Thyroid Autoantibodies - MS ENT PG Exam Notes

Overview

Key Caveat (Scott-Brown's): Thyroid antibodies are found in ~10% of the healthy, euthyroid population. Measure them only if the result will alter management.

The Three Major Thyroid Autoantibodies

1. Anti-TPO (Antithyroperoxidase / Antimicrosomal Antibody)

• Diagnose autoimmune thyroid disease (especially Hashimoto's thyroiditis)
• Predict progression from subclinical to overt hypothyroidism:
  • Isolated elevated TSH alone: 2% risk/year of progression
  • Isolated positive Anti-TPO alone: 2% risk/year
  • Both elevated TSH + positive Anti-TPO: 5% risk/year (Scott-Brown's)
  • Cummings quotes 3-5% per year in those with positive AMA + elevated TSH
• Risk factor for drug-induced thyroid problems - amiodarone, alpha-interferon, lithium use can trigger dysfunction in those who are Anti-TPO positive
• Risk factor for postpartum thyroiditis

2. Anti-Tg (Antithyroglobulin Antibody)

1. Monitoring differentiated thyroid cancer (DTC) - most important ENT use:
   • After total thyroidectomy, serum thyroglobulin (Tg) serves as a tumor marker
   • Anti-Tg antibodies interfere with radioimmunoassay (RIA) for Tg - may artificially raise or lower the Tg reading
   • Anti-Tg antibodies are present in 15-20% of DTC patients (vs 10% general population - so DTC has slightly higher prevalence)
   • When Anti-Tg is present: concurrent measurement of both Anti-Tg titer and Tg is needed
   • A gradual decline in Anti-Tg titer over time after complete thyroidectomy = good prognostic sign (suggests no residual tissue)
   • A rising or persistent Anti-Tg titer suggests persistent thyroid tissue or disease recurrence
2. Marker for malignancy risk in thyroid nodules: Presence of Anti-Tg antibodies has been associated with higher risk of malignancy in patients with a thyroid nodule (Scott-Brown's)
3. Note: Almost all patients with Tg antibodies also have TPO antibodies, but the reverse is much less common. Therefore, many labs only routinely offer TPO antibody measurement for diagnosis of autoimmune thyroid disease.

3. Anti-TSHR (TSH Receptor Antibody / TRAb)

• cAMP-generating assay = detects TSI (thyroid-stimulating immunoglobulin) - positive in 90-95% of hyperthyroid Graves' patients
• Radioreceptor / binding assay = detects TBI (TSH-binding inhibitory) antibodies - positive in 85% of hyperthyroid Graves' patients

Functional Types of TSHR Antibodies

Important: The assay for TRAb does NOT distinguish stimulating from blocking antibodies. You determine which type is acting by the clinical/biochemical picture:

• Thyrotoxic + positive TRAb = Graves' (stimulatory antibodies)
• Hypothyroid + positive TRAb = Blocking antibodies
• Occasionally a patient switches from one type to the other - can cause confusing clinical picture!

Clinical Indications for Measuring TRAb (Scott-Brown's + Cummings)

1. Confirm diagnosis of Graves' disease - especially when diagnosis is not evident clinically (e.g., no goiter, atypical presentation)
2. Planning treatment: If antithyroid drugs (ATDs) are being considered for cure, confirming Graves' disease is mandatory. (If radioiodine is being used, TRAb positivity matters less)
3. Predict neonatal Graves' disease: Measure TRAb in pregnant women previously treated for Graves' disease - transplacental passage of TRAb can cause neonatal hyperthyroidism
4. Diagnosis of euthyroid Graves' ophthalmopathy - TRAb positive even when thyroid function is normal
5. Predict remission after ATD treatment: Persistently raised TRAb = adverse sign for successful ATD-induced remission (though not specific enough for routine individual use)
6. High TRAb levels = independent risk factor for development of Graves' ophthalmopathy
7. Monitor Graves' ophthalmopathy (TSI specifically used here)

Quick Summary Table

Exam High-Yield Points

• Anti-TPO = most sensitive marker for autoimmune thyroid disease (~95%); originally called antimicrosomal antibody
• Anti-Tg interferes with thyroglobulin RIA - always co-measure in DTC follow-up
• TRAb (LATS) = originally discovered in 1950s; characterized as 7S immunoglobulin; 2 assay types: cAMP method (TSI) and radioreceptor method (TBI)
• TSH receptor antibodies are almost never required for routine diagnosis of suspected Graves' disease - but very useful in atypical cases, pregnant women with prior Graves', and euthyroid Graves' ophthalmopathy
• Thyroid antibodies found in 10% of healthy euthyroid population - positive result alone doesn't mean disease
• Declining Anti-Tg titer after thyroidectomy for DTC = good prognostic sign (no residual tissue)
• Elevated TSH + positive Anti-TPO in subclinical hypothyroidism = 5% per year risk of overt hypothyroidism