Lesson
The HPG Axis and Pulsatile Control
The reproductive axis converts a pulse pattern from the hypothalamus into gonadal steroid production, gametogenesis, and feedback that changes across age, sex, and physiologic state.
- Pulsatile GnRH
- LH and FSH
- Negative feedback
- Positive feedback
- Inhibin
Intermittent signaling preserves pituitary response
Gonadotropins distribute steroidogenic and gametogenic work
Downstream hormones reshape the next signal
Preserve the pulse
Physiologic GnRH is released in pulses. Continuous receptor stimulation eventually suppresses pituitary gonadotropin secretion, while pulse timing and amplitude help shape LH and FSH output.
Read the feedback loop
Sex steroids feed back to the hypothalamus and pituitary; inhibin selectively restrains pituitary FSH secretion. Most sex-steroid feedback is negative, but sustained estradiol near mid-cycle can produce positive feedback and the LH surge.
Interpret values as a set
A single gonadal hormone value is rarely enough. Symptoms, biologic timing, binding proteins, LH, FSH, prolactin, pregnancy status when relevant, and the clinical context determine what the value means.
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Lesson
Testicular Steroidogenesis and Spermatogenesis
Testicular function depends on coordinated LH action in Leydig cells, FSH action in Sertoli cells, high intratesticular testosterone, and intact seminiferous architecture.
- Leydig cells
- Sertoli cells
- Intratesticular testosterone
- Spermatogenesis
- Inhibin B
Generate testosterone from cholesterol-derived precursors
Support germ cells and produce inhibin B
Maintain the concentration required for spermatogenesis
Generate testosterone
LH activates its receptor on Leydig cells and supports cholesterol transport and steroidogenesis. Testosterone then acts locally and systemically and contributes negative feedback at the hypothalamus and pituitary.
Support germ cells
FSH acts on Sertoli cells, which support germ-cell development, produce inhibin B, and help maintain the specialized environment required for spermatogenesis.
Protect fertility logic
Exogenous testosterone suppresses LH and FSH. Serum testosterone may normalize while intratesticular testosterone and sperm production fall, so fertility goals must be discussed before therapy begins.
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Lesson
Ovarian Steroidogenesis and Cycle Feedback
The ovarian cycle emerges from coordinated theca and granulosa cell function, follicular recruitment, estradiol feedback, ovulation, and corpus luteum progesterone production.
- Two-cell model
- Follicular phase
- LH surge
- Corpus luteum
- Luteal phase
Supply precursor to the neighboring granulosa cell
Convert androgen substrate to estradiol
Transform and stabilize the post-ovulatory endometrium
Build estradiol
LH stimulates theca cells to produce androgen substrate. FSH increases granulosa-cell aromatase, which converts that substrate to estradiol. This is the two-cell, two-gonadotropin model.
Trigger ovulation
Rising estradiol usually suppresses gonadotropins, but sustained high estradiol from the dominant follicle switches to positive feedback and generates the mid-cycle LH surge.
Maintain the luteal phase
After ovulation, the corpus luteum produces progesterone and estradiol. Progesterone converts the proliferative endometrium toward a secretory state and suppresses another ovulatory surge.
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Lesson
Steroidogenesis and Local Hormone Metabolism
Sex steroids share a cholesterol-derived pathway, but enzyme expression in gonads and peripheral tissues determines which active hormone reaches each receptor.
- Cholesterol
- StAR
- CYP11A1
- 5-alpha-reductase
- Aromatase
CYP11A1 creates pregnenolone
Tissue enzymes determine the dominant product
5-alpha-reductase and aromatase modify local action
Enter the steroid pathway
StAR facilitates cholesterol delivery to the inner mitochondrial membrane, where CYP11A1 converts cholesterol to pregnenolone. Downstream enzymes then generate progestogens, androgens, and estrogens.
Create testosterone
Leydig-cell pathways convert steroid precursors through androstenedione, with HSD17B3 supporting testosterone formation. The exact pathway depends on tissue enzyme expression.
Modify action locally
5-alpha-reductase converts testosterone to the more potent androgen DHT in selected tissues. Aromatase converts androgen substrate to estrogen, making local metabolism part of the final pharmacologic response.
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Lesson
Sex Steroid Receptors and Tissue Response
Androgen, estrogen, and progesterone receptors are ligand-regulated transcription factors whose effects depend on receptor distribution, co-regulators, chromatin, dose, and duration.
- Androgen receptor
- Estrogen receptor alpha
- Estrogen receptor beta
- Progesterone receptor
- Selective receptor modulation
Ligand changes receptor conformation
Cell-specific proteins amplify or restrain transcription
Bone, breast, endometrium, brain, and liver can diverge
Activate nuclear signaling
A steroid enters the cell, binds its receptor, changes receptor conformation, and alters transcription at hormone-response elements or through other transcriptional partners.
Respect tissue context
Receptor subtype, co-activator and co-repressor expression, local hormone metabolism, and baseline gene programs determine whether a tissue response is strong, weak, or qualitatively different.
