Anti-TPO: The Main Marker of Hashimoto's Thyroiditis and How to Interpret It

Key points

Anti-TPO is the main marker of autoimmune thyroiditis (Hashimoto's): together with TSH, fT4 and ultrasound it helps establish the diagnosis, estimate the prognosis and start treatment on time. The table shows where the test is most useful.

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What Anti-TPO Measures: Biology and Lab Science

Anti-TPO antibodies are immunoglobulins directed against thyroid peroxidase, the enzyme that oxidizes iodide in the thyroid follicle. Without that oxidation, hormones cannot be assembled. When antibodies block the enzyme, a chronic autoimmune process begins — Hashimoto's thyroiditis (chronic autoimmune thyroiditis, AIT).

Anti-TPO is the principal marker of Hashimoto. It is positive in 90–95% of people with Hashimoto, so the diagnosis almost always rests on it. But a positive titer alone is not a diagnosis either: antibodies are found in 10–15% of healthy women and 5% of healthy men with no clinical signs and no ultrasound changes.

Thyroid peroxidase (TPO) is a membrane enzyme of the thyrocyte. It catalyzes two reactions: oxidation of iodide (I⁻ → I⁰) and attachment of iodine to tyrosine residues in thyroglobulin. This is the central step of T4 and T3 synthesis.

• The catalytic center of TPO contains a heme group — so iron is a cofactor of the enzyme. With ferritin below 70 ng/mL, TPO functions suboptimally even without antibodies.
• Hydrogen peroxide (H₂O₂) is required for the oxidation reaction. Selenium, via glutathione peroxidase, quenches excess peroxide. Without selenium, the reaction loses control and damages the tissue.
• Anti-TPO antibodies bind the catalytic domain and trigger complement-dependent and cell-mediated cytolysis of thyrocytes.

In the lab, anti-TPO is measured by ELISA or chemiluminescent immunoassay (CLIA). The reference range depends on the assay — usually "negative" is < 35 IU/mL. Some labs use different cut-offs (9, 60, 100) — always check the reference on the report.

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When to Test: Clinical Triggers

Anti-TPO testing is indicated when the result changes management. Specific situations:

• Fatigue, weight gain, hair loss with normal TSH — to separate subclinical AIT from other causes
• TSH 4–10 mIU/L — to distinguish autoimmune thyroiditis from transient TSH elevation (stress, illness, recovery from acute thyroiditis)
• Recurrent pregnancy loss, infertility, IVF preparation — even euthyroid AIT increases miscarriage risk and reduces ART success
• Diffuse hypoechogenicity and heterogeneity on thyroid ultrasound — confirm autoimmune nature of changes
• Family history of AIT or Graves' disease — risk stratification
• 3–6 months postpartum — peak of postpartum thyroiditis; 20–40% are left with permanent hypothyroidism afterwards
• Before starting amiodarone, lithium, or interferon — known triggers of autoimmune thyroid disease
• Before/during pregnancy — who should be tested and when is covered below, in the section "Anti-TPO in Pregnancy and IVF"

For background on thyroid function and iodine within the cofactor system, see Iodine and Thyroid: 5-Step Protocol.

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How to Interpret the Result: Numbers and Context

The number alone is meaningless. With context, the titer carries prognostic weight:

• < 35 IU/mL (negative) — autoimmune process not confirmed. If hypothyroidism is present, look for other causes: iodine deficiency, postsurgical, post-radiation, central.
• 35–100 IU/mL (borderline) — monitoring. Repeat at 6–12 months. Ultrasound mandatory. Often this is early subclinical AIT or transient elevation after viral infection.
• 100–500 IU/mL (moderate) — probable AIT. Monitor TSH + fT4 every 6–12 months. Progression prevention: selenium, vitamin D, anti-inflammatory diet.
• 500–1000 IU/mL (high) — AIT clinically confirmed. Prepare for hypothyroidism management.
• > 1000 IU/mL (very high) — high risk of progression to overt hypothyroidism within 5 years (risk approximately 4–5% per year with normal TSH, up to 12% per year with TSH 2.5–4.5).

Important: the titer level does not correlate with the severity of hypothyroidism. A patient with anti-TPO of 5000 IU/mL may be euthyroid, while a patient with anti-TPO of 150 IU/mL may have overt hypothyroidism. Titer predicts progression risk, not the degree of dysfunction.

