Thyroid peroxidase antibodies (TPOAb)

Pick Ordered or Not ordered to see the note.

Indicated when

• —TSH above the reference range, to identify autoimmune (Hashimoto's) thyroiditis as the cause and to help predict progression [1, 3]
• —Subclinical hypothyroidism, to help decide whether to treat or monitor, since a positive TPOAb raises the likelihood of progression to overt hypothyroidism [2, 3]
• —Pregnancy planning or early pregnancy with a borderline or mildly elevated TSH, where thyroid autoimmunity affects risk assessment [2]

Not indicated when

• —Routine screening of a normal TSH [1]
• —Repeat testing once a positive result has been established: the result does not change over time in a way that needs re-checking [1]
• —Explaining fatigue or other non-specific symptoms in a patient with a normal TSH and no other risk factors

Why not

TPO antibody positivity is common in the general population, especially in women (roughly one in ten adults; see evidence_notes). By itself it does not diagnose thyroid disease or predict who will develop it with any precision; most people with positive antibodies and a normal TSH never become hypothyroid. Ordered outside the situations above, a positive result mainly adds labelling and worry without changing management, and NICE specifically advises against repeating an already-positive test [1].

Better first step

Order TSH first. Add TPO antibodies once TSH is confirmed abnormal, or when a mildly elevated TSH needs a progression-risk estimate, such as in subclinical hypothyroidism or pregnancy planning [1, 2].

Typical ND rationale

An ND often orders TPO antibodies even with a normal TSH, reasoning that autoimmune thyroiditis (Hashimoto's) can be present and progressing for years before TSH rises, and that finding it early allows monitoring, lifestyle changes, or earlier treatment before symptoms or overt hypothyroidism develop.

Ontario coverage & CONO orderability

OHIP status

unverified

The 2026 Schedule does not list a test explicitly named 'thyroid peroxidase antibody.' The closest listing is L500, an older agglutination-method 'screen for miscellaneous agglutination reactions,' which names 'anti-thyroid (thyroglobulin or microsomal)' antibodies using pre-modern terminology ('microsomal antibody' is the historical name for TPO antibody); a separate line, L535 ('other antibody tests'), also lists 'anti-thyroid' generically. It could not be established from the Schedule text which code, if either, a modern automated TPO immunoassay is billed under, so coverage is recorded as unverified. ND-ordered testing is patient-paid regardless (Reg. 552 s.22).

CONO orderable

Yes — CONO list #118

CONO list item 118, blood: 'Thyroid Peroxidase Antibody.' A separate anti-thyroglobulin antibody assay (distinct from thyroglobulin itself, CONO 117) is not its own line item on the CONO list, so it is noted here as a related alias only, not as separately orderable.

Linked conditions

• Hashimoto's thyroiditis with a normal TSH ↗
• Hypothyroidism ↗

Counselling script

“TPO antibodies are useful once we already know your TSH is abnormal, or if it's borderline and we're deciding whether to treat or just monitor. When the TSH is normal, a positive antibody result doesn't change treatment, since most people with positive antibodies and a normal TSH never go on to develop thyroid disease. What it would do is tell us to keep a closer eye on your TSH over time, which we can do either way.”

Revisit if

• —TSH becomes abnormal or borderline on a future test
• —Pregnancy is being planned or confirmed
• —New symptoms suggestive of thyroid dysfunction develop

References

1. [1]National Institute for Health and Care Excellence (NICE) (2019). Thyroid disease: assessment and management (NG145). linkTPO antibodies are considered for adults with TSH above the reference range, and are not repeated once positive
2. [2]American Association of Clinical Endocrinologists / American Thyroid Association (2012). Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. linkolder guidelineTSH is the primary screening test; thyroid autoantibody status (including TPO) informs the management of subclinical hypothyroidism and pregnancy-related decisions
3. [3]Clinical Endocrinology (Whickham Survey twenty-year follow-up) (1995). The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. linkolder guidelinePositive anti-thyroid antibodies alone were associated with roughly 8-fold (women) to 25-fold (men) higher odds of developing hypothyroidism over 20 years of follow-up
4. [4]Ontario Association of Medical Laboratories (OAML) (2007). Guideline for the Use of Laboratory Tests to Detect Thyroid Dysfunction (CLP 015). linkolder guidelineTesting for thyroid autoantibodies, including TPO antibodies, is part of further investigation of an abnormal TSH result