Thyroid Peroxidase (TPO) Antibodies: Normal Range & What High Means

FAQ about Thyroid Peroxidase Antibodies (Anti-TPO Ab)

• What is a normal TPO antibody level?

  Normal TPO antibody levels are typically below 34 IU/mL on LabCorp and below 9 IU/mL on Quest Diagnostics. Reference ranges vary between laboratories, so always use the range printed on your own report. There is no clinically meaningful "too low" for TPO antibodies — low or undetectable results are normal and healthy.

• What does it mean if my TPO antibodies are high?

  Elevated TPO antibodies indicate the immune system is producing antibodies against the thyroid gland — most often a sign of autoimmune thyroid disease. The most common causes are Hashimoto's thyroiditis (present in approximately 90% of cases) and Graves' disease (present in 50–75% of cases). Elevated TPO antibodies can also appear in some healthy individuals and in people with other autoimmune conditions.

• Does high TPO antibody mean cancer?

  No. Elevated TPO antibodies do not indicate thyroid cancer. TPO antibodies may be present in a small percentage of people with thyroid cancer, but they are far more commonly elevated in benign autoimmune thyroid conditions. At a population level, elevated TPO antibodies alone do not increase thyroid cancer risk. The main warning sign of thyroid cancer is a painless lump in the neck — not an elevated TPO antibody result.

• What is the difference between TPO antibodies and thyroglobulin antibodies?

  Both are markers of autoimmune thyroid disease and are often tested together. TPO antibodies are more sensitive and are the first-line marker for suspected Hashimoto's thyroiditis and Graves' disease. Thyroglobulin antibodies (TgAb) are also elevated in Hashimoto's but are more commonly used to monitor thyroid cancer recurrence after surgery. When both are elevated, this strengthens the diagnosis of autoimmune thyroid disease.

• What does a TPO result of 320 IU/mL mean?

  A result of 320 IU/mL is moderately elevated and falls in the 100–500 IU/mL range on most reference scales. This level is consistent with autoimmune thyroid disease, most commonly Hashimoto's thyroiditis. Your doctor will interpret this alongside your TSH, Free T4, and symptoms to determine whether thyroid function has been affected and whether treatment is needed.

• What does a TPO result of 900 or 1300 IU/mL mean?

  Results above 500 IU/mL are significantly elevated and strongly associated with active autoimmune thyroid disease, most commonly Hashimoto's thyroiditis. Higher titres often reflect more active immune activity. Your doctor will assess your full thyroid function panel alongside these results. Very high TPO antibodies do not automatically mean your symptoms will be more severe — thyroid function tests are needed to determine clinical impact.

• Can you have high TPO antibodies with a normal TSH?

  Yes. It is possible to have elevated TPO antibodies with a completely normal TSH and normal thyroid function. This means the immune system is producing antibodies against the thyroid but thyroid hormone production has not yet been significantly affected. In this situation, elevated TPO antibodies indicate an increased future risk of developing thyroid dysfunction and usually prompt regular monitoring of TSH rather than immediate treatment.

• What is the difference between Anti-TPO Ab and Anti-TPO Ab (RDL)?

  These refer to the same test. RDL stands for "reflex dilution laboratory" and describes the testing methodology used by some laboratories — it does not indicate a different type of result or a different antibody. The interpretation is the same regardless of whether your report says "TPO Ab," "Anti-TPO Ab," or "Anti-TPO Ab (RDL)."

• What causes TPO antibodies to be elevated?

  The most common cause is autoimmune thyroid disease — either Hashimoto's thyroiditis or Graves' disease. Other causes include postpartum thyroiditis, atrophic autoimmune thyroiditis, and subclinical hypothyroidism. In approximately 10% of cases, TPO antibodies appear in people with other autoimmune conditions such as type 1 diabetes or pernicious anemia. TPO antibodies can also be mildly elevated in some healthy individuals without any thyroid disease.

• What is the difference between Hashimoto's thyroiditis and Graves' disease?

  Both are autoimmune thyroid diseases associated with elevated TPO antibodies, but they affect the thyroid in opposite ways. In Hashimoto's thyroiditis, the immune system gradually destroys thyroid tissue, leading to an underactive thyroid (hypothyroidism) with symptoms such as fatigue, weight gain, and cold intolerance. In Graves' disease, the immune system stimulates the thyroid to overproduce hormones, causing an overactive thyroid (hyperthyroidism) with symptoms such as weight loss, rapid heartbeat, and heat intolerance.

• Should I be worried if my TPO antibodies are slightly elevated?

  A mildly elevated TPO result — just above the upper limit of your lab's range — does not automatically indicate serious disease. Borderline elevations are common and are often found incidentally on routine thyroid panels. Your doctor will interpret the result alongside your TSH, Free T4, symptoms, and clinical history. Many people with mildly elevated TPO antibodies have normal thyroid function and require only periodic monitoring rather than treatment.

• Can TPO antibodies go back to normal?

  TPO antibody levels can fluctuate over time and may decrease in some people, particularly with lifestyle interventions such as optimising vitamin D levels, reducing dietary triggers in susceptible individuals, or treating underlying inflammation. However, in established Hashimoto's thyroiditis, antibodies rarely normalise completely. The level of TPO antibodies does not always correlate directly with symptoms or thyroid function — TSH and Free T4 remain the primary measures of how well the thyroid is functioning.