Thyroid antibodies are proteins your immune system produces that mistakenly target your own thyroid gland. In a healthy immune response, antibodies attack foreign invaders like bacteria and viruses. But in autoimmune thyroid disease, these antibodies lock onto proteins in or on your thyroid, either damaging the gland or forcing it to work overtime. About 10 to 15% of people with normal thyroid function test positive for at least one type of thyroid antibody, but higher levels are strongly linked to conditions like Hashimoto’s disease and Graves’ disease.
The Main Types of Thyroid Antibodies
There are four thyroid antibodies that doctors test for, and each one points toward a different problem. Understanding which antibody showed up on your lab work tells you a lot about what’s happening inside your thyroid.
Thyroid peroxidase antibodies (TPO antibodies) are the most commonly tested and the primary marker for Hashimoto’s disease. They target an enzyme your thyroid needs to produce hormones. When these antibodies bind to that enzyme, they trigger a destructive immune response that gradually damages thyroid tissue. Over 90% of people with Hashimoto’s test positive for TPO antibodies. Even if your thyroid hormone levels are currently normal, a positive TPO result can signal that you may develop an underactive thyroid later in life.
Thyroglobulin antibodies (TgAb) target thyroglobulin, a protein your thyroid uses as a building block for hormones. Like TPO antibodies, they can indicate Hashimoto’s disease. They also play a special role in thyroid cancer monitoring: if you’ve had thyroid surgery and radioactive iodine treatment for thyroid cancer, rising thyroglobulin antibody levels may be an early sign the cancer has returned.
TSH receptor antibodies (TRAb) behave differently from the first two. Instead of destroying thyroid tissue, they attach to the receptors that normally respond to thyroid-stimulating hormone (TSH). About 95% of people with Graves’ disease have elevated TRAb. When combined with a low TSH level, a positive TRAb result is a strong indicator of Graves’ disease.
Thyroid-stimulating immunoglobulin (TSI) is actually a specific subtype of TRAb and the primary marker for Graves’ disease. TSI mimics TSH, effectively telling your thyroid to keep producing hormones when it shouldn’t. In studies comparing the two tests, TSI detected untreated Graves’ disease with roughly 99% sensitivity, while TRAb came in at about 97%.
How Thyroid Antibodies Cause Damage
The destruction isn’t random. In Hashimoto’s disease, TPO antibodies activate a part of the immune system called complement, a cascade of proteins that punches holes in thyroid cells and kills them. Over months and years, this steady assault shrinks the functional tissue of your thyroid until it can no longer produce enough hormones. This is why Hashimoto’s often develops slowly: you can have positive antibodies for years before your thyroid hormone levels drop low enough to cause symptoms like fatigue, weight gain, and cold sensitivity.
In Graves’ disease, the mechanism is almost the opposite. TSI antibodies stimulate the thyroid rather than destroy it, pushing hormone production into overdrive. The result is hyperthyroidism, with symptoms like rapid heart rate, weight loss, anxiety, and heat intolerance. Your thyroid may also enlarge visibly, forming what’s called a goiter.
Positive Antibodies With Normal Thyroid Levels
One of the most confusing results you can get is a positive antibody test when your thyroid hormones look perfectly normal. This happens more often than you might think. Roughly 10 to 15% of the general population carries detectable thyroid antibodies without any clinical thyroid dysfunction. For some of these people, the antibodies will never cause a problem. For others, it’s the earliest stage of autoimmune thyroid disease that hasn’t progressed far enough to affect hormone production yet.
If you’re in this category, your doctor will likely monitor your TSH periodically rather than start treatment. The antibodies themselves aren’t treated directly in most cases. The goal is to catch any shift in thyroid function early.
Why Thyroid Antibodies Matter in Pregnancy
Thyroid antibodies carry particular significance during pregnancy, even when thyroid hormone levels are normal. Multiple studies have found that women who test positive for thyroid antibodies have roughly double the risk of miscarriage compared to antibody-negative women. One large study of 1,500 euthyroid (normal thyroid function) women found a miscarriage rate of 36.3% in the antibody-positive group compared to 1.8% in those without antibodies, along with higher rates of premature birth (26.8% versus 8.0%).
The research also offers a hopeful finding. In a study by Negro and colleagues, untreated antibody-positive women had a miscarriage rate of 13.8%, while those who received thyroid hormone supplementation brought their rate down to 3.5%, nearly identical to the 2.4% rate in antibody-negative controls. This is one reason many doctors now screen for thyroid antibodies early in pregnancy or when evaluating recurrent pregnancy loss.
For women with Graves’ disease, TRAb levels are typically checked around 24 to 28 weeks of pregnancy. If levels are more than three times the upper limit of normal, closer fetal monitoring is recommended because these antibodies can cross the placenta and affect the baby’s thyroid.
When Antibodies Don’t Tell the Whole Story
About 5% of people diagnosed with Hashimoto’s disease based on clinical symptoms and ultrasound findings test negative for all thyroid antibodies. This is sometimes called seronegative Hashimoto’s. In these cases, doctors can still make the diagnosis using thyroid ultrasound, which shows a characteristic pattern of reduced echogenicity (the gland appears darker than normal), combined with two elevated TSH readings taken a few months apart. People with antibody-negative Hashimoto’s tend to have a milder course of disease, according to research from the American Thyroid Association.
Can You Lower Thyroid Antibody Levels?
Selenium supplementation is the most studied intervention for reducing thyroid antibodies. A 2025 meta-analysis found that selenium significantly lowered TPO antibody levels after three months of supplementation, with even greater reductions at six months. Thyroglobulin antibodies also dropped at three months, though the effect didn’t persist at six months. TSH levels showed a modest but statistically significant improvement at six months.
These results are encouraging but come with caveats. The reductions, while real, were moderate. Selenium didn’t normalize antibody levels in most patients, and the clinical benefit (whether people actually felt better or avoided thyroid damage) remains less clear. Still, for people with Hashimoto’s who live in areas with low dietary selenium, supplementation is a reasonable conversation to have with a doctor.
How Antibody Levels Guide Treatment Decisions
For Hashimoto’s disease, antibody levels are most useful at diagnosis. Once you’re confirmed and on thyroid hormone replacement, treatment is guided by your TSH and symptom response rather than by tracking antibody numbers over time.
For Graves’ disease, the picture is different. TRAb levels can help predict whether the disease will relapse after stopping medication. Patients with persistently medium to high TRAb levels are counseled that stopping anti-thyroid medication will almost certainly lead to a quick return of hyperthyroidism. These patients are typically given the choice between continuing medication with antibody retesting every six to twelve months or pursuing definitive treatment such as radioactive iodine or surgery. A dropping TRAb level, on the other hand, suggests the immune attack may be fading and medication withdrawal is more likely to succeed.