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The Full Panel: What to Order Alongside Anti-TPO

An isolated anti-TPO without accompanying markers is half-information. A complete thyroid autoimmune workup includes:

• Anti-TPO — primary marker of Hashimoto and postpartum thyroiditis
• Anti-TG (anti-thyroglobulin) — less specific for Hashimoto, but valuable in suspected differentiated thyroid cancer as a marker of tumor/residual tissue after thyroidectomy. Positive in 60–80% of Hashimoto cases, but also in 10% of healthy women.
• TSI / TRAb (TSH receptor antibodies) — marker of Graves' disease (diffuse toxic goiter), not Hashimoto. Order when hyperthyroidism or orbitopathy is suspected.
• TSH + free T4 + free T3 — functional picture
• Thyroid ultrasound — diffuse hypoechogenicity and heterogeneity confirm the autoimmune process when anti-TPO is positive

Minimum diagnostic set: TSH + fT4 + anti-TPO + ultrasound. If TSH is elevated and anti-TPO positive, TRAb is not needed. If TSH is suppressed, TRAb is mandatory for differential diagnosis of Graves vs the thyrotoxic phase of Hashimoto.

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What Is Commonly Missed

• Ferritin and iron. TPO is an iron-dependent enzyme. Iron deficiency worsens hypothyroidism even at adequate T4 levels. Target ferritin: 70–100 ng/mL, not "within reference range."
• Vitamin D. Deficiency below 30 ng/mL is associated with higher anti-TPO titers and earlier progression to overt hypothyroidism. Target level in AIT: 50–80 ng/mL.
• Red blood cell selenium. Selenium deficiency is the leading cause of AIT "not responding to standard therapy."
• T4 → T3 conversion. Many patients on L-thyroxine complain of fatigue at "normal TSH." Often this is impaired conversion — see T4–T3 Conversion and Functional Hypothyroidism.

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Monitoring: Why Anti-TPO Is Not Retested

Anti-TPO is a static marker. Once elevated, antibodies remain elevated for decades. Antibody dynamics clinically do not change AIT management.

L-thyroxine does not reduce anti-TPO. It compensates hypothyroidism but does not treat the autoimmune process. This must be made clear to the patient: the goal of therapy is not to "zero out the antibodies" but to keep TSH and fT4 in the optimal range and resolve symptoms.

The only intervention with proven impact on titer is selenium 200 mcg/day in the form of selenomethionine or sodium selenite. It works best where selenium deficiency exists, so red blood cell selenium is checked before the course.

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Selenium 200 mcg/day: Form, Course and Safety

Selenium has a narrow therapeutic window: deficiency impairs glutathione peroxidase and iodothyronine deiodinases, excess causes selenosis. 200 mcg/day is the standard dose in AIT.

Selenium does not replace L-thyroxine in overt hypothyroidism: it is an adjunct for lowering antibodies and possibly slowing progression, not replacement therapy.

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When the Test Is NOT Needed

• Screening in asymptomatic men under 40 with no family history and normal TSH — base rate is 4–5%, downstream management does not change.
• Repeat testing in already confirmed AIT, including every 3 months "for monitoring" — the marker is static, antibody dynamics do not change therapy. Exception: assessing response to a 6-month selenium course (optional).
• "Just in case" with normal TSH, normal fT4, no symptoms, no family history, normal ultrasound — low pretest probability, any result will not change management.

ATA Guideline principle: anti-TPO testing is indicated when the result changes patient management. If management will not change, the test is not needed.

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What to Do with Anti-TPO Positive and Normal TSH

This is the most common scenario — subclinical AIT. TSH is normal, fT4 is normal, antibodies are positive, symptoms include fatigue, weight gain, hair loss, cold hands. What to do:

• Monitor TSH + fT4 every 6–12 months. Progression to overt hypothyroidism: 4–5% per year with anti-TPO positive. Goal: do not miss the moment when L-thyroxine becomes necessary.
• Thyroid ultrasound every 1–2 years — track nodules and volume changes.
• Selenium 100–200 mcg/day for 6 months with confirmed RBC selenium deficiency. Anti-TPO recheck optional.
• Vitamin D — cholecalciferol 2000–5000 IU/day.
• Iron if deficient — iron bisglycinate 25–50 mg.
• Anti-inflammatory diet — reduce gluten when celiac or intolerance is confirmed, dairy when sensitivity is confirmed.
• Stress triggers — cortisol suppresses deiodinases and amplifies the autoimmune process. Sleep 7–9 h, HPA-axis control — see Cortisol and the HPA Axis.

When to start L-thyroxine: TSH > 10 mIU/L unequivocally. TSH 4–10 — individualized, considering symptoms, planned pregnancy, titer level, TSH trajectory over time.

