What is the most common type of thyroid cancer associated with Hashimoto (Hashimoto's thyroiditis)?

Papillary Thyroid Cancer is the Most Common Thyroid Cancer Associated with Hashimoto's Thyroiditis

The answer is A. Papillary thyroid cancer is the most common type of thyroid cancer associated with Hashimoto's thyroiditis. This association has been well-documented in multiple studies, with significant evidence supporting this relationship.

Evidence for Papillary Thyroid Cancer and Hashimoto's Association

• Systematic reviews have found a relative risk (RR) of Hashimoto's thyroiditis among papillary thyroid cancer of 2.36 (95% CI 1.55-3.29, p < 0.001), and an RR of papillary thyroid cancer among Hashimoto's thyroiditis of 1.40 (95% CI 1.07-1.85, p = 0.016) 1
• In a large retrospective cohort study of 9,210 patients with papillary thyroid cancer, 1,751 (19%) had coexistent Hashimoto's thyroiditis, demonstrating the frequent co-occurrence of these conditions 2
• Multiple studies have consistently shown that papillary thyroid carcinoma (PTC) is the main type of malignancy associated with Hashimoto's thyroiditis, accounting for up to 96.2% of thyroid cancers in patients with this autoimmune condition 3

No Association with Other Thyroid Cancer Types

• No significant association has been found between Hashimoto's thyroiditis and follicular, medullary, or anaplastic thyroid cancer 1
• While thyroid lymphoma has been associated with Hashimoto's thyroiditis (RR of 9.74,95% CI 3.93-24.13), it is much rarer than papillary thyroid cancer in patients with Hashimoto's 1

Clinical Characteristics of Papillary Thyroid Cancer with Hashimoto's Thyroiditis

• Patients with coexistent Hashimoto's thyroiditis and papillary thyroid cancer typically present with less aggressive disease characteristics compared to those without Hashimoto's 2

• These patients show:

  • Smaller primary tumor size (less likely to have tumors ≥4 cm) 2
  • Less frequent gross extrathyroidal extension 2
  • Less frequent extranodal extension 2
  • Lower rates of distant metastasis 2

• However, in some endemic goiter areas, papillary thyroid cancer with Hashimoto's may still exhibit aggressive features, including lymph node invasion and extrathyroidal extension 3

Prognostic Implications

• Patients with coexistent Hashimoto's thyroiditis and papillary thyroid cancer generally have better outcomes:

  • Superior 10-year disease-specific survival (99.9% vs 96.6%) 2
  • Better recurrence-free survival (92.0% vs 87.6%) 2
  • Decreased PTC-related mortality (HR 0.19,95% CI 0.05-0.76) 2

• The protective effect of Hashimoto's thyroiditis appears particularly strong in patients with extrathyroidal extension or after total thyroidectomy 2

Clinical Implications

• When evaluating patients with Hashimoto's thyroiditis, clinicians should maintain vigilance for thyroid nodules given the increased risk of papillary thyroid cancer 4
• Although papillary thyroid cancer with Hashimoto's generally has a better prognosis, these tumors are more frequently multifocal (46.2%), which may influence surgical planning 3, 4
• The presence of Hashimoto's thyroiditis should be considered a relevant clinical factor when assessing risk and prognosis in patients with thyroid nodules or confirmed papillary thyroid cancer 5