A 35-year-old woman from Mumbai presents with a 2-month history of a painless, progressively enlarging thyroid nodule. On examination, the nodule is firm, fixed, and approximately 3 cm. There is ipsilateral cervical lymphadenopathy. Thyroid function tests are normal. Fine-needle aspiration cytology (FNAC) shows follicular cells arranged in a microfollicular pattern with increased mitotic activity and areas of necrosis. What is the most likely diagnosis?

Explanation

## Diagnosis: Follicular Thyroid Carcinoma ### Clinical Presentation **Key Point:** Follicular thyroid carcinoma (FTC) typically presents as a solitary, firm, fixed thyroid nodule with slow growth over months to years. Cervical lymphadenopathy is common but distant metastases (bone, lung) are more frequent than nodal involvement. ### Cytological Features **High-Yield:** FNAC findings in FTC include: - Microfollicular or solid architecture - Increased cellularity and mitotic activity - Areas of necrosis - Absence of papillary structures (which would indicate papillary carcinoma) - Nuclear grooves and pseudoinclusions are NOT present (these are papillary carcinoma features) ### Diagnostic Distinction | Feature | Follicular | Papillary | Medullary | Anaplastic | |---------|-----------|-----------|-----------|------------| | **FNAC Pattern** | Microfollicular, solid | Papillary fronds, nuclear grooves | Spindle cells, amyloid | High cellularity, pleomorphism | | **Age at Presentation** | 40–50 years | 30–40 years | Any age (familial) | 60+ years | | **Lymph Node Involvement** | Uncommon | Common (50%) | Common | Common | | **Distant Metastases** | Bone, lung (early) | Late | Bone, liver | Early, aggressive | | **Prognosis (10-year)** | 85% | 93% | 50–80% | <5% | ### Key Pathological Findings in FTC **Clinical Pearl:** FTC is diagnosed by **capsular and/or vascular invasion** on histology, NOT cytology alone. FNAC cannot reliably differentiate benign follicular adenoma from FTC — this requires surgical histopathology. **Mnemonic: FTC Features — "MINC"** - **M**icrofollicular architecture - **I**nvasion (capsular or vascular) - **N**ecrosis and increased mitoses - **C**ellular atypia ### Why This Case Points to FTC 1. Firm, fixed nodule with slow progression 2. Microfollicular pattern on FNAC with increased mitoses and necrosis 3. Absence of papillary structures rules out papillary carcinoma 4. Age (35 years) and presentation fit FTC better than anaplastic (which is aggressive in elderly) 5. Medullary carcinoma would show spindle cells and amyloid, not microfollicles [cite:Robbins 10e Ch 24]