Lean PCOS

What Is It?

Diagnosis (Rotterdam Criteria, updated 2023)

1. Irregular or absent menses (oligo- or anovulation)
2. Clinical or biochemical hyperandrogenism - elevated total/free testosterone, modified Ferriman-Gallwey (mFG) score ≥ 4-6 depending on ethnicity, acne, hirsutism
3. Polycystic-appearing ovaries on ultrasound - ≥ 20 antral follicles or ovarian volume ≥ 10 cm³ in at least one ovary, OR elevated AMH

• Other causes must be excluded: hypothyroidism, hyperprolactinemia, non-classical congenital adrenal hyperplasia (17-OHP elevated), androgen-secreting tumors
• In adolescents (< 3 years post-menarche): ultrasound and AMH criteria are NOT used; only irregular menses + hyperandrogenism are diagnostic

How Lean PCOS Differs from Obese PCOS

Key implication

• Harrison's 22E notes: "Lean oligo-ovulatory patients with PCOS generally have high LH levels in the presence of normal to low levels of FSH and estradiol"

Pathophysiology

1. GnRH pulsatility dysregulation - Abnormally rapid GnRH pulses increase LH secretion disproportionately to FSH, driving excess ovarian androgen (theca cell) production
2. Insulin resistance - Present even in lean PCOS (though milder), particularly in skeletal muscle and adipose tissue, causing insulin-stimulated ovarian androgen excess. This can occur without frank obesity or glucose intolerance
3. Elevated 11-oxygenated androgens - An alternate androgen source that may also contribute
4. Genetic factors - ~19 GWAS loci associated with PCOS; cluster analyses show both reproductive and metabolic sub-phenotypes

Clinical Features

• Menstrual irregularity - Oligomenorrhea (< 8 cycles/year) or amenorrhea
• Hyperandrogenism - Hirsutism (excess hair in male-pattern distribution), acne, androgenic alopecia
• Infertility - Anovulation is the most common cause of ovulatory infertility overall
• Polycystic ovaries on ultrasound - Though ovarian cysts alone are not required; normal-appearing ovaries do not exclude PCOS if other criteria are met
• Psychological comorbidities - High prevalence of depression, anxiety, disordered eating, and body image distress
• Endometrial risk - Chronic anovulation causes unopposed estrogen exposure, increasing endometrial hyperplasia and cancer risk 2-6 fold

Lab Workup

Management

Not Seeking Pregnancy

• Combined oral contraceptive (COC) - First-line for menstrual regulation and reduction of androgens. COCs increase SHBG, thereby reducing free testosterone. Allow 6 months before assessing full effect on hirsutism/acne
• Antiandrogens - If inadequate COC response after 6 months: spironolactone (most widely used), flutamide
• Endometrial protection - If COC not used: cyclic progestin (medroxyprogesterone 10 mg or progesterone 200 mg for 10-14 days every 3 months) or levonorgestrel IUD
• Metformin - Considered primarily for overweight/obese women with PCOS for cardiometabolic protection. In lean PCOS, role is less established; it is NOT recommended alone for endometrial protection, hyperandrogenism, or preventing gestational diabetes
• Lifestyle modification - Recommended for all PCOS patients regardless of weight

Seeking Pregnancy (Ovulation Induction)

1. Letrozole (aromatase inhibitor, 2.5 mg cycle days 3-7) - First-line for ovulation induction; higher live birth rates than clomiphene (27.5% vs 19.1% in the pivotal multicenter RCT of 750 women)
2. Clomiphene citrate (50 mg) - Second-line; effectiveness is reduced by obesity and hyperandrogenism (note: less applicable in lean PCOS, so lean patients may respond better to clomiphene than obese PCOS patients)
3. Injectable gonadotropins - Used carefully by specialists; PCOS increases hyperstimulation risk
4. Weight management pre-conception - Even a 5% weight loss in overweight/obese PCOS improves pregnancy rates

Long-Term Risks (even in lean women)

• Endometrial cancer (2-6x increased risk from chronic anovulation)
• Gestational diabetes, gestational hypertension, preeclampsia, preterm birth
• Type 2 diabetes and metabolic syndrome (risk lower in lean vs obese PCOS but still elevated vs. general population)
• Cardiovascular disease (particularly with obese phenotype; cardiovascular risk assessment recommended for all PCOS)
• Depression and anxiety

Clinical Pearls for Lean PCOS

• Do not dismiss PCOS because the patient is not overweight - 20-30% of PCOS is lean
• LH/FSH ratio is elevated in lean PCOS more often than obese PCOS, which can be a useful supportive clue even though it is not a diagnostic criterion
• Metformin has a smaller role in lean PCOS compared to obese PCOS since insulin resistance is less pronounced
• Screening for eating disorders is important - lean women with PCOS have higher rates of disordered eating and body image issues, partly driven by hyperandrogenism symptoms (acne, hirsutism, hair loss)
• Symptoms may improve with age - By the fourth decade, most women with PCOS may have regular menses and normalizing androgens