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Difficulty: HardOrdering Laboratory Tests

A 45-year-old man presents to the clinic with a 6-month history of decreased libido, fatigue, and erectile dysfunction. His physical examination reveals normal secondary sex characteristics, normal visual fields, and bilaterally descended testes with a volume of 18 mL (normal). Arrange the following laboratory tests in the most appropriate chronological sequence for the step-wise diagnostic evaluation of his symptoms, assuming each sequential test result supports the progression toward a diagnosis of secondary (central) hypogonadism.

  1. 1Initial measurement of a fasting morning serum total testosterone level
  2. 2Confirmatory repeat measurement of morning serum total testosterone
  3. 3Measurement of serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels
  4. 4Measurement of serum prolactin level and transferrin saturation

Answer

The correct step-wise sequence is: Initial morning testosterone → Repeat morning testosterone → LH and FSH → Prolactin and iron panel.
The diagnostic evaluation of male hypogonadism follows a strict, step-wise laboratory sequence. The first step is screening with a morning total testosterone level, as levels peak between 8:00 AM and 10:00 AM. Because transient illnesses, sleep disturbances, and normal diurnal variations can suppress testosterone, a single low value must be confirmed with a second morning measurement. Once true hypogonadism is established, the defect must be localized by measuring the gonadotropins LH and FSH. Elevated levels indicate primary (testicular) hypogonadism, while low or inappropriately normal levels indicate secondary (central) hypogonadism. Finally, if secondary hypogonadism is diagnosed, specific laboratory tests such as serum prolactin (to screen for prolactinoma) and an iron panel (to screen for hemochromatosis) are ordered to determine the underlying etiology prior to pursuing pituitary magnetic resonance imaging.

Step-by-Step Solution

1
Identify the appropriate initial screening test for suspected male hypogonadism.
Initial measurement of a fasting morning serum total testosterone level.
Testosterone exhibits a strong diurnal rhythm with peak levels in the early morning; thus, the initial screening must be performed between 8:00 AM and 10:00 AM.
2
Determine the required confirmatory step following an initially low result.
Confirmatory repeat measurement of morning serum total testosterone.
Up to 30% of men with an initially low testosterone level will have a normal level on repeat testing. Diagnosis requires confirmation with a second morning sample.
3
Select the laboratory test to localize the defect within the hypothalamic-pituitary-gonadal axis.
Measurement of serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels.
Gonadotropins differentiate primary testicular failure (elevated LH/FSH) from secondary pituitary or hypothalamic failure (low or inappropriately normal LH/FSH).
4
Choose the targeted laboratory tests to investigate the etiology of central hypogonadism.
Measurement of serum prolactin level and transferrin saturation.
Once secondary hypogonadism is established, checking a prolactin level (for prolactinoma) and transferrin saturation (for hemochromatosis) identifies common, potentially reversible specific etiologies.

Key Concept

The diagnostic evaluation of male hypogonadism requires a strict step-wise progression: biochemical screening with morning testosterone, confirmation with a repeat morning testosterone, anatomic localization using LH and FSH, and finally etiologic investigation of the specific defect.
Estimated Time:1m 30s
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Ordering Laboratory Tests

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