Life stages ↗GnRH pulses drive LH and FSH
Intermittent hypothalamic signals stimulate pituitary gonadotropins; testosterone, estradiol, and inhibin feed back at different parts of the axis.
LIFE STAGES / 03
One axis.
Several distinct outputs.
Testosterone production, spermatogenesis, sperm transport, secondary traits, bone, muscle, blood, sexual function, and fertility overlap but cannot be reduced to one hormone number or one symptom.
ESTABLISHEDHPG-axis physiology.
NORMAL VARIATIONAge and individual context.
DISEASE CONTEXTHypogonadism and infertility.
LIMITED / MIXEDNonspecific symptom attribution.
LH activates Leydig cells
LH receptor signaling supports conversion of cholesterol into testosterone in the testicular interstitium.
FSH activates Sertoli support
FSH and high intratesticular testosterone support Sertoli-cell barrier, metabolic, paracrine, and germ-cell functions.
Feedback stabilizes rather than freezes
Axis output changes with pulses, sleep, time of day, age, illness, energy state, medicines, and external hormones.
Germ cells pass through staged divisions
Spermatogonia proliferate, meiotic cells reduce chromosome number, and spermatids remodel into sperm over a process lasting many weeks.
The epididymis supports maturation
After leaving seminiferous tubules, sperm acquire motility and membrane features while moving through epididymal regions.
Accessory glands create seminal fluid
Seminal vesicle, prostate, and other secretions contribute volume, substrates, buffering, enzymes, and transport context at ejaculation.
Semen analysis samples several variables
Concentration, total number, motility, morphology, volume, and other measures vary. No single feature guarantees or excludes reproductive potential.
Local metabolism changes androgen signals
Testosterone may act directly, convert to dihydrotestosterone, or aromatize to estradiol, making effects tissue-specific.
Age-related change is usually gradual
Testosterone and reproductive function can shift with age, health, sleep, adiposity, medications, testicular function, and central signaling; there is no universal abrupt male menopause.
Symptoms are not hormone-specific
Fatigue, mood change, strength, body composition, desire, and erectile function have multiple possible contributors.
Diagnosis joins symptoms, levels, and cause
Appropriately timed repeat measurement, assay context, LH and FSH, health history, examination, fertility goals, and treatment risks inform clinical evaluation.