Understand selective modulators
A selective estrogen receptor modulator can act as an agonist in one tissue and an antagonist in another because the ligand-bound receptor recruits a different regulatory complex.
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Lesson
Diagnosing and Classifying Hypogonadism
Hypogonadism is a clinical and biochemical diagnosis, not a symptom label or a single low laboratory result.
- Compatible symptoms
- Repeat morning testosterone
- Accurate assay
- Primary failure
- Secondary failure
Do not diagnose from a nonspecific symptom alone
Exclude transient illness and invalid timing
Separate primary gonadal from central signaling failure
Require agreement
In men, diagnosis requires compatible symptoms or signs plus unequivocally and consistently low testosterone measured with an accurate assay. Confirm a low result with a separate fasting morning total-testosterone measurement, using appropriate assay reference limits. Avoid diagnosing from a transient low value during acute illness; symptoms and reproducible biochemical findings must agree.
Resolve binding-protein uncertainty
When total testosterone is borderline or SHBG is altered, assess free testosterone by equilibrium dialysis or a validated calculation using total testosterone, SHBG and albumin. Direct analog free-testosterone immunoassays are inaccurate.
Investigate central warning signs
Severe secondary hypogonadism, persistent hyperprolactinemia, other pituitary deficits, new headache or visual-field symptoms can warrant pituitary imaging. Replacement alone does not address a possible central lesion.
Locate the level
Low testosterone with elevated LH and FSH supports primary testicular failure. Low testosterone with low or inappropriately normal gonadotropins supports central disease and may require prolactin measurement or pituitary evaluation.
Find reversible contributors
Acute illness, obesity, sleep disruption, opioid or glucocorticoid exposure, nutritional stress, and selected systemic disorders can lower testosterone. Treating the context may be more appropriate than immediate replacement.
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Lesson
Testosterone Formulations and Exposure
Testosterone products differ in absorption, peak-to-trough variation, administration burden, transfer risk, food requirements, injection interval, and ease of dose adjustment.
- Topical gel
- Transdermal system
- Intramuscular ester
- Subcutaneous ester
- Oral undecanoate
Flexible adjustment with transfer precautions
Longer intervals with product-specific peaks and monitoring
Follow formulation-specific administration and blood-pressure guidance
Compare routes
Topical products provide daily exposure and easier dose adjustment but can transfer to others. Injectable esters may simplify adherence but can produce product-specific peak and trough patterns. Oral testosterone undecanoate has formulation-specific administration and monitoring requirements.
Use product instructions
Application site, skin contact precautions, injection technique, timing of serum measurement, meal instructions, and dose-adjustment rules differ by product. One testosterone label cannot safely stand in for another.
Apply a specific gel example
AndroGel 1.62% starts at 40.5 mg each morning, equivalent to two pump actuations. Apply to shoulders and upper arms, not the abdomen or genitals. Prime a new pump with three discarded actuations before first use. Avoid washing or swimming for at least two hours. Dose adjustment uses pre-dose morning testosterone at approximately days 14 and 28 after initiation or adjustment; other gels can use different sites, doses and sampling times.
Recognize the Aveed safety setting
Aveed is intramuscular testosterone undecanoate: 750 mg in 3 mL at initiation, four weeks later, then every ten weeks. Certified prescribers and healthcare settings participate in its REMS. Observe for 30 minutes after every injection, even after prior uneventful doses, because pulmonary oil microembolism and anaphylaxis can occur. It is not a home subcutaneous injection.
Distinguish oral undecanoate
Jatenzo starts at 237 mg twice daily with food. Its dose-adjustment sample is drawn six hours after the morning dose, after at least seven days on the initial or changed dose. Do not reuse a gel pre-dose sampling schedule. Jatenzo requires hematocrit checks about every three months, periodic blood-pressure monitoring and avoidance in uncontrolled hypertension. Other oral products have their own schedules.
Prevent secondary exposure
Patients using topical testosterone should wash hands after application, cover the site after drying, and wash the application area before anticipated skin contact. If someone else touches unwashed gel-treated skin, wash their exposed skin promptly with soap and water. Keep flames away until alcohol-based gel dries.
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Lesson
Testosterone Monitoring, Safety, and Fertility
Safe testosterone therapy pairs symptom response with surveillance for erythrocytosis, blood-pressure elevation, prostate concerns, edema, sleep apnea, thromboembolic symptoms, and misuse.
- Hematocrit
- Blood pressure
- PSA
- Fertility
- Misuse
Confirm appropriate product-specific exposure
Detect erythrocytosis and pressure elevation
Investigate new risk rather than escalating automatically
Monitor exposure
Assess clinical response, adherence, serum testosterone at the product-specific time, hematocrit, and blood pressure. Product labeling and patient risk determine the exact interval and additional tests.
Act on hematocrit
The general guideline schedule checks hematocrit before treatment, at three to six months and annually thereafter; use more frequent product-specific monitoring when required, such as Jatenzo every three months. The Endocrine Society advises stopping therapy above 54%, assessing hypoxia and sleep apnea, then considering a lower dose once safe. An unexplained high baseline value also needs evaluation.
