TPO Antibodies: What High Levels Mean for Hashimoto's

If you've been diagnosed with Hashimoto's thyroiditis, you've probably had your TPO antibodies tested. You may have gotten a number that seemed alarming — sometimes in the hundreds, sometimes in the thousands — without much explanation of what it means or what to do about it. This article explains what TPO antibodies actually are, why they fluctuate, and what the research says about reducing them.

What Are TPO Antibodies?

Thyroid peroxidase (TPO) is an enzyme inside your thyroid cells that plays a central role in producing thyroid hormones. It catalyzes the process of attaching iodine to thyroglobulin to create T3 and T4^[1].

In Hashimoto's thyroiditis, the immune system mistakenly identifies TPO as a threat and produces antibodies against it. These TPO antibodies (also called anti-TPO or TPOAb) are the immune system's attack markers — their presence confirms that autoimmune activity is targeting your thyroid.

Research suggests that 80–90% of people with Hashimoto's will have elevated TPO antibodies, elevated thyroglobulin antibodies (TgAb), or both^[2].

What a Positive Test Means

A positive TPO antibody test — generally considered anything above 35 IU/mL, though lab ranges vary — is one of the primary diagnostic markers for Hashimoto's thyroiditis. When elevated TPO antibodies appear alongside a clinical picture of hypothyroid symptoms or an abnormal TSH, Hashimoto's is typically the diagnosis.

Elevated TPO antibodies have also been associated with anxiety and mood symptoms independent of thyroid hormone levels, suggesting the autoimmune process itself may affect wellbeing beyond just the hormonal impact^[2].

Importantly: a high TPO antibody number does not automatically mean you need thyroid medication. Many people have elevated antibodies with a normal TSH and no symptoms. The antibodies indicate immune activity; they don't always indicate that the thyroid has been sufficiently damaged to require hormone replacement.

Why Your Number Fluctuates

TPO antibody levels are not static. They rise and fall based on immune activity, stress, illness, hormonal shifts, and other factors. This is why a single reading should not cause panic, and why tracking your trend over multiple tests is more meaningful than any individual result.

If your antibodies were 400 IU/mL six months ago and are now 250 IU/mL, that downward trend is clinically meaningful — even if 250 IU/mL still looks like a high number in isolation.

Can TPO Antibodies Be Reduced?

This is one of the most common questions in the Hashimoto's community, and the evidence is more encouraging than many patients are told.

Selenium

The most studied intervention for TPO antibody reduction is selenium supplementation. A 2025 meta-analysis of 21 randomized controlled trials including 1,610 patients found that selenium supplementation significantly reduced TPO antibody levels at both 3 months (SMD = −0.46, p = .001) and 6 months (SMD = −0.80, p = .008)^[3]. A separate systematic review of 35 RCTs with 2,358 participants confirmed a significant decrease in TPOAb with selenium supplementation (SMD −0.96), with the effect being strongest in patients using doses above 100 µg/day and those using selenomethionine specifically^[4].

That said, it's worth noting that approximately 68% of individual cohorts in one large review found no significant effect, while 32% did show a significant decrease^[4]. The benefit appears real but variable by individual. Most researchers recommend discussing selenium supplementation with your doctor before starting, as excess selenium carries its own risks.

Vitamin D

Some evidence suggests that correcting vitamin D deficiency may also modestly reduce TPO antibodies in patients who are deficient, though the effect is less robust than selenium^[5].

Levothyroxine

For patients who do require thyroid hormone replacement, starting levothyroxine treatment appears to modestly reduce antibody levels over time in some patients, likely by reducing the TSH-driven stimulation of the thyroid gland.

What to Ask Your Doctor

• "Can we track my TPO antibodies over time rather than treating each result in isolation?"
• "Have my TgAb (thyroglobulin antibodies) been tested as well?"
• "Is selenium supplementation appropriate for my situation?"
• "At what antibody level, if any, would you recommend starting treatment?"

TPO antibodies are a useful signal, not a sentence. Tracking the trend is the clinical value.