According to the Rotterdam 2003 criteria, PCOS requires at least 2 of 3 criteria. A woman has hirsutism and oligo-ovulation but a normal pelvic ultrasound (no polycystic ovarian morphology). Which additional investigation is MOST important to exclude before diagnosing PCOS?

• A Serum LH:FSH ratio
• B Serum total testosterone and SHBG
• C Thyroid stimulating hormone (TSH)
• D Non-classical congenital adrenal hyperplasia (NC-CAH) — serum 17-OH progesterone ✓

Explanation

Non-classical (late-onset) CAH due to 21-hydroxylase deficiency presents with hyperandrogenism and oligo-ovulation identical to PCOS and can satisfy Rotterdam criteria. The diagnostic test is early morning (follicular phase) serum 17-OH progesterone: a basal level >200 ng/dL or stimulated (ACTH) level >1000 ng/dL confirms NC-CAH. This distinction is crucial because NC-CAH is treated with glucocorticoids rather than PCOS-specific therapy. TSH is checked to exclude hypothyroidism-related anovulation, which is also important but specifically addresses the distinction PCOS vs androgens-driven NC-CAH.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.