Thyroid Antibodies – what you need to know
There are three major types of antibody that are relevant in thyroid disease:
- Anti-thyroid peroxidase (TPOAb),
- Anti-thyroglobulin antibody (TgAb)
- TSH receptor antibodies (TrAb)
The presence of antibodies implies autoimmune thyroid disease – a problem in which the body’s immune system incorrectly produces antibodies to its own, healthy tissue within the thyroid gland.
These antibodies then attack the thyroid tissue preventing it from functioning correctly.
Anti-thyroid peroxidase antibodies (TPOAb)
This is a type of autoimmune disease that results from the abnormal production of TPO antibodies by the immune system.
The presence of TPO antibodies is associated with Hashimoto’s disease. GPs will usually test for this if your TSH and free T4 levels are found to be abnormal.
TPO antibodies attack the healthy tissue of the thyroid gland and prevent it from producing the thyroid hormones, rendering the person ‘hypothyroid’.
Up to 95% of people with Hashimoto’s will have TPO antibodies in their blood and these are therefore the best marker for the disease.
Anti-thyroglobulin antibodies (TgAb)
Thyroglobulin antibodies (TgAb) are another type of antibody associated with thyroid disease.
TPO antibodies are positive in nearly 95% of Hashimoto’s patients whereas anti-thyroglobulin antibodies (TgAb) are found in 60-70% of people with Hashimoto’s disease.
Most people with an autoimmune thyroid disease have TPO antibodies, so it would be very rare for a patient to be TgAb positive and TPOAb negative. For this reason, the presence of TPOAb in the blood is more widely used to diagnose the disease.
Thyroid receptor antibodies (TRAb)
Graves’ disease is caused by the abnormal presence of antibodies to the TSH receptor found on the surface of the pituitary.
The Thyroid Stimulating Hormone acts by binding onto TSH receptors found on the surface of the pituitary gland. In some people, antibodies can form against these TSH receptors, and are found in 90% of patients with Graves’ disease. TSH receptor antibodies are used to help diagnose Graves’ disease, and monitor remission of the condition.
Note that 80% of Graves’ disease patients will also have TPO antibodies, though their role in the disease process is unclear.
Why are antibodies important?
There is evidence to suggest that the presence of antibodies can precede the change in hormone levels and that having antibodies increases your risk of developing thyroid disease later down the line.
If a patient has confirmed ‘subclinical’ hypothyroidism, the presence of antibodies increases the likelihood of progression to clinical hypothyroidism. The risk is 4% per year in this scenario.
The evidence for the risk of progression to overt hypothyroid disease is stronger for TPOAb than it is for TgAb.
Lastly, autoimmune conditions are associated with one another. If you have a diagnosis of Hashimoto’s you are also at risk of developing diseases including type 1 diabetes, Addison’s, coeliac disease and pernicious anaemia.
The presence of antibodies to the thyroid may prompt your clinician to screen for these other diseases depending on your symptoms.
A note on antibodies in pregnancy
Thyroid disease is associated with poor outcomes in pregnancy, the risk of which is increased in the presence of antibodies. If you are pregnant or planning a pregnancy and are concerned about thyroid disease it is important to discuss this with your GP.
Note that the blood test ranges used in pregnancy are different and are trimester-specific.