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Differential Diagnosis: AIT and Other Thyroiditides

Positive anti-TPO plus thyroid dysfunction ≠ automatically Hashimoto. Several conditions look alike in the lab but need their own management; the key is the combination of antibodies, disease phase, ultrasound and scintigraphy.

Thyrotoxic phase of Hashimoto (Hashitoxicosis). Resembles Graves in the lab, but TRAb is negative, anti-TPO is high and uptake on scintigraphy is reduced: the thyrotoxicosis is destructive — preformed hormone leaks from damaged follicles rather than being overproduced. Antithyroid drugs are not indicated — the phase resolves on its own within 2–8 weeks; symptomatically — a beta-blocker.

Postpartum thyroiditis is a variant of painless thyroiditis. It occurs in 5–10% of women within 12 months after delivery and in 30–50% of women anti-TPO positive during pregnancy. The classic course has three phases: thyrotoxic (1–6 months postpartum, lasting 1–3 months), hypothyroid (3–8 months postpartum, lasting 4–6 months) and recovery of euthyroidism. 20–40% are left with permanent hypothyroidism. TRAb is negative; reduced uptake on scintigraphy in the thyrotoxic phase distinguishes it from a postpartum relapse of Graves. Antithyroid drugs are contraindicated; in the hypothyroid phase with symptoms — L-thyroxine, with withdrawal assessed after 6–12 months.

Subacute de Quervain thyroiditis (granulomatous). Flu-like prodrome with fever, painful neck radiating to the jaw and ear, ESR often > 50 mm/h, CRP elevated. Ultrasound — focal hypoechogenicity with ill-defined borders, not diffuse; uptake on scintigraphy is reduced. Anti-TPO is usually negative or low. Thyrotoxic phase 4–8 weeks, then euthyroidism, permanent hypothyroidism in 5–15%. Treatment — NSAIDs, in severe cases prednisolone 15–40 mg/day tapered over 4–8 weeks. L-thyroxine is not indicated in this scenario — the disease resolves on its own.

IgG4-related thyroiditis and the sclerosing variant of Hashimoto. Part of histologically confirmed Hashimoto turns out to be an IgG4-positive form with faster fibrotic progression and a tendency to compress the trachea. Serum IgG4 is elevated in 40–50% of these patients, IgG4+ plasma cells in the biopsy > 20/HPF. Not a screening diagnosis — but with rapid goiter growth, stridor or dysphagia in an anti-TPO positive patient, this is a direction for biopsy.

In hypothyroidism the differential is simpler: TSH↑ + fT4↓ + anti-TPO positive + typical ultrasound = AIT. With anti-TPO negative, besides iodine deficiency, postsurgical and post-radiation hypothyroidism, rule out central hypothyroidism (low or normal TSH + low fT4 — pituitary tumor, empty sella syndrome) and thyroid hormone resistance (rare, THRB mutations). In older adults, an isolated TSH rise is often an age norm: at TSH 4–7 mIU/L without anti-TPO and symptoms, the need for L-thyroxine is reassessed.

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Atrophic and Hypertrophic Forms of Hashimoto

Hashimoto is not a single phenotype. Histologically and clinically two variants are distinguished, and their monitoring differs.

Hypertrophic (goitrous) form — classic Hashimoto. Thyroid volume is increased (> 18 mL in women, > 25 mL in men); on palpation the gland is firm, heterogeneous and painless. Ultrasound — diffuse hypoechogenicity, pseudonodular structure, hypervascularity in the active phase. Histology — lymphocytic infiltration, germinal centers, Hürthle cells (oncocytic metaplasia). Anti-TPO is usually high (often > 500 IU/mL). Progression to overt hypothyroidism is slow — years. Some patients stay euthyroid for decades.

Atrophic form (primary myxedema) — the end stage or an alternative course. The thyroid is small (< 6 mL in women, < 8 mL in men), fibrotic and poorly secreting. Overt hypothyroidism is almost inevitable. Anti-TPO may be moderate (100–500 IU/mL) — little gland mass is left for immune attack. On ultrasound — hypoechoic, poorly distinguishable tissue, sometimes a fibrous strand instead of the organ. Blocking-type TSH receptor antibodies (TBII) are present in some patients — a separate mechanism without goiter formation.

Clinical difference:

• Hypertrophic form — priority on monitoring, selenium/vitamin D/iron, postponing L-thyroxine in a subclinical course.
• Atrophic form — replacement therapy is almost always needed. The L-thyroxine dose per body weight is higher (1.6–1.8 mcg/kg vs 1.4–1.6 mcg/kg in the hypertrophic form), since residual production is absent.
• Ultrasound volume is the only reliable way to separate the forms at the first visit. A report saying "diffuse changes" without volume carries no such information.