Follow prostate findings
When prostate monitoring is chosen, assess risk before treatment and again at three to twelve months. During the first year, a confirmed PSA rise above 1.4 ng/mL from baseline, PSA above 4 ng/mL or an abnormal prostate examination warrants urologic consultation.
Screen important exclusions
The 2018 Endocrine Society guideline advises against initiation with elevated hematocrit, untreated severe sleep apnea, severe urinary symptoms, uncontrolled heart failure, recent myocardial infarction or stroke within six months, or thrombophilia. Apply current product labeling and specialist assessment for cancer-related restrictions, including the evolving FDA prostate-label revisions.
Respond to safety signals
A marked hematocrit rise, new venous thromboembolic symptoms, uncontrolled blood pressure, worsening edema, severe sleep-apnea concerns, or a clinically important prostate finding requires reassessment.
Protect reproductive goals
Discuss current and future fertility before prescribing. Exogenous testosterone can suppress sperm production; someone pursuing fertility needs a reproductive evaluation and an appropriate alternative rather than routine replacement.
Distinguish a label request from an updated product
FDA requested further revisions in June 2026 concerning age-related hypogonadism, prostate cancer and benign prostatic hyperplasia. The proposal would narrow the prostate-cancer contraindication to metastatic disease while retaining assessment and monitoring. It is not evidence that every product label has already changed, nor a blanket assurance of long-term prostate safety. Check the current selected product and specialist assessment.
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Lesson
Estrogen and Progestogen Pharmacology Foundations
Estrogens and progestogens differ by molecule and route, but their major clinical effects follow receptor biology, hepatic exposure, endometrial context, and patient-specific thrombotic or cancer risk.
- Estradiol
- Ethinyl estradiol
- Progesterone
- Progestins
- Endometrial protection
Support target tissues while accounting for hepatic and thrombotic context
Oppose persistent endometrial estrogen exposure when needed
Contraception, menopause, and gynecologic care require separate risk frameworks
Distinguish molecules and routes
Estradiol, conjugated estrogens, ethinyl estradiol, micronized progesterone, and synthetic progestins have different potency, metabolism, receptor profiles, and route-dependent effects. They should not be treated as a single interchangeable class.
Connect physiology to therapy
Estrogen supports endometrial proliferation, bone, urogenital tissue, and multiple systemic functions. Progesterone transforms the estrogen-primed endometrium and supports the luteal reproductive state.
Frame safety correctly
Risk depends on indication, dose, route, duration, age, uterus status, thrombotic history, cancer history, and other comorbidities. Detailed selection belongs in the relevant condition-specific module.
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Lesson
GnRH Modulation and Pathway-Directed Therapy
Gonadal hormone pathways can be altered at the hypothalamic-pituitary signal, steroid-synthesis enzymes, local conversion enzymes, or the final receptor.
- GnRH agonist
- GnRH antagonist
- Aromatase inhibitor
- 5-alpha-reductase inhibitor
- Receptor antagonist
Change pituitary gonadotropin output
Reduce formation of an active tissue hormone
Change tissue action after the hormone is produced
Differentiate GnRH strategies
Continuous GnRH agonist therapy initially increases LH, FSH, and gonadal steroids before receptor desensitization produces suppression. GnRH antagonists block the receptor directly and suppress gonadotropins without the agonist flare.
Block hormone production
Aromatase inhibitors reduce estrogen synthesis, 5-alpha-reductase inhibitors reduce DHT formation, and selected steroidogenesis inhibitors reduce androgen output at earlier enzymatic steps.
Block hormone action
Estrogen, progesterone, and androgen receptor antagonists or selective modulators change tissue response without necessarily eliminating hormone production. Their clinical use and safety depend on indication and tissue context.
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Module practice
Check the connections.
Each attempt draws 10 questions from the complete 33 question bank.
Each attempt draws a fresh set and rearranges the answer choices.
References
Current clinical foundation.
Core source material was synthesized with the following contemporary guidance. Verify local policy and current guidance before applying clinical information.
- The Menopause Society. Hormone therapy: systemic routes and endometrial protection.
- Bose et al. Rapid regulation of steroidogenesis by mitochondrial protein import (2002).
- DailyMed. Aveed prescribing information.
- DailyMed. Jatenzo prescribing information.
- Shang and Brown. Molecular determinants for the tissue specificity of SERMs (2002).
- Miller and Auchus. Human steroidogenesis and its disorders (2011).
- Serum inhibin B as a marker of spermatogenesis (1998).
- Endocrine Society. Testosterone replacement therapy statement, July 16, 2026.
- Bhasin et al. Testosterone therapy in men with hypogonadism: full guideline (2018).
- HHS. Requested testosterone labeling updates, June 18, 2026.
- Endocrine Society Clinical Practice Guideline: Testosterone Therapy for Hypogonadism
- FDA Testosterone Information and June 2026 Labeling Update
- FDA Class-Wide Testosterone Labeling Changes, February 2025
- DailyMed. AndroGel 1.62% prescribing information.
- FDA ORGOVYX Prescribing Information, 2025