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Difficulty: EasyOrdering Laboratory Tests

A 42-year-old man comes to the physician reporting a 4-month history of worsening fatigue, unintentional 5-kg (11-lb) weight loss, and a strong craving for salty foods. His blood pressure is 92/58 mm Hg. Physical examination reveals generalized hyperpigmentation, particularly prominent over his knuckles and palmar creases. A diagnosis of primary adrenal insufficiency is suspected. Arrange the following laboratory tests in the most appropriate chronological sequence for the diagnostic workup, from initial biochemical evaluation to confirmation of the most common etiology.

  1. 1Measurement of morning serum cortisol and plasma corticotropin (ACTH) levels
  2. 2Cosyntropin (synthetic ACTH) stimulation test
  3. 3Measurement of serum 21-hydroxylase antibodies

Answer

The correct sequence is: 1) Measurement of morning serum cortisol and plasma corticotropin (ACTH) levels, 2) Cosyntropin (synthetic ACTH) stimulation test, 3) Measurement of serum 21-hydroxylase antibodies.
The correct sequence follows the standard diagnostic algorithm for suspected primary adrenal insufficiency: initial biochemical assessment, followed by dynamic confirmatory testing, and finally etiological determination. Morning (8 AM) serum cortisol and plasma ACTH levels are the first-line screening tests. A low cortisol with a high ACTH points to primary adrenal insufficiency. The cosyntropin (synthetic ACTH) stimulation test is then used to definitively confirm the diagnosis by demonstrating an absent or blunted cortisol response. Finally, once the diagnosis is biochemically established, 21-hydroxylase antibody testing is indicated to identify autoimmune adrenalitis, which accounts for the vast majority of primary cases.

Step-by-Step Solution

1
Identify the suspected diagnosis based on the clinical vignette.
The patient's fatigue, weight loss, salt craving, hypotension, and generalized hyperpigmentation strongly suggest primary adrenal insufficiency (Addison's disease).
Recognizing the clinical syndrome determines the specific diagnostic laboratory algorithm to follow.
2
Determine the first step in the diagnostic testing algorithm.
The initial evaluation requires simultaneous measurement of morning (8 AM) serum cortisol and plasma ACTH.
A low morning cortisol paired with an elevated ACTH provides immediate evidence for primary adrenal insufficiency before proceeding to more complex dynamic testing.
3
Identify the appropriate confirmatory test.
The high-dose cosyntropin (synthetic ACTH) stimulation test is the standard confirmatory test.
It assesses the adrenal gland's maximal capacity to produce cortisol and definitively confirms the diagnosis if the initial screening is equivocal or requires verification.
4
Determine the final test to establish the underlying etiology.
Measurement of 21-hydroxylase antibodies is the final step in this sequence.
Once the functional diagnosis is confirmed, establishing the underlying cause is necessary for management. Autoimmune adrenalitis is the most common cause of primary adrenal insufficiency and is identified by these antibodies.

Key Concept

Diagnostic sequencing for primary adrenal insufficiency begins with initial morning cortisol and ACTH levels, proceeds to a cosyntropin stimulation test for confirmation, and concludes with 21-hydroxylase antibody testing to determine the etiology.
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