GnRH Stimulation Test

The gonadotrophin releasing hormone (GnRH) stimulation test evaluates pituitary gonadotroph reserve and the hypothalamic-pituitary-gonadal axis by measuring luteinising hormone (LH) and follicle stimulating hormone (FSH) after GnRH or a GnRH agonist.

In practice, it is used to confirm central precocious puberty (gonadotrophin dependent) and to separate it from peripheral (gonadotrophin independent) causes, and to distinguish constitutional delay of puberty from hypogonadotrophic hypogonadism.

The test has become less important in adults, where basal LH, FSH and sex steroids usually decide.

Precocious puberty is defined as onset of breast development before 8 years in girls or testicular enlargement before 9 years in boys. Central precocious puberty accounts for 90% of cases in girls (mostly idiopathic) and about half in boys (many with a central nervous system lesion). The prevalence is about 1 in 5000 children.

Delay of puberty (no breast development by 13 years in girls, no testicular enlargement by 14 years in boys) is most commonly constitutional (about 60 to 65% of boys), but functional hypogonadotrophic hypogonadism (chronic illness, undernutrition) and permanent hypogonadotrophic hypogonadism (including Kallmann syndrome) must be separated.

Indications. Children with early signs of puberty, advanced bone age and accelerated growth (precocious puberty). Adolescents with delayed puberty to separate constitutional delay from hypogonadotrophic hypogonadism, when basal LH is low. Suspected pituitary gonadotroph disease in adults. Monitoring of GnRH analogue treatment suppression (leuprorelin or triptorelin) in central precocious puberty.

Contraindications and cautions. Hypersensitivity to GnRH analogues. Pregnancy. A pituitary tumour with apoplexy risk (rare reports of apoplexy after GnRH testing). Repeated testing is avoided.

Preparation. No fasting is required. The child is rested and calm. Baseline clinical assessment (Tanner stage, growth, bone age). Test is carried out in the morning in a day unit.

Procedure (GnRH test).

• Intravenous cannula. Baseline samples for LH and FSH (and oestradiol or testosterone).
• Give gonadorelin 100 microg intravenously (or 2.5 microg/kg to a maximum of 100 microg) as a bolus.
• Take samples for LH and FSH at 20, 40 and 60 minutes (some centres at 30 and 60 minutes).

Procedure (GnRH agonist test). Subcutaneous triptorelin 100 microg (or leuprorelin 20 microg/kg), with LH at 0 and 3 hours (and 24 hours for oestradiol or testosterone). This replaced the intravenous test where gonadorelin is unavailable. A single LH at 40 minutes after intravenous GnRH, or at 3 hours after the agonist, is often enough.

Central precocious puberty (pubertal response).

• Peak stimulated LH over 5 IU/L (some centres over 3.3 to 5 IU/L by immunochemiluminometric assays) indicates central activation. The LH to FSH ratio is over 0.66 (some centres 1.0). Basal LH over 0.3 IU/L (ultrasensitive assay) also points to puberty.
• A prepubertal response shows peak LH under 3 to 5 IU/L with FSH predominance (FSH exceeds LH), indicating no central puberty (premature thelarche, peripheral precocious puberty).
• In peripheral precocious puberty (adrenal tumour, ovarian cyst, McCune-Albright syndrome, congenital adrenal hyperplasia, gonadal tumour, hCG-secreting tumour), LH is suppressed and unresponsive to GnRH.

Delayed puberty.

• Constitutional delay: LH rises into the pubertal range (peak over 5 IU/L), indicating an intact axis that is maturing late.
• Hypogonadotrophic hypogonadism: LH response is blunted (peak under 5 IU/L, under 4 IU/L in the stricter definitions), but the test has limited ability to separate it from constitutional delay (sensitivity about 70 to 90%). Inhibin B, anti-Müllerian hormone, a hCG stimulation test and gene testing add discrimination, and watchful follow up often decides.
• Primary hypogonadism: basal FSH and LH are high and the response is exaggerated.

Adults. GnRH testing is rarely informative. Basal LH, FSH and sex steroids identify primary or secondary hypogonadism. Delayed or exaggerated FSH responses suggest the site of the defect.

Monitoring treatment. In central precocious puberty treated with GnRH analogues, suppressed stimulated LH (under 4 IU/L, or basal LH under 1 IU/L) shows adequate pituitary suppression.

Critical values and findings needing immediate action.

• Central precocious puberty in a young girl (under 6 years) or in a boy: MRI of the brain and hypothalamic pituitary region to look for tumours (hamartoma, glioma), and urgent paediatric endocrine referral.
• Rapid virilisation or breast development with suppressed LH: adrenal or gonadal tumour, and imaging.
• Visual disturbance, headache or other pituitary deficits with low LH: pituitary mass, urgent MRI.
• Hypogonadotrophic hypogonadism with anosmia: Kallmann syndrome, genetic counselling and hormone induction of puberty.
• Hypotension or collapse after test (very rare): anaphylaxis.

Interferences and pitfalls.

• Assay type matters: ultrasensitive immunochemiluminometric assays give higher results than immunofluorometric or older radioimmunoassays, so assay specific cut offs are essential.
• Early puberty may show a prepubertal response, and repeat testing 3 to 6 months later is needed when clinical progression continues.
• Pubertal stage, sex, age and nutrition affect responses.
• Recent use of sex steroids, GnRH analogues, oral contraceptives, glucocorticoids or opioids suppresses gonadotrophins and invalidates the result.
• Obesity, anorexia nervosa, chronic illness, intense exercise and hypothyroidism lower LH.
• The test needs several samples and is stressful in children, so single timed LH after agonist stimulation is often used.
• Gonadorelin availability is limited, and agonist testing is a practical alternative.
• GnRH testing alone does not separate delayed puberty types in all cases, and follow up observation is essential.

Special groups.

• Girls: breast development before 8 years requires bone age, uterine ultrasound and basal sex steroid levels.
• Boys: testicular volume over 4 mL indicates central puberty, and testicular enlargement with asymmetry suggests tumour.
• Adults: test is not routine.
• Regions: late puberty from malnutrition and chronic disease is more frequent, and access to MRI and assay is limited.

Follow up tests. Basal LH, FSH, oestradiol or testosterone, bone age radiograph, pelvic ultrasound (uterine and ovarian volume), 17-hydroxyprogesterone and DHEA-S, hCG and alpha fetoprotein in boys, TSH and free T4, prolactin, IGF-1, brain MRI, inhibin B, anti-Müllerian hormone, karyotype, and genetic testing (KAL1, FGFR1, GNRHR) in hypogonadotrophic hypogonadism.