Gonadotropin-releasing hormone agonist analogs are structural modifications of the native gonadotropin-releasing hormone decapeptide that bind the gonadotropin-releasing hormone receptor with higher affinity and resist peptidase degradation, producing prolonged non-pulsatile receptor occupancy. At initiation, this triggers an indistinguishable Gq-mediated signal: phospholipase C beta activation, inositol trisphosphate and diacylglycerol generation, calcium mobilization, and protein kinase C activation, driving an initial surge of luteinizing hormone and follicle-stimulating hormone lasting 1 to 2 weeks. In men with prostate cancer, serum testosterone rises 50 to 80% above baseline during this flare period.

The transition to suppression proceeds in two phases. Phase one: within hours, protein kinase C-mediated phosphorylation uncouples the receptor from Gq without significant surface receptor loss. Phase two: over days to weeks, receptor internalization via a clathrin-independent pathway reduces surface receptor density by 80 to 95%. Testosterone falls to castrate levels (below 50 nanograms per deciliter) within 3 to 4 weeks of the first depot injection.

The testosterone flare poses clinical risk in prostate cancer patients with vertebral metastases (spinal cord compression), bulky nodal disease, or severe urinary obstruction. Standard management is co-administration of a non-steroidal anti-androgen beginning 7 to 14 days before the first depot injection and continued for 4 weeks. Bicalutamide 50 milligrams daily is most commonly used. Combined androgen blockade — continuous gonadotropin-releasing hormone agonist plus anti-androgen — provides only marginal additional benefit over agonist alone but substantially increases adverse effects, so most guidelines limit anti-androgen use to flare prevention only.

Gonadotropin-releasing hormone antagonists (degarelix, oral relugolix) avoid the flare entirely by competitively blocking the receptor without any initial activation and are the preferred choice when rapid testosterone suppression is needed or flare risk is high.

Degarelix (Firmagon) is the first gonadotropin-releasing hormone antagonist approved specifically for prostate cancer. It competitively blocks the gonadotropin-releasing hormone receptor without any initial activation, producing castrate testosterone levels (below 50 nanograms per deciliter) in more than 96% of patients within 3 days of the first injection — without any testosterone flare. The loading dose is 240 milligrams given as two 120-milligram subcutaneous injections on day 1, followed by monthly maintenance doses of 80 milligrams subcutaneous. After injection, degarelix forms a depot gel at the subcutaneous site, releasing drug over approximately 28 days. Elimination is primarily through hepatic peptide metabolism.

The key limitation of degarelix is injection site reactions: pain, erythema, and nodule formation occur in approximately 35 to 40% of patients, substantially more than with intramuscular leuprolide or goserelin. These are generally self-limited but may require site rotation and warm compresses. Unlike gonadotropin-releasing hormone agonist depots, there is no long-acting formulation of degarelix beyond the monthly subcutaneous injection, which some patients find inconvenient compared to quarterly depot agonist formulations.

Pulsatile gonadotropin-releasing hormone therapy is the therapeutic opposite of depot agonist therapy. A portable infusion pump delivers native gonadotropin-releasing hormone in subcutaneous or intravenous pulses of 2.5 to 20 micrograms every 60 to 120 minutes, mimicking physiological hypothalamic secretion. This approach is used exclusively in patients with hypogonadotropic hypogonadism caused by isolated gonadotropin-releasing hormone deficiency (Kallmann syndrome or normosmic idiopathic hypogonadotropic hypogonadism), where absent pulsatile secretion leads to prepubertal gonadal function despite an intact pituitary.

Pulsatile gonadotropin-releasing hormone therapy reliably induces spermatogenesis in men (adequate sperm counts for natural conception in approximately 75 to 80% of treated patients) and ovulation in women (cumulative pregnancy rates of 80 to 90% over multiple treatment cycles). Because it restores physiological signaling, it avoids the supraphysiological gonadotropin levels and multiple follicular development that complicate exogenous gonadotropin therapy in women with hypogonadotropic hypogonadism.

Androgen deprivation therapy with a gonadotropin-releasing hormone agonist or antagonist is the cornerstone of treatment for metastatic hormone-sensitive prostate cancer, biochemically recurrent disease after local therapy, and locally advanced disease in combination with radiotherapy. The clinical goal is sustained testosterone below 50 nanograms per deciliter (or preferably below 20 nanograms per deciliter by current guidelines). Intermittent androgen deprivation therapy — alternating treatment cycles and treatment holidays guided by prostate-specific antigen levels — is an option for biochemically recurrent disease without metastases, offering similar overall survival in selected patients while allowing partial testosterone recovery between cycles. Continuous androgen deprivation therapy is required for metastatic disease. Duration combined with radiotherapy for high-risk localized disease is typically 18 to 36 months based on randomized trial data. The HERO trial established relugolix as an alternative to leuprolide with cardiovascular advantage, which is clinically relevant because cardiovascular disease is the leading non-cancer cause of death in men with prostate cancer receiving androgen deprivation therapy.

In endometriosis, gonadotropin-releasing hormone agonist depots (leuprolide 3.75 milligrams monthly or 11.25 milligrams every 3 months; goserelin 3.6 milligrams monthly) reduce dysmenorrhea, pelvic pain, and dyspareunia in approximately 85% of patients, comparable to surgical management in symptom control. Treatment is limited to 6 months of continuous use without add-back therapy owing to hypoestrogenic bone mineral density loss of approximately 4 to 6% at the lumbar spine over 6 months. Add-back therapy with low-dose estrogen plus progestin (norethindrone acetate 5 milligrams daily, or conjugated equine estrogen 0.625 milligrams plus norethindrone acetate 5 milligrams daily) maintains pain control while attenuating bone mineral density loss by keeping estradiol in the range of 20 to 40 picograms per milliliter — below the threshold for endometriosis stimulation but adequate for bone protection. Elagolix 150 milligrams daily (partial suppression) may be used for up to 24 months; elagolix 200 milligrams twice daily is limited to 6 months without add-back but up to 12 months with norethindrone acetate 1 milligram daily add-back.

In uterine fibroids, gonadotropin-releasing hormone agonists reduce fibroid volume by 35 to 65% over 3 to 6 months through hypoestrogenic shrinkage, useful preoperatively to reduce surgical bleeding. Fibroid regrowth occurs within 3 to 6 months of stopping therapy in most patients. The oral combination product Myfembree (relugolix 40 milligrams plus estradiol 1 milligram plus norethindrone acetate 0.5 milligrams once daily) provides sustained fibroid symptom control with built-in add-back therapy, enabling use beyond 6 months with bone mineral density monitoring.

In central precocious puberty — defined as hypothalamic-pituitary-gonadal axis activation before age 8 in girls and age 9 in boys — depot gonadotropin-releasing hormone agonist therapy is the standard of care. Continuous hypothalamic-pituitary-gonadal suppression halts pubertal progression and protects final adult height by extending the period for skeletal growth before epiphyseal fusion. Leuprolide depot (Lupron Depot-Ped) at 0.3 milligrams per kilogram intramuscularly every 4 weeks (minimum 7.5 milligrams) is most commonly used. Histrelin acetate subcutaneous implant (Supprelin LA) provides continuous agonist delivery from an annually replaced subcutaneous implant. Adequacy of suppression is confirmed by stimulated luteinizing hormone below 2 international units per liter after gonadotropin-releasing hormone agonist stimulation at 30 to 60 minutes.