Polycystic Ovary Syndrome (PCOS) is a common endocrine disorder affecting women of reproductive age. A characteristic feature is a hormonal imbalance, specifically an elevated ratio of Luteinizing Hormone (LH) compared to Follicle-Stimulating Hormone (FSH). This altered ratio is a significant indicator of the neuroendocrine disturbance. Understanding why LH is preferentially higher than FSH is important because this imbalance directly contributes to the reproductive and metabolic symptoms experienced by people with PCOS.
The Standard Roles of LH and FSH
The reproductive cycle is regulated by the Hypothalamic-Pituitary-Ovarian (HPO) axis, a complex communication system involving the brain and the ovaries. The hypothalamus releases Gonadotropin-Releasing Hormone (GnRH) in regular, pulsatile bursts. This GnRH travels to the pituitary gland, stimulating it to release the two gonadotropins: FSH and LH.
These two hormones have distinct but cooperative roles within the ovary. FSH acts primarily on the granulosa cells surrounding the developing egg, promoting follicular growth and stimulating the conversion of precursor hormones into estrogen. LH stimulates the theca cells, which produce androgens, the precursors that the granulosa cells then process. In a healthy cycle, a surge of LH ultimately triggers ovulation.
The frequency of GnRH pulses from the hypothalamus determines which gonadotropin is favored for release by the pituitary gland. A slower pulse frequency promotes the synthesis and release of FSH. Conversely, a faster, more frequent pulse pattern preferentially stimulates the pituitary to release LH.
Defining the LH/FSH Imbalance in PCOS
In PCOS, the hypothalamus increases the pulse frequency of Gonadotropin-Releasing Hormone (GnRH). This persistently rapid GnRH pulsing is the primary cause for the disproportionately high Luteinizing Hormone levels and the elevated LH/FSH ratio observed in many individuals with PCOS.
The pituitary gland responds to this accelerated signaling by synthesizing and releasing more LH. This rapid pulse frequency simultaneously suppresses the release of FSH. The result is a ratio that is often two or three times higher than the typical 1:1 balance seen in healthy women during the early follicular phase.
Several factors contribute to this accelerated GnRH pulse frequency. Hyperandrogenism, the excess male hormones produced by the ovaries, reduces the sensitivity of the GnRH pulse generator to negative feedback signals from sex steroids. Elevated insulin levels, often present due to insulin resistance, also influence the HPO axis and contribute to the increased GnRH pulse frequency.
How High LH Drives Androgen Production
The elevated Luteinizing Hormone level has profound consequences at the level of the ovary. High LH excessively stimulates the theca cells. This over-stimulation leads to an excessive production of androgens, such as testosterone and androstenedione.
This androgen excess is directly responsible for many visible symptoms of PCOS, including hirsutism (excess body and facial hair), acne, and male-pattern hair thinning. The relatively lower level of FSH cannot properly stimulate the granulosa cells, which are unable to effectively convert the large amount of androgen precursors into estrogen.
This hormonal environment, characterized by high androgens and insufficient FSH, impairs normal follicular development. Follicles are recruited but stall at an immature stage, failing to develop a dominant follicle that can be ovulated. This follicular arrest leads to the accumulation of many small, undeveloped follicles, giving the ovary the characteristic “string of pearls” appearance often seen on ultrasound. The chronic lack of ovulation resulting from this follicular arrest is a major cause of irregular periods and fertility challenges.
Managing the Hormonal Imbalance
Medical interventions for PCOS are designed to mitigate the effects of the elevated LH/FSH ratio and resulting hyperandrogenism. Hormonal contraceptives are a common first-line treatment for managing menstrual irregularities. These medications contain synthetic estrogen and progestin that suppress the release of Luteinizing Hormone and Follicle-Stimulating Hormone.
By suppressing these hormones, contraceptives effectively lower high LH levels, which decreases ovarian androgen production and alleviates symptoms like hirsutism and acne. Insulin sensitizers, like metformin, address a root cause by improving insulin sensitivity and lowering circulating insulin levels. This reduction in hyperinsulinemia may slow the rapid GnRH pulse frequency, helping restore a more balanced LH/FSH ratio and regularizing the menstrual cycle.