1. Overview
Gonadotropin Releasing Hormone (GnRH) is a decapeptide hormone produced in the hypothalamus. It acts on GnRH receptors located on gonadotroph cells of the anterior pituitary gland, stimulating the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
These pituitary hormones subsequently regulate the production and release of testosterone in males and estrogen in females from the gonads and placenta. GnRH is secreted in a pulsatile manner, and this pulsatility is essential for normal reproductive function.
The synthesis and release of GnRH are regulated by circulating levels of sex steroids—testosterone in men and estrogens in women—through feedback mechanisms. GnRH serves as the key regulator of the hypothalamic–pituitary–gonadal (HPG) axis and is essential for sexual development, reproduction, and endocrine feedback control.
2. Physiological Role and Function
GnRH functions by initiating and maintaining reproductive hormone signaling. Pulsatile GnRH release stimulates LH and FSH secretion, which in turn regulate gametogenesis and sex steroid production.
Continuous or non-pulsatile exposure to GnRH suppresses LH and FSH release, a principle that is clinically used in GnRH analogue therapy. Proper GnRH signaling is therefore critical for normal puberty, fertility, and reproductive health.
3. Clinical Indications
GnRH testing and stimulation studies are used in several clinical scenarios:
- Central precocious puberty, defined as the onset of puberty before 8 years in girls and 9 years in boys
- Delayed puberty, to differentiate constitutional delay from hypogonadotropic hypogonadism
- Monitoring therapy in precocious puberty, particularly to assess suppression achieved with GnRH analogues
- Hypogonadotropic hypogonadism, where pituitary or hypothalamic dysfunction is suspected
- Differential diagnosis of pubertal disorders, to distinguish central precocious puberty from peripheral causes
- Assessment in congenital adrenal hyperplasia, to detect abnormal pubertal progression or associated central precocious puberty
4. Sample Collection and GnRH Stimulation Test
Before testing, patients are advised to discontinue steroids, ACTH, gonadotropins, or estrogen medications, if possible, for at least 48 hours before blood collection. Blood samples are collected in a plain red-capped tube.
The GnRH stimulation test is used to evaluate pituitary function and pubertal status. A baseline blood sample is collected for LH and FSH estimation, followed by intravenous administration of GnRH as a bolus dose of 2.5 µg/kg. Additional blood samples are collected at 15, 30, 45, and 60 minutes after injection.
Interpretation is based on the hormonal response:
- High hormonal response indicates activation of the HPG axis
- Low hormonal response suggests a lack of activation
- No response is consistent with hypogonadotropic hypogonadism
5. Reference Ranges and Interpretation
Key reference values used in interpretation include:
- Basal LH (prepubertal): <0.3–0.5 IU/L
- Basal LH (pubertal onset): >0.3 IU/L
- Peak LH post-GnRH (central precocious puberty): ≥5 IU/L
- Peak LH/FSH ratio: >0.66–1.0, indicating pubertal axis activation
These values help distinguish between prepubertal and pubertal states and identify central activation of puberty.
6. Causes of Abnormal GnRH Levels
Low GnRH levels may result from congenital conditions such as Kallmann syndrome and congenital hypogonadotropic hypogonadism, acquired causes like pituitary tumors, head trauma, or functional causes, including stress, excessive exercise, eating disorders, and chronic illness.
High GnRH levels are seen in primary gonadal failure, including Turner syndrome, Klinefelter syndrome, premature ovarian failure, gonadal dysgenesis, post-surgical states, chemotherapy, and aging (menopause).
7. Limitations
GnRH testing has several limitations. It is time-consuming, requires multiple blood samples, and can be uncomfortable, particularly for children. Interpretation may require repeat or adjunct endocrine tests.
There is limited standardization of baseline and stimulated cut-off values, and results may be less reliable in obese children or during very early stages of precocious puberty. Recent hormonal therapy and improper sample handling may also affect results.
