TPO Antibodies

Anti-thyroid peroxidase antibodies

Anti-TPO antibodies