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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 48-year-old man presents to the ambulatory clinic complaining of excessive sweating, diffuse joint pain, and an increase in his shoe size over the past two years. He notes that he recently had to have his wedding ring resized because it no longer fit his finger. He has no significant past medical history and takes no medications. Vital signs show a blood pressure of 158/96 mm Hg, pulse of 76/min, and respiratory rate of 14/min. Physical examination reveals coarse facial features, a prominent forehead, and macroglossia. There is a 'doughy' consistency to the skin on his hands. Laboratory testing reveals a serum insulin-like growth factor-1 (IGF-1) concentration of 780 ng/mL (reference range: 90-250 ng/mL). Which of the following is the most appropriate next step in this patient's diagnostic workup?

  1. Measurement of serum growth hormone concentration following an oral glucose loadAnswer
  2. B
    Magnetic resonance imaging of the pituitary gland
  3. C
    Measurement of a single random serum growth hormone concentration
  4. D
    Formal visual field testing by perimetry
  5. E
    Measurement of 24-hour urinary free cortisol excretion

Answer

Measurement of serum growth hormone concentration following an oral glucose load
The correct sequence for diagnosing acromegaly begins with a clinical suspicion, followed by measuring serum insulin-like growth factor-1 (IGF-1) as the initial screening test. Because growth hormone (GH) is secreted in a pulsatile manner, its levels fluctuate widely, making IGF-1 (which provides a stable, integrated marker of GH activity) the preferred initial test. If the IGF-1 level is elevated, the diagnosis MUST be confirmed biochemically before proceeding to imaging. The gold standard confirmatory test is the measurement of serum GH following an oral glucose load (oral glucose tolerance test, or OGTT). Normal individuals will suppress GH to undetectable levels after glucose ingestion, whereas patients with acromegaly will exhibit non-suppressible, elevated GH levels. Only after this biochemical confirmation is achieved should anatomical imaging (MRI of the pituitary) be performed to locate the suspected somatotroph adenoma.

Step-by-Step Solution

1
Identify the suspected diagnosis based on the clinical presentation and initial laboratory findings.
The patient has classical features of acromegaly (acral enlargement, coarse facial features, hyperhidrosis) and an elevated screening IGF-1 level.
To determine the current stage in the diagnostic algorithm.
2
Determine the standard diagnostic sequence for suspected acromegaly following an abnormal initial screening test.
Biochemical confirmation is strictly required before proceeding to anatomical imaging.
To prevent false-positive diagnoses caused by incidental pituitary adenomas, which are present in up to 20% of the general population.
3
Select the appropriate confirmatory biochemical test.
The oral glucose tolerance test (OGTT) with growth hormone measurement is chosen.
In a healthy individual, a glucose load suppresses growth hormone release. In a patient with acromegaly, growth hormone remains autonomously elevated (non-suppressible), confirming the diagnosis.

Key Concept

In the diagnostic test sequencing for acromegaly, an elevated screening serum IGF-1 level must be followed by confirmatory biochemical testing with an oral glucose tolerance test (OGTT) prior to obtaining a pituitary MRI.
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Diagnostic Workup and Test Sequencing

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