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Difficulty: MediumOrdering Laboratory Tests

A 4242-year-old woman presents to the clinic after noticing a lump in her neck while applying makeup. She reports feeling generally well and denies any recent weight changes, palpitations, heat intolerance, or difficulty swallowing. Vital signs are within normal limits. Physical examination reveals a solitary, firm, non-tender, 2.52.5 cm nodule in the right lobe of the thyroid gland that moves with swallowing. No cervical lymphadenopathy is appreciated. Which of the following is the most appropriate next step in laboratory evaluation?

  1. Serum thyroid-stimulating hormone (TSH) levelAnswer
  2. B
    Serum free thyroxine (T4) and triiodothyronine (T3) levels
  3. C
    Serum thyroglobulin measurement
  4. D
    Anti-thyroid peroxidase and anti-thyroglobulin antibodies
  5. E
    Serum calcitonin measurement

Answer

The most appropriate initial laboratory test is a serum thyroid-stimulating hormone (TSH) level to determine the functional status of the nodule.
The correct answer is a serum thyroid-stimulating hormone (TSH) measurement. According to clinical guidelines from the American Thyroid Association, the initial evaluation of a newly discovered thyroid nodule should always include a serum TSH level. The TSH level determines the subsequent diagnostic algorithm. If the TSH is subnormal (indicating hyperthyroidism), a radionuclide thyroid scan is the next step to evaluate for an autonomously functioning 'hot' nodule, which rarely harbors malignancy and generally does not require fine-needle aspiration (FNA). If the TSH is normal or elevated, a diagnostic neck ultrasound should be performed (if not already done) to assess sonographic features and determine if FNA biopsy is indicated based on size and suspicious characteristics. Measuring TSH is the most cost-effective and sensitive initial step.

Step-by-Step Solution

1
Identify the primary clinical problem from the patient's presentation.
The patient has a newly discovered, asymptomatic, solitary 2.52.5 cm thyroid mass without clinical signs of hyperthyroidism or hypothyroidism.
Establishing the clinical baseline helps determine the pre-test probability of various thyroid pathologies and rules out immediate acute complications.
2
Recall the standard diagnostic algorithm for evaluating a new thyroid nodule.
Guidelines dictate that both structural assessment (via ultrasound) and functional assessment (via laboratory testing) are required.
The functional status of the nodule dictates the subsequent diagnostic pathway, specifically whether a radionuclide scan or fine-needle aspiration (FNA) is indicated next.
3
Select the single best initial laboratory test to assess thyroid function.
Serum TSH is selected as the most appropriate laboratory evaluation.
TSH is the most sensitive screening test for primary thyroid dysfunction. A subnormal TSH prompts a radionuclide scan, while a normal or elevated TSH typically prompts progression to diagnostic ultrasound and potential FNA.

Key Concept

In the initial laboratory evaluation of a newly discovered thyroid nodule, measurement of serum thyroid-stimulating hormone (TSH) is the most appropriate first step to guide further diagnostic imaging and biopsy decisions.
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Ordering Laboratory Tests

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