A 47-year-old woman comes to the physician because of fatigue, difficulty falling asleep, and night sweats for the past 6 months. Over the past year, her menstrual cycle has become irregular and her last menstrual period was 2 months ago. She quit smoking 2 years ago. Pelvic exam shows vulvovaginal atrophy. A pregnancy test is negative. Which of the following changes is most likely to occur in this patient's condition?

Increased follicle-stimulating hormone

Decreased gonadotropin-releasing hormone

A 15-year-old boy is brought to the physician for a well-child visit. His parents are concerned that he has not had his growth spurt yet. As a child, he was consistently in the 60th percentile for height; now he is in the 25th percentile. His classmates make fun of his height and high-pitched voice. His parents are also concerned that he does not maintain good hygiene. He frequently forgets to shower and does not seem aware of his body odor. As an infant, he had bilateral orchidopexy for cryptorchidism and a cleft palate repair. He is otherwise healthy. Vital signs are within normal limits. On physical exam, axillary and pubic hair is sparse. Genitals are Tanner stage 1 and the testicles are 2 mL bilaterally. Which of the following is the most likely diagnosis?

Constitutional delay of puberty

A 52-year-old G3P3 presents to her gynecologist complaining of painful intercourse. She reports a 6-month history of intermittent dryness, itching, and burning during intercourse. Upon further questioning, she also reports poor sleep and occasional periods during which she feels very warm and sweats profusely. Her past medical history is significant for poorly controlled hypertension and a 10 pack-year smoking history. She takes hydrochlorothiazide and enalapril. Her temperature is 99.3°F (37.4°C), blood pressure is 135/85 mmHg, pulse is 90/min, and respirations are 18/min. On examination, she is a healthy female in no distress. Pelvic examination reveals no adnexal or cervical motion tenderness. Which of the following sets of hormone levels are most likely to be found in this patient?

Decreased estrogen, increased FSH, increased LH, increased GnRH

Increased estrogen, decreased FSH, decreased LH, decreased GnRH

Decreased estrogen, decreased FSH, decreased LH, increased GnRH

Normal estrogen, normal FSH, normal LH, normal GnRH

Increased estrogen, increased FSH, increased LH, increased GnRH

A 24-year-old woman comes to her primary care physician because she has not had a menstrual period for 6 months. She is a competitive runner and has been training heavily for the past year in preparation for upcoming races. She has no family or personal history of serious illness. She has not been sexually active for the past 9 months. Her temperature is 36.9°C (98.4° F), pulse is 51/min, respirations are 12/min, and blood pressure is 106/67 mm Hg. Her BMI is 18.1 kg/m2. Which of the following is the most likely cause of her amenorrhea?

Decreased frequency of GnRH release from the hypothalamus

Poor synthetic response of ovarian cells to circulating LH and FSH

Increased LH release and increased ovarian androgen production

A 23-year-old woman comes to the emergency department because of a 5-day history of nausea and vomiting. There is no associated fever, abdominal pain, constipation, diarrhea, or dysuria. She is sexually active and uses condoms inconsistently. Her last menstrual period was 10 weeks ago. Her temperature is 37°C (98.6°F), pulse is 90/min, respirations are 18/min, and blood pressure is 110/70 mm Hg. There is no rebound tenderness or guarding. A urine pregnancy test is positive. Ultrasonography shows an intrauterine pregnancy consistent in size with an 8-week gestation. The hormone that was measured in this patient's urine to detect the pregnancy is also directly responsible for which of the following processes?

Maintenance of the corpus luteum

Hypertrophy of the uterine myometrium

Stimulation of uterine contractions at term

A 27-year-old woman comes to the physician for the evaluation of infertility. She has been unable to conceive for the past 2 years. Menses occur at 45 to 80-day intervals. She is 168 cm (5 ft 6 in) tall and weighs 77 kg (170 lb); BMI is 27.4 kg/m2. Physical examination shows facial acne and pigmented hair on the upper lip. Serum studies show elevated levels of testosterone and an LH:FSH ratio of 4:1. Treatment with the appropriate drug for this patient's infertility is begun. Which of the following is the primary mechanism of action of this drug?

Inhibition of hypothalamic estrogen receptors

Activation of pituitary dopamine receptors

Activation of granulosa cell aromatase

Activation of ovarian luteinizing hormone receptors

Inhibition of endometrial progesterone receptors

A 33-year-old man comes to the physician with his wife for evaluation of infertility. They have been unable to conceive for 2 years. The man reports normal libido and erectile function. He has smoked one pack of cigarettes daily for 13 years. He does not take any medications. He has a history of right-sided cryptorchidism that was surgically corrected when he was 7 years of age. Physical examination shows no abnormalities. Analysis of his semen shows a low sperm count. Laboratory studies are most likely to show which of the following?

