If you have had thyroid blood tests, you may have seen results for antibodies. These tests look for immune proteins that target the thyroid, and they help explain why the thyroid is misbehaving — not just how much hormone it is making.
What are thyroid antibodies?
Antibodies are proteins the immune system makes to recognise and neutralise foreign threats. In autoimmune thyroid disease, the immune system mistakenly produces antibodies against the thyroid’s own proteins. These antibodies are markers of autoimmunity and, in some cases, directly affect thyroid function.
The main thyroid antibodies
Anti-TPO (thyroid peroxidase) antibodies
Anti-TPO antibodies are the most common and most useful marker. They are found in the great majority of people with Hashimoto’s thyroiditis and in many with Graves’ disease. A positive result supports an autoimmune cause for an underactive or overactive thyroid.
Anti-thyroglobulin (anti-Tg) antibodies
Anti-Tg antibodies target thyroglobulin, the protein the thyroid uses to store hormone. They often accompany anti-TPO antibodies. They matter in thyroid cancer follow-up, because if present they can interfere with thyroglobulin measurement as a tumour marker.
TRAb (TSH receptor antibodies)
TRAb are the antibodies behind Graves’ disease. They stimulate the TSH receptor, driving the thyroid to overproduce hormone. TRAb testing helps diagnose Graves’ disease, can predict relapse, and is important in pregnancy because these antibodies can cross the placenta and affect the baby.
What a positive result means
A positive antibody test indicates an autoimmune process affecting the thyroid. It does not, by itself, tell you how severe your condition is or how you will feel. Many people have positive antibodies with completely normal thyroid function.
Antibody levels can fluctuate and are not usually used to guide treatment — treatment is based on thyroid hormone levels and symptoms.
What a negative result means
A negative antibody test does not entirely exclude autoimmune thyroid disease, and it does not rule out a thyroid problem. Some people with Hashimoto’s have low or undetectable antibodies. Diagnosis rests on the overall picture: hormone levels, symptoms, examination and, where relevant, imaging.
Why antibodies are tested
- To identify the cause of an under- or overactive thyroid
- To support a diagnosis of Hashimoto’s or Graves’ disease
- To guide decisions in pregnancy (TRAb in particular)
- In cancer follow-up, to interpret thyroglobulin results
The bottom line
Thyroid antibodies are clues to an autoimmune cause, not a measure of disease severity. If your results show positive antibodies, the important questions are about your thyroid hormone levels and symptoms — and those are what guide treatment. Discuss what your specific results mean with your clinician.