Analysis of the Clinical Case

The Critical Diagnostic Twist: Striae

• Cushing syndrome can mimic PCOS almost perfectly: hirsutism, menstrual irregularity, weight gain, mood changes
• Striae (especially purple/violaceous, wide striae) are a hallmark of Cushing syndrome
• PCOS is a diagnosis of exclusion - other causes of hyperandrogenism must be ruled out before confirming it

Most Appropriate Next Step: 24-hour Urine Free Cortisol (or Low-dose Dexamethasone Suppression Test)

• Those tests move toward confirming PCOS, but PCOS is a diagnosis of exclusion
• Cushing syndrome must be ruled out first when striae are present
• The Textbook of Family Medicine states: "Proceed with a workup for PCOS, late-onset congenital adrenal hyperplasia, or Cushing syndrome if there are features consistent with these illnesses."

1. Striae (abdominal)
2. Weight gain
3. Hirsutism + menstrual irregularity + mood change

• 24-hour urine free cortisol - most commonly cited initial test
• Low-dose overnight dexamethasone suppression test (1 mg dexamethasone at 11 PM, serum cortisol at 8 AM; normal suppression to <1.8 mcg/dL rules out Cushing)
• Late-night salivary cortisol - equally sensitive

Supporting Framework: Why This Isn't "Just PCOS"

Summary: Most Appropriate Next Step

• A) Serum LH/FSH ratio
• B) Pelvic ultrasound
• C) Serum testosterone levels (to document hyperandrogenism)
• D) 24-hour urine free cortisol
• E) Dexamethasone suppression test

• Best answer = 24-hour urine free cortisol or overnight dexamethasone suppression test (to exclude Cushing)
• If no Cushing option is present: Serum testosterone (to document biochemical hyperandrogenism as the first lab step in suspected PCOS)

Key references: Harrison's Principles of Internal Medicine, 22e (2025); Goldman-Cecil Medicine; Textbook of Family Medicine 9e

Why Serum Testosterone is the Best First Step

Breaking Down the Case

Why C is Correct: Serum Testosterone

• Testosterone is the initial screening test for hyperandrogenism, the core pathophysiologic abnormality driving this presentation.
• It establishes biochemical hyperandrogenism as the basis for further workup.
• The level of testosterone elevation guides the differential:
  • Mildly elevated (e.g., 60-150 ng/dL): consistent with PCOS or functional ovarian hyperandrogenism
  • Markedly elevated (>200 ng/dL): raises strong suspicion for an androgen-secreting tumor (ovarian or adrenal) - prompting CT/MRI next
• PCOS is a diagnosis of exclusion, and testosterone level is needed before any imaging or further workup is justified.

Why NOT the Other Options

• This is a tempting distractor because of the abdominal striae - a classic Cushing finding.
• However, a random or fasting morning cortisol is not a validated screening test for Cushing syndrome. It has poor sensitivity and specificity because cortisol is pulsatile and diurnal.
• The proper screen for Cushing is either a 24-hour urine free cortisol, late-night salivary cortisol, or overnight 1-mg dexamethasone suppression test - none of which are listed here.
• More importantly, this patient lacks the classic Cushing features: no buffalo hump, no moon face, no proximal muscle weakness, no easy bruising, no hypertension. The striae alone in a teenager gaining weight are more consistent with PCOS than Cushing.
• Cortisol measurement would not be the first step even if Cushing were suspected.

• Hyperprolactinemia can cause menstrual irregularity, but it does not cause hirsutism or androgen excess.
• This patient's primary complaint is facial hair growth - prolactin elevation does not explain this.
• Prolactin would be appropriate if she had galactorrhea, headaches, or visual field defects (suggesting prolactinoma) - none of which are present.

• Imaging is premature at this stage.
• CT is reserved for after testosterone levels are measured; specifically, if testosterone is markedly elevated (>200 ng/dL), CT/MRI would then be used to localize an adrenal or ovarian androgen-secreting tumor.
• Jumping to CT without biochemical confirmation is inappropriate and exposes the patient to unnecessary radiation.

• Would be appropriate if a pituitary adenoma (e.g., prolactinoma, Cushing disease from an ACTH-secreting tumor) were suspected.
• This patient has no headaches, no visual changes, no galactorrhea, and no symptoms pointing to the pituitary.
• Again, premature without biochemical workup first.

Diagnostic Algorithm for This Patient

Hirsutism + Irregular menses + Weight gain
              ↓
  STEP 1: Serum testosterone (+ DHEAS)   ← THIS IS THE ANSWER
              ↓
   Mildly elevated         Markedly elevated (>200 ng/dL)
        ↓                          ↓
  Consider PCOS           CT abdomen/pelvis
  (+ TSH, prolactin,      (rule out androgen-
   17-OHP to exclude       secreting tumor)
   other causes)
        ↓
  2 of 3 Rotterdam criteria → PCOS diagnosis
  (Hyperandrogenism + Oligo-ovulation + PCO on US)

Key Teaching Points

1. Serum testosterone is the first biochemical test in any workup for hirsutism - it both confirms androgen excess and stratifies the differential diagnosis.
2. The striae are a distractor designed to make you think of Cushing syndrome - but a morning cortisol is not the right Cushing screen, and classic Cushingoid features are absent here.
3. PCOS is the most likely diagnosis, but you arrive there through testosterone measurement, not by assuming it upfront.
4. No imaging before labs - CT and MRI are second-line steps triggered by abnormal biochemical results.

Sources: Harrison's Principles of Internal Medicine, 22e (2025); Goldman-Cecil Medicine; Textbook of Family Medicine, 9e