TPO Antibodies: The Blood Test for Hashimoto's Thyroiditis

What Is Thyroid Peroxidase?

Thyroid peroxidase (TPO) is the key enzyme responsible for producing thyroid hormones. It catalyses the incorporation of iodine into thyroglobulin to form T3 and T4 — the active thyroid hormones. When the immune system mistakenly identifies TPO as a foreign protein and produces antibodies against it, these anti-TPO antibodies (TPO-Ab) interfere with thyroid hormone production and cause chronic inflammation within the thyroid gland.

This immune attack is the pathological basis of Hashimoto's thyroiditis. Over time, inflammatory damage progressively destroys thyroid tissue, reducing the gland's ability to produce hormones. Initially, the pituitary compensates by increasing TSH output. Eventually, even maximum TSH stimulation is insufficient and hypothyroidism develops — a process that may take years or decades.

TPO Antibody Reference Ranges

Result	TPO-Ab Level	Interpretation
Negative	<34–35 IU/mL (varies by lab)	No detectable autoimmune thyroid activity
Borderline	35–60 IU/mL	Low-level positivity — monitor, may or may not progress
Mildly positive	60–200 IU/mL	Consistent with Hashimoto's; monitor TSH, free T4 annually
Moderately positive	200–500 IU/mL	Active autoimmune thyroid disease; closer monitoring warranted
Strongly positive	>500 IU/mL	High autoimmune activity; symptoms and thyroid function changes likely

Reference ranges vary between laboratories, so always check what your lab's upper limit of normal is. The exact number matters less than the trend — TPO-Ab levels that rise over time indicate ongoing autoimmune activity, while stable or falling levels suggest the attack has plateaued or responded to intervention.

What It Means to Have High TPO Antibodies

A positive TPO antibody result means that your immune system is producing antibodies against your own thyroid gland. This confirms an autoimmune process is occurring. It does not tell you how damaged the gland currently is — that requires TSH and free T4 testing — but it establishes the underlying cause and predicts future risk.

People with positive TPO antibodies and normal thyroid function (subclinical Hashimoto's) have approximately 2–5% annual risk of progressing to overt hypothyroidism — roughly 5 times higher than the general population. Over a 20-year period, the majority will develop at least subclinical hypothyroidism requiring monitoring or treatment.

Symptoms of Hashimoto's: What People Experience

It is well documented that Hashimoto's patients can experience significant symptoms while TSH remains in the reference range. This is because TSH measures pituitary response, not necessarily what is happening at the tissue level with thyroid hormone conversion and receptor sensitivity. Some Hashimoto's patients also have low-normal free T3 despite normal free T4, because inflammation impairs the conversion of T4 to active T3.

Other Thyroid Autoimmune Tests

TPO-Ab is the most sensitive marker for Hashimoto's — positive in about 95% of cases. Thyroglobulin antibodies (TgAb) are present in about 80% of Hashimoto's cases and may be positive when TPO-Ab is negative (particularly in early disease or when TPO is borderline). Having both tests positive increases the certainty of the diagnosis. Thyroid-stimulating immunoglobulins (TSI) or TSH receptor antibodies (TRAb) are associated with Graves' disease (overactive thyroid) rather than Hashimoto's, and their presence alongside positive TPO-Ab suggests a mixed or overlap condition.

TPO Positive but Normal TSH: What Now?

This is the most common scenario and the most confusing for patients. You have an autoimmune attack confirmed by positive TPO-Ab, but your thyroid is currently producing enough hormones that TSH remains in range. The appropriate management is monitoring, not immediate treatment with thyroid hormone (which would not be appropriate if thyroid function is normal).

The recommended monitoring schedule is TSH and free T4 annually, or sooner if new symptoms develop. Some practitioners also monitor thyroid ultrasound every 2–3 years to assess for changes in thyroid texture and volume consistent with Hashimoto's (heterogeneous, hypoechoic pattern). Selenium supplementation (200 µg/day — only under medical guidance, as selenium has a narrow therapeutic window and excess causes toxicity) has randomised controlled trial evidence for reducing TPO-Ab levels over 6–12 months in Hashimoto's patients — it is the most evidence-backed non-pharmacological intervention for the autoimmune process itself.

When Treatment Starts

Levothyroxine (synthetic T4) is the standard treatment for Hashimoto's-related hypothyroidism, but treatment is only warranted when thyroid function tests confirm inadequate hormone production. The threshold varies: TSH above 10 mIU/L is a clear indication; TSH between 4.5 and 10 (subclinical hypothyroidism) is treated when symptoms are present, or when the patient is pregnant or trying to conceive. Testing TPO antibodies at this stage also informs treatment decisions — positive TPO-Ab increases the likelihood that subclinical hypothyroidism is Hashimoto's-related and will progress, supporting earlier treatment.