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Difficulty: MediumOrdering Laboratory Tests

During a routine health maintenance visit, a 48-year-old woman reports a two-year history of progressive joint pain and generalized fatigue. She casually mentions that she recently had to have her wedding band resized and was forced to purchase larger shoes. On review of systems, she notes persistent, excessive daily diaphoresis. Her blood pressure is 148/92 mmHg, and her heart rate is 76/min. Physical examination is notable for prominent supraorbital ridges, a widened jawline, and enlarged, doughy hands. Which of the following is the most appropriate initial step in laboratory evaluation?

  1. A
    Magnetic resonance imaging of the pituitary gland
  2. B
    Measurement of a random serum growth hormone level
  3. Measurement of serum insulin-like growth factor-1Answer
  4. D
    Oral glucose tolerance testing with growth hormone measurement
  5. E
    Measurement of serum thyroid-stimulating hormone

Answer

Measurement of serum insulin-like growth factor-1
The patient's presentation of enlarged extremities, structural facial changes, hypertension, and diaphoresis is classic for acromegaly, a condition caused by a growth hormone-secreting pituitary adenoma. The initial step in the diagnostic workup is always the measurement of serum insulin-like growth factor-1 (IGF-1). Because growth hormone is secreted in a pulsatile fashion and has a short half-life, random levels are unreliable. IGF-1, which is produced by the liver in response to growth hormone, has a long half-life and accurately reflects total integrated growth hormone secretion. If the IGF-1 level is elevated, the diagnosis is subsequently confirmed with an oral glucose tolerance test demonstrating a failure to suppress growth hormone.

Step-by-Step Solution

1
Analyze the clinical vignette to identify the underlying syndrome.
The combination of acral enlargement (increased ring and shoe size), coarse facial features (prognathism, prominent ridges), diaphoresis, and secondary hypertension is highly specific for acromegaly.
Accurate syndrome recognition is required before ordering targeted diagnostic investigations.
2
Determine the standard diagnostic algorithm for suspected acromegaly.
The workup requires a highly sensitive biochemical screening test as the first step, followed by a confirmatory dynamic suppression test if the screen is abnormal.
Test sequencing must progress from non-invasive screening to definitive diagnostic confirmation.
3
Select the correct initial laboratory screening test.
Serum insulin-like growth factor-1 (IGF-1) is chosen as the initial test.
IGF-1 accurately reflects integrated growth hormone secretion over time and avoids the pitfalls of pulsatile hormone release.

Key Concept

Initial laboratory evaluation of suspected acromegaly requires measurement of serum insulin-like growth factor-1 (IGF-1).
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Ordering Laboratory Tests

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