Special case — transition from one form to the other. In some patients the hypertrophic form becomes atrophic over 10–20 years through progressive fibrosis. This explains a "normal volume" on an old ultrasound and a "small thyroid" 15 years later in the same patient. A single ultrasound is not enough for a long-term prognosis — follow-up once every 1–2 years is needed.

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Anti-TPO in Pregnancy and IVF: Specific Thresholds

Euthyroid anti-TPO positivity is a risk factor, not a diagnosis of AIT in pregnancy: the risk of miscarriage, preterm birth and postpartum thyroiditis is increased. The reference is the ATA Guideline 2017 (PMID 28056690):

• TSH < 2.5 mIU/L + anti-TPO positive, euthyroid — L-thyroxine 25–50 mcg/day may be considered, especially with recurrent pregnancy loss or previous ART failures.
• TSH 2.5–4.0 mIU/L + anti-TPO positive — L-thyroxine 25–50 mcg/day may be considered (weak ATA recommendation) during pregnancy or in an ART cycle. Target — TSH < 2.5 mIU/L before conception and in the first trimester.
• TSH > 4.0 mIU/L + anti-TPO positive — during pregnancy L-thyroxine is recommended. Starting dose 50–75 mcg/day; in overt hypothyroidism (TSH > 10 mIU/L or fT4 below the reference) — full replacement dose of 1.6–1.8 mcg/kg/day.

Monitoring during pregnancy:

• TSH + fT4 every 4 weeks in the first and second trimesters, then every 6–8 weeks.
• TSH target — 0.1–2.5 mIU/L in the first trimester, 0.2–3.0 mIU/L in the second and 0.3–3.0 mIU/L in the third. (Local trimester-specific reference ranges, when available, take priority over universal ones.)
• In women with hypothyroidism known before pregnancy, the L-thyroxine dose is usually increased by 25–30% right after a positive hCG test (rule of thumb — two extra tablets per week), because estrogens raise thyroxine-binding globulin.
• Keep at least 4 hours between L-thyroxine and iron / calcium / magnesium preparations. ART stimulation drugs (progesterone, hCG) have no clinically significant interaction with L-thyroxine.
• Untreated overt hypothyroidism in the first trimester is associated with a 5–10-point lower IQ in the child — this is the most critical window.

After delivery:

• The L-thyroxine dose returns to the preconception level within 4–6 weeks; TSH is checked after 6 weeks.
• Anti-TPO + TSH at 3 and 6 months is mandatory. The risk of postpartum thyroiditis in women anti-TPO positive during pregnancy is 30–50%.
• In the thyrotoxic phase of postpartum thyroiditis — a beta-blocker (propranolol), no antithyroid drugs. In the hypothyroid phase with symptoms or when planning another pregnancy — L-thyroxine; after 6–12 months withdrawal is assessed: the dose is reduced by 50% and TSH is checked 6 weeks later.
• L-thyroxine is compatible with breastfeeding — only traces reach the milk.

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Associated Autoimmune Diseases: What to Look For with Anti-TPO Positive

AIT rarely exists in isolation. Autoimmune polyglandular syndrome type 2 (APS-2, Schmidt) combines Hashimoto + adrenal insufficiency (Addison's disease) + type 1 diabetes in any combination. The prevalence of APS-2 among AIT patients is 2–5%, but related "single" autoimmune conditions are much more common.

What to check at the first diagnosis of AIT:

