A 52-year-old man presents with a 3 cm solitary thyroid nodule, fine-needle aspiration cytology (FNAC) shows Bethesda Category VI (malignant), and imaging confirms no distant metastases. He undergoes total thyroidectomy. Histopathology reveals follicular thyroid carcinoma (FTC) with vascular invasion. What is the drug of choice for post-operative adjuvant therapy?

Explanation

## Post-Operative Management of Follicular Thyroid Carcinoma ### Risk Stratification in FTC **Key Point:** Follicular thyroid carcinoma with vascular invasion is classified as **intermediate to high-risk disease**. The presence of vascular invasion significantly increases the risk of distant metastases (particularly to bone and lung) and warrants aggressive adjuvant therapy. ### Why Radioactive Iodine (I-131)? 1. **FTC is radioiodine-avid:** Unlike anaplastic or medullary thyroid carcinoma, FTC cells retain the ability to concentrate iodine, making RAI an effective ablative therapy. 2. **Vascular invasion indicates higher recurrence risk:** RAI reduces recurrence and improves survival in intermediate/high-risk FTC. 3. **Dual benefit:** RAI ablates residual thyroid tissue AND treats occult metastases (which may not be visible on imaging). 4. **Standard protocol:** RAI is given 4–6 weeks post-operatively after TSH preparation (as described in Question 1), followed by long-term levothyroxine suppression. ### Comparison of Adjuvant Strategies in FTC | Risk Category | Vascular Invasion | Age | Tumor Size | Recommended Therapy | | --- | --- | --- | --- | --- | | Low-risk | Absent | <45 | <4 cm | Levothyroxine suppression alone | | Intermediate-risk | Present OR absent | Any | 4–10 cm | **RAI + levothyroxine suppression** | | High-risk | Present | >45 | >4 cm | **RAI + levothyroxine suppression** | **High-Yield:** **Vascular invasion is the single most important adverse prognostic factor in FTC.** Its presence mandates RAI therapy regardless of other factors. ### Long-Term Suppressive Therapy After RAI, levothyroxine is given at suppressive doses (target TSH 0.1–0.5 mIU/L) for 5–10 years to: - Inhibit TSH-driven proliferation of any remaining cancer cells - Reduce recurrence risk - Improve overall survival **Clinical Pearl:** Levothyroxine suppression alone (without RAI) is insufficient for intermediate/high-risk FTC because it does not ablate occult metastases that may not concentrate iodine until TSH is elevated. [cite:Robbins 10e Ch 24; Harrison 21e Ch 397]