Interactive Case: Thyroid Nodule

A guided walkthrough of a benign thyroid aspirate.

Clinical History

A 73-year-old woman with an incidental 1.7 cm left thyroid nodule found on imaging. A fine-needle aspiration was performed.

Step 1: Background Population

What cell type makes up the predominant background population in this smear?

Lymphocytes Neutrophils Atypical follicular cells Red blood cells

Rationale: The background is composed of a mixed population of small, mature lymphocytes and scattered larger lymphoid cells. This finding is the hallmark of lymphocytic (Hashimoto's) thyroiditis.

Step 2: Epithelial Population

Mixed with the lymphocytes, there are clusters of large epithelial cells. What are these cells called?

Oncocytes / Hürthle cells Papillary carcinoma cells Squamous metaplasia Parathyroid cells

Rationale: These are Hürthle cells, also known as oncocytes. They are metaplastic follicular cells that become enlarged and filled with mitochondria in response to the chronic inflammation of thyroiditis. They are a very common finding in this condition.

Step 3: Oncocytic Cell Details

What are the classic cytologic features of these Hürthle cells? (Select all that apply)

Abundant, granular, grey-blue cytoplasm Round, centrally located nucleus Prominent central nucleolus Nuclear grooves and pseudoinclusions Scant, clear cytoplasm

Rationale: Hürthle cells are defined by their abundant, granular cytoplasm (due to mitochondria), round nuclei, and prominent, often cherry-red, central nucleoli. The absence of nuclear features of papillary carcinoma (grooves, pseudoinclusions) is a key feature for a benign diagnosis.

Step 4: Other Findings

What is this thick, amorphous material with a "cracked" appearance?

Colloid Amyloid Fibrin Necrotic debris

Rationale: This is colloid, the proteinaceous material stored in thyroid follicles. Its presence is a reassuring feature of a benign process. The "cracked" artifact is common in air-dried smears.

Step 5: Final Diagnosis

Considering the background lymphocytes, Hürthle cells, and colloid, what is the final diagnosis?

Chronic Lymphocytic Thyroiditis (Bethesda II) Hürthle Cell Neoplasm (Bethesda IV) Papillary Thyroid Carcinoma (Bethesda VI) Atypia of Undetermined Significance (Bethesda III)

Rationale: The triad of background lymphocytes, Hürthle cells, and colloid is classic for Chronic Lymphocytic (Hashimoto's) Thyroiditis. Since this is a benign inflammatory condition, the diagnosis falls under The Bethesda System for Reporting Thyroid Cytopathology as Category II (Benign).

Final Diagnosis: Benign Thyroid Nodule (Bethesda II)

These findings are consistent with chronic lymphocytic thyroiditis (Hashimoto's thyroiditis).

The diagnosis is established by the presence of a prominent lymphocytic infiltrate, sheets of benign Hürthle cells, and background colloid. Therefore, the nodule is categorized as Bethesda Category II (Benign).