A 41-year-old woman is referred to the endocrine surgery clinic after her primary care phy… — USMLE Step 2 CK

A $41$ -year-old woman is referred to the endocrine surgery clinic after her primary care physician palpated a swelling in her lower neck. She has no palpitations, heat intolerance, or unexpected weight changes. Vital signs are within normal limits. Physical examination reveals a $3.5\text{-cm}$ , firm, mobile nodule in the right thyroid lobe. Cervical lymph nodes are not palpable. Laboratory testing shows a thyroid-stimulating hormone (TSH) level of $1.8 \text{ mU/L}$ (normal: $0.4 - 4.0 \text{ mU/L}$ ) and a free thyroxine (T4) level of $1.3 \text{ ng/dL}$ (normal: $0.9 - 1.7 \text{ ng/dL}$ ). A neck ultrasound confirms a $3.5\text{-cm}$ solid, isoechoic nodule in the right lobe with a thin hypoechoic halo. A fine-needle aspiration (FNA) biopsy is performed, and cytology yields a highly cellular specimen composed of follicular cells arranged in microfollicles with scant colloid. No nuclear grooves or pseudoinclusions are identified. Which of the following is the most appropriate next step in management?

Diagnostic right thyroid lobectomyAnswer
B
Total thyroidectomy
C
Radionuclide thyroid scintigraphy
D
Repeat fine-needle aspiration in 6 months
E
Administration of radioactive iodine (I-131) therapy

Answer

Diagnostic right thyroid lobectomy

The correct answer involves performing a diagnostic right thyroid lobectomy. This patient's FNA biopsy reveals a follicular neoplasm (Bethesda Category IV). The distinction between a benign follicular adenoma and a malignant follicular carcinoma relies entirely on identifying transcapsular or vascular invasion under a microscope. Because FNA only samples free cells and cannot assess tissue architecture or the nodule's capsule, cytology alone is insufficient for a definitive diagnosis. Surgical excision, typically via lobectomy, is required to provide the intact nodule capsule for definitive histopathological evaluation. Lobectomy is preferred over total thyroidectomy as the initial step because 70% to 80% of these lesions prove to be benign, and lobectomy minimizes the risks of bilateral recurrent laryngeal nerve injury and permanent hypoparathyroidism.

Step-by-Step Solution

Analyze the Fine-Needle Aspiration (FNA) cytology results.

The cytology demonstrates a highly cellular specimen with microfollicles and scant colloid, lacking nuclear features of papillary carcinoma. This defines a follicular neoplasm (Bethesda Category IV).

Establishing the specific cytologic category dictates the subsequent diagnostic algorithm.

Identify the inherent diagnostic limitation of the cytology.

FNA samples individual cells but cannot evaluate the nodule's architecture. It cannot distinguish between a benign follicular adenoma and a malignant follicular carcinoma.

Malignancy in follicular neoplasms is defined exclusively by capsular and/or vascular invasion, which requires examination of the entire intact tumor capsule.

Determine the safest and most definitive next step in management.

A diagnostic thyroid lobectomy must be performed to provide the pathologist with the entire nodule and its capsule.

Lobectomy provides definitive histopathologic diagnosis while minimizing the morbidity (e.g., recurrent laryngeal nerve injury, hypoparathyroidism) associated with an upfront total thyroidectomy, given that the majority of these lesions are benign.

Key Concept

Management of indeterminate thyroid nodules (Bethesda IV/Follicular Neoplasm)

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic