Anti-TPO and Anti-Tg (Thyroid Antibodies)
Definition
Measures autoantibodies directed against the thyroid gland. Three main antibodies are tested:
| Antibody | What it indicates |
|---|---|
| TPOAb (Anti-TPO) | A high level is a sign that Hashimoto's disease is causing hypothyroidism |
| TgAb (Anti-Tg) | A high level is a sign of Hashimoto's disease |
| TRAb (TSH receptor antibody) | May be a sign of Graves' disease |
Purpose
To determine whether thyroid dysfunction, when thyroid hormone levels (T3, T4, TSH) indicate it, is due to an autoimmune cause. Symptoms that prompt the test: weight change, fatigue, tremor, heat/cold intolerance, irregular heartbeat.
Results
- Negative, the symptoms are not of autoimmune origin
- TPOAb and/or TgAb positive, possible Hashimoto's disease
- TPOAb and/or TRAb positive, possible Graves' disease
The more thyroid antibodies there are, the greater the likelihood of an autoimmune thyroid disorder.
Normal Values
Laboratory-specific; refer to the reference range on your report.
Risks and Limitations
Antibody positivity is also seen in healthy people with normal thyroid function, and this is the point that most often causes concern:
In a study of 4,775 people with normal thyroid function (euthyroid) based on data from the Korea National Health and Nutrition Examination Survey:
| Group | TPOAb positivity |
|---|---|
| Euthyroid people without metabolic syndrome | 5% |
| Euthyroid people with metabolic syndrome | 7% |
In other words, about one in every 20 people whose thyroid hormones are completely normal tests positive for TPOAb.
What the patient needs to know: Anti-TPO positivity on its own does not mean "you have thyroid disease". In a person who is antibody positive but has normal TSH/T4, the diagnosis is not autoimmune thyroiditis but a state of "thyroid autoimmunity", which requires follow-up and may not require treatment.
For this reason, testing antibodies for screening purposes in people without symptoms may cause unnecessary anxiety. The test is ordered when hormone levels indicate dysfunction.