Measuring serum gonadotropins (FSH, LH) along with serum oestradiol and progesterone levels provides information to aid in the diagnosis of hypothalamic, pituitary, and ovarian causes of amenorrhea or oligomenorrhea. It can rule out causes such as ovarian in sufficiency when elevated serum FSH and LH levels accompany low serum oestradiol (<20 pg/ ml) and progesterone (<1 ng/ ml) levels, indicating low or absent ovarian reserve.
Finding a transient elevation in serum FSH and LH levels with serum oestradiol levels <2 ng/ ml can indicate the midcycle ovulatory gonadotropin surge.
Compared to normal women, women with functional hypothalamic amenorrhea have a 53% reduction in LH pulse frequency but comparable LH pulse amplitude. In these women, the diagnosis of functional hypothalamic amenorrhea is made after excluding other causes such as severe weight loss, excessive exercise resulting in low body fat, severe psychological stress, and physical wasting from severe systemic diseases (i.e. disseminated malignancies and pituitary or central nervous system lesions). Serum LH and FSH levels are often normal; however, very low and often undetectable serum LH and FSH levels suggest organic hypothalamic amenorrhea due to genetic mutations affecting gonadotropin- releasing hormone (GnRH) production and function or central causes, such as pituitary, hypothalamic, or other brain tumours, and infiltrative lesions. Patients with hypothalamic amenorrhea characteristic ally have a low level of serum oestradiol and low or low- to- normal levels of LH and FSH, whereas the gonadotropin response to GnRH stimulation can be preserved.
Serum oestradiol assessment can be limited by factors related to the assay used, such as poor sensitivity, variation among assays, and the fact that a measurement reflects a single time point.
Low serum oestradiol and progesterone ( <1 ng/ ml) levels are consistent with anovulation and may be seen in women with underlying polycystic ovary syndrome (PCOS). An elevated serum LH to FSH ratio previously has been reported in PCOS patients, but neither circulating levels of LH and FSH, nor the ratio of serum LH to FSH contribute significantly to the diagnosis of PCOS, particularly since LH secretion is pulsatile in nature and one- third of PCOS patients have normal circulating LH levels. Moreover, LH secretion and pulse amplitude are inversely related to body mass index (BMI) and percent body fat, suggesting that obesity in PCOS patients further modifies gonadotropin secretion.
Serum FSH and oestradiol checked on cycle days 2– 4 can help to measure ovarian reserve. Serum FSH concentration starts to increase early in the menstrual cycle with diminished ovarian reserve and in reproductive ageing, but the assays have significant inter- and intracycle variability limiting their reliability.
Basal oestradiol alone should not be used to screen for diminished ovarian reserve. The test can aid to correctly interpret basal serum FSH value. With lower ovarian reserve as normally occurs with reproductive ageing, there is an early rise in serum oestradiol levels that can lower an otherwise elevated basal serum FSH con centration into the normal range
Anti- Müllerian hormone is another measure for ovarian reserve and is discussed later in this section.
