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Difficulty: MediumOrdering Imaging Studies

A 48-year-old woman presents to the outpatient clinic for evaluation of a newly discovered neck lump. She has felt increasingly anxious over the past month and reports a 4-kg (8.8-lb) weight loss despite a normal appetite. Vital signs include a blood pressure of 132/76 mm Hg and a heart rate of 105/min. Physical examination reveals a firm, mobile, non-tender nodule on the right side of the anterior neck. A targeted bedside diagnostic ultrasound confirms the presence of a 2.5-cm solid, well-circumscribed nodule in the right thyroid lobe, with no suspicious sonographic features and no cervical lymphadenopathy. Laboratory testing reveals a thyroid-stimulating hormone (TSH) concentration of 0.05 μ\mu U/mL (normal: 0.5–5.0 μ\mu U/mL) and a free thyroxine (T4) concentration of 2.8 ng/dL (normal: 0.9–1.7 ng/dL).

Which of the following imaging modalities is the most appropriate next step in the diagnostic workup?

  1. Radionuclide thyroid scintigraphyAnswer
  2. B
    Ultrasound-guided fine-needle aspiration biopsy
  3. C
    Computed tomography of the neck with intravenous contrast
  4. D
    Magnetic resonance imaging of the neck without contrast
  5. E
    18F-fluorodeoxyglucose positron emission tomography scan

Answer

Radionuclide thyroid scintigraphy
The most appropriate next diagnostic step is radionuclide thyroid scintigraphy. When a thyroid nodule is discovered, the initial evaluation mandates a serum TSH measurement and a dedicated neck ultrasound (which this patient has already had). Because this patient's TSH is subnormal (indicating primary hyperthyroidism), the next step is to determine if the nodule is autonomously producing thyroid hormone. A radionuclide scan using radioactive iodine (I-123) or technetium-99m will show whether the nodule is 'hot' (hyperfunctioning, high uptake) or 'cold' (nonfunctioning, low uptake). Hot nodules are almost universally benign and do not require biopsy, whereas cold nodules carry a higher malignancy risk and generally require fine-needle aspiration.

Step-by-Step Solution

1
Identify the primary clinical problem from the vignette.
The patient has a 2.5-cm thyroid nodule with overt primary hyperthyroidism, characterized by a suppressed TSH and elevated free T4.
Establishing the baseline clinical and hormonal status dictates the subsequent diagnostic imaging algorithm.
2
Apply the evidence-based diagnostic algorithm for evaluating a thyroid nodule.
If the serum TSH is normal or elevated, one proceeds directly to biopsy criteria based on size and ultrasound features. However, if the TSH is subnormal, the functional status of the nodule must be assessed first.
Hyperfunctioning (toxic) nodules are rarely malignant, whereas nonfunctioning nodules carry a higher risk of malignancy and require further evaluation.
3
Select the appropriate imaging modality to evaluate the functional status of the nodule.
Radionuclide thyroid scintigraphy is selected.
This study differentiates a hyperfunctioning 'hot' nodule (typically a benign toxic adenoma) from a nonfunctioning 'cold' nodule (which would require subsequent fine-needle aspiration).

Key Concept

Evaluation of a thyroid nodule with a suppressed TSH requires radionuclide thyroid scintigraphy to determine the functional status of the nodule before considering any invasive testing.
Estimated Time:1m 15s
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Ordering Imaging Studies

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