A 29-year-old primigravida at 15 weeks of gestation is referred to the surgery clinic for … — USMLE Step 2 CK

A $29$ -year-old primigravida at $15$ weeks of gestation is referred to the surgery clinic for evaluation of a prominent neck lump. The patient noticed the swelling three weeks ago. She reports mild heat intolerance and fatigue but feels otherwise well. Her pregnancy has been uncomplicated. Vital signs show a blood pressure of $118/74\text{ mmHg}$ and a pulse of $92\text{/min}$ . Physical examination reveals a palpable, firm, $2.4\text{-cm}$ nodule in the right thyroid lobe that moves with swallowing. No cervical lymphadenopathy is present.

Laboratory results are as follows:
Thyroid-stimulating hormone (TSH): $0.05\text{ \mu U/mL}$ (Reference: $0.4\text{--}4.0\text{ \mu U/mL}$ )
Free thyroxine (T4): $1.9\text{ ng/dL}$ (Reference: $0.8\text{--}1.8\text{ ng/dL}$ )

An ultrasound of the neck shows a $2.4\text{-cm}$ completely solid, hypoechoic nodule with irregular borders and internal microcalcifications in the right lobe.

Which of the following is the most appropriate next step in the management of this patient?

A
Radionuclide thyroid scintigraphy
Fine-needle aspiration biopsy of the noduleAnswer
C
Right thyroid lobectomy
D
Initiation of levothyroxine therapy
E
Observation with repeat ultrasound in $6$ months

Answer

The most appropriate next step in management is a fine-needle aspiration biopsy of the nodule.

The standard evaluation of a thyroid nodule accompanied by a suppressed TSH involves radionuclide thyroid scintigraphy to determine if the nodule is autonomously functioning (a 'hot' nodule), which carries an exceptionally low risk of malignancy and does not typically require biopsy. However, radioactive isotopes cross the placenta and can permanently destroy the developing fetal thyroid gland. Therefore, radionuclide scanning is absolutely contraindicated during pregnancy. In the absence of a scan to confirm a benign functioning adenoma, the physician must rely on sonographic features to guide management. This patient's ultrasound demonstrates multiple features highly suspicious for malignancy, including a solid composition, hypoechogenicity, irregular borders, and microcalcifications. Consequently, the correct next step is to perform a fine-needle aspiration (FNA) biopsy to establish a tissue diagnosis and rule out thyroid cancer.

Step-by-Step Solution

Analyze the laboratory results in the context of the clinical presentation.

The patient has a suppressed TSH and elevated free T4, indicating thyrotoxicosis, which may be gestational or related to an autonomously functioning thyroid nodule.

Establishing the functional status of a thyroid nodule dictates the diagnostic algorithm.

Recall the standard diagnostic algorithm for a thyroid nodule with a suppressed TSH.

The standard next step is radionuclide thyroid scintigraphy to differentiate a 'hot' (hyperfunctioning, rarely malignant) nodule from a 'cold' nodule.

This avoids unnecessary biopsies for benign functioning adenomas.

Adjust the standard algorithm based on the patient's physiological state (pregnancy).

Radionuclide imaging (I-123, I-131, or Tc-99m) is strictly contraindicated in pregnancy due to the risk of irreversible fetal thyroid ablation.

Pregnant patients cannot undergo standard radioactive diagnostic pathways.

Determine the alternative next step based on ultrasound findings.

Because a scan cannot be safely performed to rule out malignancy, and the ultrasound shows highly suspicious features (solid, hypoechoic, microcalcifications, irregular margins), a fine-needle aspiration (FNA) biopsy must be performed.

Tissue diagnosis is mandated to rule out thyroid cancer (e.g., papillary thyroid carcinoma) when suspicious sonographic features are present and a benign 'hot' nodule cannot be radiographically confirmed.

Key Concept

Management of suspicious thyroid nodules in pregnancy

Estimated Time:1m 30s

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