A 42-year-old woman from Mumbai presents with a 2-month history of a painless, firm nodule in the lower pole of the right thyroid lobe. She denies dysphagia, dyspnea, or voice changes. Physical examination reveals a solitary 1.5 cm nodule with no cervical lymphadenopathy. TSH is normal. Fine-needle aspiration cytology (FNAC) shows follicular cells arranged in a microfollicular pattern with increased mitotic activity and focal areas of capsular invasion. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: Follicular Carcinoma of Thyroid ### Clinical Presentation The patient presents with a solitary thyroid nodule without lymphadenopathy or distant metastases—a classic presentation of follicular carcinoma. The absence of voice changes or dysphagia suggests no local invasion at presentation. ### FNAC Findings **Key Point:** Follicular carcinoma cannot be definitively diagnosed by FNAC alone because cytology cannot reliably distinguish between follicular adenoma and follicular carcinoma. However, the presence of **capsular invasion** on FNAC is highly suggestive of malignancy. **High-Yield:** The distinction between follicular adenoma and follicular carcinoma is **histological**, not cytological: - **Follicular adenoma:** Encapsulated, no capsular or vascular invasion - **Follicular carcinoma:** Encapsulated WITH capsular and/or vascular invasion The microfollicular pattern with increased mitotic activity and focal capsular invasion described in this case is diagnostic of follicular carcinoma. ### Pathological Features of Follicular Carcinoma | Feature | Follicular Adenoma | Follicular Carcinoma | |---------|-------------------|----------------------| | Capsule | Present, intact | Present, invaded | | Vascular invasion | Absent | Present (diagnostic) | | Mitotic activity | Low | Increased | | Lymph node involvement | No | Rare | | Distant metastases | No | Common (bone, lung) | | Prognosis | Excellent | Good to fair | ### Clinical Pearl Follicular carcinoma typically spreads **hematogenously** (to bone and lung) rather than lymphatically, explaining the absence of cervical lymphadenopathy in this case. This contrasts sharply with papillary carcinoma, which spreads to regional lymph nodes early. ### Management Once follicular carcinoma is diagnosed on histology (after thyroidectomy), the patient requires: 1. Total thyroidectomy (if not already done) 2. Radioactive iodine (RAI) ablation for intermediate/high-risk tumors 3. TSH suppression with levothyroxine 4. Long-term follow-up with thyroglobulin monitoring [cite:Robbins 10e Ch 24]