Thyroid Antibodies Explained: TPO, Thyroglobulin and TRAb

Why this matters

The thyroid is a small gland at the front of the neck. It makes hormones that control how the body uses energy. It can become underactive, making too little hormone, or overactive, making too much. Autoimmune thyroid disease, where the immune system attacks the thyroid, is the most common cause of both in the UK. Tiredness, weight change, feeling too hot or too cold, and low mood can all be linked to the thyroid, but they have many other causes too. Antibody tests help show whether the immune system may be involved. Antibodies are proteins the immune system makes to fight germs; in autoimmune thyroid disease, some are directed at the thyroid instead.

Function tests and antibody tests answer different questions

TSH (thyroid stimulating hormone, the signal from the brain that tells the thyroid how hard to work), free T4 and sometimes free T3 (the main hormones the thyroid makes) measure how well the thyroid is working right now. Antibody tests look for a possible cause. Someone can have positive antibodies with normal thyroid function, and someone can have an underactive thyroid without raised antibodies. Reading both together gives a clearer picture.

Thyroid peroxidase antibodies (TPO)

Thyroid peroxidase is a protein the thyroid needs to make its hormones. TPO antibodies are found in most people with Hashimoto's thyroiditis, a condition where the immune system gradually damages the thyroid and the most common cause of an underactive thyroid. They are also found in many people with Graves' disease, an autoimmune cause of an overactive thyroid.

They are also found in some people whose thyroid works normally. In that situation, positive TPO antibodies suggest a higher chance of developing an underactive thyroid in future, so your clinician may suggest checking thyroid function from time to time.

Thyroglobulin antibodies (TgAb)

Thyroglobulin is a protein the thyroid uses to store and make hormones. Antibodies against it are often found alongside TPO antibodies in autoimmune thyroid disease, but on their own they are less clearly linked to thyroid disease. They are most useful as part of a wider group of tests rather than as a single result.

TSH-receptor antibodies (TRAb)

TSH-receptor antibodies attach to the receptor on thyroid cells, the part that normally receives the brain's signal (TSH) to make hormones. In Graves' disease they switch that signal on, so the thyroid becomes overactive. A positive TRAb result can help distinguish Graves' disease from other causes of an overactive thyroid, which can look very similar on symptoms alone.

What you could do

Thyroid results make most sense when they are read together and over time.

• Ask for your TSH and free T4 results alongside any antibody results
• Tell your clinician if you take biotin (often found in hair, skin and nail supplements), as it can interfere with some thyroid tests
• If you have positive antibodies and are pregnant or planning a pregnancy, mention this to your GP or midwife, as thyroid function may need closer monitoring

This article is for general information only and is not a substitute for medical advice. If you are worried about your health or your results, speak to your GP or specialist team. In an emergency, call 999 or go to A&E.

References:

NICE, 'Thyroid disease: assessment and management' (NG145) https://www.nice.org.uk/guidance/ng145

NHS, 'Underactive thyroid (hypothyroidism)' https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/

NHS, 'Overactive thyroid (hyperthyroidism)' https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/