What is the likely diagnosis for a female patient of reproductive age with hair on the chin and neck, normal Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), and Dehydroepiandrosterone Sulfate (DHEAS) levels, and no other mentioned medical history?

Likely Diagnosis: Idiopathic Hirsutism

The most likely diagnosis is idiopathic hirsutism, which accounts for less than 20% of hirsute women and is defined by the presence of terminal hair growth in androgen-dependent areas (chin, neck) with normal ovulatory function and normal circulating androgen levels including FSH, LH, and DHEAS. 1

Diagnostic Confirmation Required

Before finalizing this diagnosis, you must confirm several critical elements that distinguish idiopathic hirsutism from other causes:

Verify True Ovulatory Function

• Document ovulatory cycles with mid-luteal progesterone measurement, as up to 40% of women with regular menses are actually anovulatory, which would instead suggest PCOS 1
• Regular menstrual history alone is insufficient for excluding ovulatory dysfunction 1

Complete the Androgen Panel

• Measure total testosterone by LC-MS/MS (sensitivity 74%, specificity 86%) and free testosterone by LC-MS/MS (sensitivity 89%, specificity 83%) as first-line hormonal testing 2
• While FSH, LH, and DHEAS are normal, testosterone levels must also be documented as normal to confirm idiopathic hirsutism 3
• If DHEAS >600 μg/dL, immediately obtain adrenal CT imaging to exclude adrenocortical carcinoma, which accounts for more than half of androgen hypersecretion cases 2

Exclude Other Endocrine Disorders

• Measure 17-hydroxyprogesterone on day 4 of the menstrual cycle to rule out non-classical congenital adrenal hyperplasia (21-hydroxylase deficiency), particularly since this can present with normal DHEAS 2, 4
• Check TSH and prolactin levels, as hyperprolactinemia can mimic PCOS presentation and primary hypothyroidism can elevate prolactin 5
• Consider androstenedione measurement as a second-line test if clinical suspicion remains high despite normal first-line androgens 3

Differential Diagnosis Considerations

PCOS Remains Most Common Overall

• PCOS accounts for 95% of all hyperandrogenism cases and affects 10-13% of women globally, making it statistically the most likely diagnosis in any woman presenting with hirsutism 2
• However, PCOS requires 2 of 3 Rotterdam criteria: androgen excess (clinical or biochemical), ovulatory dysfunction, or polycystic ovaries on ultrasound 3
• Obtain pelvic ultrasound to assess for polycystic ovarian morphology before excluding PCOS 5

Functional Hypothalamic Amenorrhea with PCOM

• If the patient has low body weight (BMI <20 kg/m²), history of eating disorder, excessive exercise, or stress, consider functional hypothalamic amenorrhea (FHA) with underlying PCOS features 3
• FHA can mask PCOS features through hypothalamic suppression, with FSH and LH both <2 IU/L and estradiol <100 pmol/L, but polycystic ovarian morphology may persist 3

Pathophysiology of Idiopathic Hirsutism

When strictly defined with confirmed normal ovulation and normal androgens:

• Primary mechanism is increased skin 5α-reductase activity (both type 1 and type 2 isoenzymes), which converts testosterone to the more potent dihydrotestosterone (DHT) locally in hair follicles 1
• Possible alterations in androgen receptor function at the hair follicle level 1
• Strong familial predilection due to genetic factors regulating androgen receptor activity and 5α-reductase activity 6, 7

Treatment Approach

Combination therapy is required for optimal results, as idiopathic hirsutism responds poorly to monotherapy:

First-Line Pharmacologic Treatment

• Antiandrogen therapy with spironolactone (50-200 mg daily), which blocks peripheral androgen receptors 6, 1
• 5α-reductase inhibitor (finasteride 5 mg daily) to block conversion of testosterone to DHT, though this may be less effective than antiandrogens 1, 4
• Combined oral contraceptives provide additional benefit by suppressing ovarian androgen production, even though baseline levels are normal 6, 1

Mechanical Hair Removal

• Laser hair removal offers the fastest method of hair reduction, though effectiveness depends on skin type, hair color, practitioner skill, and hair growth cycle timing 7
• Electrolysis, depilatory creams, plucking, and waxing are alternative mechanical options 7
• Eflornithine hydrochloride 13.9% topical cream can ameliorate unwanted facial hair growth 6

Critical Pitfalls to Avoid

• Do not diagnose idiopathic hirsutism without documenting ovulatory function with mid-luteal progesterone, as regular menses are unreliable 1
• Do not miss virilizing tumors by failing to check total and free testosterone; rapid progression or testosterone >1.5 ng/mL warrants imaging 4
• Do not overlook non-classical congenital adrenal hyperplasia, which requires specific 17-hydroxyprogesterone testing 2, 4
• Do not assume normal DHEAS excludes all pathology; testosterone measurement is essential as DHEAS is only a second-line test 2