• Celiac disease. Shared HLA-DQ2/DQ8 predisposition. Anti-tTG IgA + total IgA (IgA deficiency gives a false-negative tTG; then — anti-tTG IgG or anti-DGP IgG); if screening is positive — duodenal biopsy. Prevalence of celiac disease in AIT is 2–5%, 4–10 times higher than in the general population. With established celiac disease, a gluten-free diet lowers anti-TPO in some patients; without celiac disease it is not routinely indicated.
• Type 1 diabetes. Fasting glucose + HbA1c. With risk factors (family history of T1D, BMI < 25, urinary ketones with hyperglycemia) — antibodies to GAD-65, IA-2, ZnT8. In AIT patients the risk of T1D is low but not zero, especially at a young age.
• Addison's disease. Morning cortisol (8:00) + ACTH. With cortisol < 5 mcg/dL and ACTH > 100 pg/mL — antibodies to 21-hydroxylase (anti-CYP21); if suspected — a stimulation test with 250 mcg tetracosactide. Symptoms — unexplained fatigue, hyperpigmentation of palmar creases and mucosa, orthostatic hypotension, hyponatremia, hyperkalemia. Critically important: with AIT + Addison's, adrenal insufficiency is compensated first, and only then is L-thyroxine started — otherwise an Addisonian crisis is provoked.
• Vitiligo, alopecia areata, autoimmune gastritis / pernicious anemia. Clinical examination. Gastric parietal cell antibodies in AIT — 20–30%, overt pernicious anemia — 2–5%, B12 deficiency — 10–15%. With macrocytic anemia (MCV > 100) or paresthesias — B12 + folate + homocysteine and methylmalonic acid + anti-parietal antibodies + anti-IF (intrinsic factor). Symptoms overlap with hypothyroidism, so hypothyroidism "not responding to L-thyroxine" is often unrecognized B12 deficiency.
• Rheumatoid arthritis, SLE, Sjögren's syndrome, multiple sclerosis, autoimmune hepatitis. The risk of each in AIT is 2–5 times higher than in the general population. With arthralgias, sicca symptoms or rash — ANA, RF, anti-CCP. Not routine screening, only with symptoms.

Special scenario — patients with type 1 diabetes. The prevalence of anti-TPO positivity in this group is 15–30%, so anti-TPO + TSH screening at T1D diagnosis is recommended by the American Diabetes Association, with retesting every 2–3 years.

Turner and Down syndromes are genetic syndromes with a high rate of AIT (up to 30% in Down syndrome, up to 50% in Turner syndrome). Anti-TPO + TSH screening every year from childhood.

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Laboratory Artifacts That Distort Anti-TPO

Apart from inter-laboratory variability of assays, several technical factors give falsely high or falsely low anti-TPO results.

Heterophile antibodies and HAMA (human anti-mouse antibodies). In 0.05–0.5% of the general population and up to 3–5% of patients who work with laboratory mice or received mouse monoclonal antibodies, there are antibodies that bind mouse immunoglobulins in the assay. The result is falsely high anti-TPO with zero real activity. Suspect it when the anti-TPO titer does not match the clinical picture and ultrasound. Solution — retest in another lab on a different assay or with a blocking reagent (heterophile blocking tube).

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Who should see a doctor first

• Isolated anti-TPO positive without ultrasound findings and clinical signs is not a diagnosis of AIT. It is a lab finding. Do not start L-thyroxine based on antibodies alone.
• Postpartum period. Transient thyrotoxicosis 1–3 months postpartum is often mistaken for Graves. Differentiation: TRAb negative, anti-TPO positive, phase self-limited. Do not start antithyroid drugs.
• Pregnancy. With anti-TPO positive, miscarriage risk is increased even at euthyroidism. Target TSH in the first trimester: < 2.5 mIU/L. Monitoring — per the schedule in the pregnancy section, and at 3 and 6 months postpartum.
• Amiodarone, lithium, interferon — provocative drugs. Before initiation — anti-TPO + TSH, during treatment — every 3–6 months.
• Iodine without cofactor preparation in anti-TPO positive patients worsens the autoimmune process. Selenium for 4–8 weeks first, then iodine. Without preparation, "iodine supplements" flare Hashimoto.
• Suspected adrenal insufficiency. L-thyroxine is not started without prior hydrocortisone replacement — otherwise an adrenal crisis is provoked.
• Signs of selenium excess — hair loss (AIT itself causes it too — tell them apart), garlic breath, brittle nails, peripheral neuropathy: check plasma selenium, and at > 190 mcg/L stop intake until it normalizes.
• Glucocorticoids are not indicated for lowering the anti-TPO titer — side effects outweigh the benefit.

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Bottom line

Anti-TPO is the key to diagnosis, not the target of therapy.

The diagnosis of autoimmune thyroiditis per ATA Guideline is a combination: anti-TPO positive + typical ultrasound pattern (diffuse hypoechogenicity, heterogeneity) + clinical (or subclinical) hypothyroidism. If any one of the three is missing, the diagnosis is not yet established. You can go over your own results at a consultation.

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About the Author

I am Dr. Vladimir Pereligyn, endocrinologist and researcher. I specialize in endocrine, metabolic, and autoimmune protocols with a holistic approach and individualized lab diagnostics. Book a consultation — universum.earth/consultation. Daily clinical breakdowns — @md_pereligyn_thyroid.

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Sources

• Alexander EK et al. 2017 Guidelines of the American Thyroid Association for Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017. PMID 28056690