Which autoimmune antibody is most specific for Hashimoto's thyroiditis and correlates with disease activity?

Question

Accepted Answer

A. Anti-thyroid peroxidase (anti-TPO) antibody. ## Autoimmune Antibodies in Thyroiditis

**Key Point:** Anti-thyroid peroxidase (anti-TPO) antibody is the most specific and sensitive marker for Hashimoto's thyroiditis, present in 90–95% of patients and correlating with disease severity and progression.

### Antibody Profile in Thyroid Autoimmunity

| Antibody | Hashimoto's | Graves' | Subacute | Sensitivity | Specificity |
|----------|-------------|---------|----------|-------------|-------------|
| **Anti-TPO** | +++++ | + | − | 90–95% | Very high |
| **Anti-thyroglobulin** | ++++ | + | − | 60–70% | Moderate |
| **TRAb (anti-TSH-R)** | − | +++++ | − | 90% | Very high |
| **Anti-smooth muscle** | − | − | − | Non-specific | Low |

**High-Yield:** TPO (thyroid peroxidase) is the enzyme responsible for iodination of tyrosine residues in thyroglobulin. Antibodies against TPO are the primary driver of thyroid destruction in Hashimoto's thyroiditis through complement-dependent and antibody-dependent cellular cytotoxicity (ADCC).

**Mnemonic:** **TPO = Thyroid Peroxidase = Hashimoto's** — Remember that anti-TPO is the hallmark of Hashimoto's, while anti-TSH receptor (TRAb) is the hallmark of Graves' disease.

### Clinical Significance

1. **Diagnostic value**: Anti-TPO positivity confirms autoimmune thyroiditis
2. **Disease activity**: Titers correlate with degree of lymphocytic infiltration and thyroid dysfunction
3. **Predictive value**: Presence of anti-TPO in euthyroid individuals predicts future development of hypothyroidism
4. **Pregnancy implications**: Anti-TPO positivity increases risk of postpartum thyroiditis

**Clinical Pearl:** Anti-thyroglobulin antibodies are also present in Hashimoto's but are less specific and less sensitive than anti-TPO. Both may coexist, but anti-TPO alone is sufficient for diagnosis.

Answer

B. Anti-thyroglobulin antibody

Answer

C. Anti-TSH receptor antibody (TRAb)

Answer

D. Anti-smooth muscle antibody (ASMA)

Which autoimmune antibody is most specific for Hashimoto's thyroiditis and correlates with disease activity?

Explanation

Autoimmune Antibodies in Thyroiditis

Antibody Profile in Thyroid Autoimmunity

Clinical Significance

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Antibody	Hashimoto's	Graves'	Subacute	Sensitivity	Specificity
Anti-TPO	+++++	+	−	90–95%	Very high
Anti-thyroglobulin	++++	+	−	60–70%	Moderate
TRAb (anti-TSH-R)	−	+++++	−	90%	Very high
Anti-smooth muscle	−	−	−	Non-specific	Low