Are colloid cysts of the thyroid (thyroid gland) benign?

Colloid Cysts of the Thyroid Are Benign

Colloid cysts of the thyroid are benign lesions that represent the most common type of thyroid nodule. According to the National Comprehensive Cancer Network (NCCN) guidelines, thyroid nodules are extremely common, with approximately 50% of thyroids showing nodules when examined at autopsy, surgery, or with ultrasonography, and these nodules are almost always benign 1.

Characteristics of Colloid Cysts

• Colloid cysts are a variant of colloid nodules, which are categorized as benign (Category II) in the Bethesda System for Reporting Thyroid Cytopathology 1
• They typically contain thick colloid material and may have varying degrees of cystic degeneration
• On ultrasound, they often exhibit:
  • Comet tail artifacts (characteristic finding)
  • Anechoic or mixed composition
  • Well-defined margins
  • TI-RADS 2 classification (risk of malignancy <2%) 2

Diagnostic Evaluation

Fine-needle aspiration (FNA) is the preferred diagnostic procedure for evaluating thyroid nodules 1, 2:

• For cystic nodules, aspirated fluid should be sent for cytological examination
• Bloody fluid is common in cystic lesions (found in 81% of cases) and does not necessarily indicate malignancy 3
• The presence of follicular cells in the aspirate is associated with lower recurrence rates 4

Potential Diagnostic Pitfalls

1. Insufficient sampling: Cystic lesions may yield insufficient material for diagnosis in up to 20% of cases, compared to solid nodules 3
2. False negatives: FNA has slightly lower sensitivity for cystic lesions (88%) compared to solid nodules (100%) 3
3. Cyst recurrence: Absence of follicular cells on initial cytology is associated with higher recurrence rates (odds ratio 3.18) 4

Management Considerations

For confirmed benign colloid cysts:

• Most require no specific treatment if asymptomatic

• Surgical intervention is indicated if the cyst:

  • Causes compressive symptoms
  • Shows significant growth despite benign cytology
  • Recurs after multiple aspirations
  • Is ≥2cm with gradual increase in size 2

• Follow-up recommendations:

  • Initial follow-up one month after any procedure
  • Subsequent assessments at 3,6, and 12 months during the first year
  • Annual follow-up for stable, benign nodules 2

Natural History

• Most benign thyroid nodules, including colloid cysts, tend to grow over time
• An estimated 89% of benign nodules show an increase in volume of ≥15% after 5 years 5
• Cystic nodules generally grow less rapidly than solid nodules 5
• Important: Growth alone is not a reliable predictor of malignancy 5

Conclusion

While colloid cysts of the thyroid are benign, they should be properly evaluated with ultrasound and FNA to confirm their benign nature. Regular monitoring is recommended for any changes in size or characteristics that might warrant intervention.