Thyroid Antibody Tests Explained: Learn what thyroid antibody tests mean, why doctors order them, and how they help diagnose autoimmune thyroid conditions like Hashimoto’s disease and Graves’ disease.
Antibodies are proteins made by your immune system.
Their normal job is to attack things like:
Viruses
Bacteria
Germs
But sometimes the immune system gets confused and attacks healthy parts of your body.
When it attacks the thyroid, it can cause autoimmune thyroid disease.
Standard thyroid tests (TSH, Free T4, Free T3) tell us:
If the thyroid is too slow
If the thyroid is too fast
But they don’t tell us why.
Thyroid antibody tests help answer that question.
There are three main ones doctors check:
TPO Antibodies (Thyroid Peroxidase Antibodies)
Thyroglobulin Antibodies (TgAb)
TSI or TRAb (Thyroid Stimulating Immunoglobulins / TSH Receptor Antibodies)
Each one points to different thyroid conditions.
TPO antibodies are the most commonly checked thyroid antibodies.
A positive TPO test usually means:
➡ Hashimoto’s disease
(the most common cause of hypothyroidism)
If TPO antibodies are positive:
Your immune system is attacking your thyroid
Over time this can make the thyroid underactive
It confirms an autoimmune cause
Many people with high TPO antibodies eventually develop hypothyroidism.
This test is often checked along with TPO.
Positive TgAb can also suggest:
Other autoimmune thyroid conditions
It is especially useful when TPO is negative but autoimmune thyroid disease is still suspected.
These antibodies are different.
They are usually checked when doctors suspect:
➡ Graves’ disease (overactive thyroid)
If TSI or TRAb is positive:
The immune system is stimulating the thyroid
The thyroid is pushed to make too much hormone
This confirms autoimmune hyperthyroidism
This is the most common cause of high thyroid levels.
Here are common combinations:
High TSH + Positive TPO
Confirms Hashimoto’s hypothyroidism
Low TSH + Positive TSI
Confirms Graves’ disease (hyperthyroidism)
Normal TSH + Positive TPO
This can mean:
Early autoimmune thyroid disease
Higher risk of future thyroid problems
Need for occasional monitoring
Yes!
Some people have positive antibodies but:
Normal TSH
Normal Free T4
No symptoms
This means the immune system is active, but thyroid function is still okay.
Doctors often just monitor in these cases.
Usually:
Antibodies can stay positive for years
Levels may go up and down
Treatment focuses on thyroid function, not the antibody number
Doctors rarely treat based only on antibody levels.
Antibodies are commonly checked when:
TSH is abnormal
Symptoms suggest autoimmune thyroid disease
A cause needs to be confirmed
Someone has a family history of thyroid problems
A person is pregnant or planning pregnancy
If antibodies are positive:
Doctors look at TSH and Free T4
Decide whether treatment is needed
Plan regular monitoring
Discuss long-term expectations
Antibody tests help find the cause of thyroid problems
Positive TPO → usually Hashimoto’s
Positive TSI → usually Graves’ disease
Antibodies can be positive even with normal thyroid levels
Treatment depends on hormone levels, not just antibodies
The next page will explain:
➡ Hypothyroidism Explained (Low Thyroid)
We’ll connect symptoms, tests, and treatment all together.