Decreased inhibin B concentration

Increased prolactin concentration

Increased placental ALP concentration

Decreased testosterone concentration

Hypothalamic-pituitary-gonadal axis for USMLE Step 3: High-Yield Physiology Lessons

HPG Axis - The Hormone Orchestra

Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH) in pulses.
Anterior Pituitary: GnRH stimulates release of:
- Luteinizing Hormone (LH) → Theca cells (ovary) / Leydig cells (testis).
- Follicle-Stimulating Hormone (FSH) → Granulosa cells (ovary) / Sertoli cells (testis).
Gonads: Produce sex hormones (estrogen, progesterone, testosterone), which exert negative feedback on the hypothalamus and pituitary.

Female and Male Hypothalamic-Pituitary-Gonadal (HPG) Axis

⭐ Clinical Pearl: Continuous (non-pulsatile) administration of GnRH agonists like Leuprolide initially causes a surge in LH/FSH, but ultimately leads to downregulation and suppression of the axis, used to treat conditions like endometriosis or prostate cancer.

Menstrual Cycle - Rhythmic Hormonal Dance

A ~28-day cycle orchestrated by HPG axis feedback loops, preparing the uterus for potential pregnancy.

Hormonal changes during the menstrual cycle

Follicular Phase (Days 1-14, variable):
- Starts with menses (endometrial shedding).
- ↑ FSH stimulates follicular growth.
- Dominant follicle produces ↑ estrogen, causing endometrial proliferation.
- Estrogen initially inhibits LH/FSH, then switches to positive feedback at high levels.
Ovulation (~Day 14):
- Triggered by a massive LH surge 24-36 hours prior.
- Oocyte is released from the follicle.
Luteal Phase (Days 14-28, constant):
- Corpus luteum (CL) forms, secretes ↑ progesterone (and some estrogen).
- Progesterone promotes endometrial vascularization and glycogen storage (secretory phase).
- Progesterone exerts strong negative feedback on GnRH/LH/FSH.
- No pregnancy → CL degrades → ↓ progesterone/estrogen → menses.

⭐ The switch from negative to positive feedback by estrogen on GnRH/LH secretion is the critical event that triggers the ovulatory LH surge.

Feedback Loops - Axis Control System

Negative Feedback (Tonic): The primary control mechanism.
- Estrogen, progesterone, and testosterone inhibit GnRH (hypothalamus) and LH/FSH (pituitary).
- Inhibin B from granulosa/Sertoli cells provides selective FSH suppression.
Positive Feedback (Mid-Cycle Switch):
- Sustained high estrogen (>200 pg/mL for >48 hours) reverses feedback on the pituitary.
- This induces the LH surge, which is essential for triggering ovulation.

⭐ The switch from negative to positive feedback by estrogen is concentration- and duration-dependent, a critical event for ovulation.

Clinical Correlations - When the Axis Fails

Hypogonadotropic Hypogonadism (Central): Failure of hypothalamus or pituitary. ↓GnRH or ↓FSH/LH leads to ↓estrogen/testosterone.
- Causes: Kallmann syndrome (impaired migration of GnRH neurons, anosmia), functional hypothalamic amenorrhea (stress, excessive exercise, low body weight).
Hypergonadotropic Hypogonadism (Primary): Gonadal failure. ↓estrogen/testosterone & ↓inhibin leads to a compensatory ↑FSH/LH.
- Causes: Turner syndrome (45,XO), Klinefelter syndrome (47,XXY), premature ovarian insufficiency.

⭐ In Polycystic Ovary Syndrome (PCOS), dysregulation leads to a persistently high LH/FSH ratio (often >2:1), driving excess androgen production and anovulation.

High‑Yield Points - ⚡ Biggest Takeaways

Pulsatile GnRH release is essential for pituitary stimulation; continuous release is inhibitory.

The mid-cycle LH surge is the direct trigger for ovulation.

Estrogen exerts dual feedback: mostly negative, but a sustained high level causes a positive feedback LH surge.

Progesterone provides powerful negative feedback to the hypothalamus and pituitary.

Inhibin B, from granulosa cells, selectively inhibits FSH secretion.

hCG mimics LH to maintain the corpus luteum in early pregnancy.

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