Sign up to access the complete lesson and track your progress!
Unlock This CourseThe menstrual cycle is controlled by a complex system of hormones working together like a well-orchestrated team. Two key players in this system are FSH (Follicle Stimulating Hormone) and LH (Luteinising Hormone). These hormones, produced by the pituitary gland in your brain, act as chemical messengers that control when eggs develop and when ovulation occurs.
Think of FSH and LH as the conductors of an orchestra - they don't make the music themselves, but they tell all the other parts when to start, stop and change tempo. Without these hormones working properly, the menstrual cycle would be chaos!
Key Definitions:
FSH is like a gardener tending to seeds. It travels from the pituitary gland to the ovaries and encourages several follicles to start growing. Each follicle contains an immature egg. FSH levels are highest at the beginning of the menstrual cycle, typically during days 1-7, when the body is preparing for the next potential pregnancy.
While FSH gets things started, LH delivers the final punch. LH levels surge dramatically around day 14 of a typical 28-day cycle, causing the dominant follicle to burst open and release its mature egg. This is ovulation - the moment when pregnancy becomes possible.
The LH surge is one of the most dramatic hormone changes in the human body. Within 24-48 hours of this surge, ovulation occurs. It's so reliable that home ovulation tests work by detecting LH in urine. The surge happens because oestrogen levels from the growing follicle reach a critical point, triggering the brain to release a massive amount of LH.
LH levels remain relatively low while FSH stimulates follicle growth. The dominant follicle produces increasing amounts of oestrogen.
LH levels spike dramatically, often increasing by 5-10 times normal levels within 24 hours. This triggers ovulation.
LH levels drop quickly after ovulation. The empty follicle becomes the corpus luteum, which produces progesterone.
The menstrual cycle operates on negative feedback loops - like a thermostat controlling temperature. When hormone levels get too high, the body automatically reduces production. When they're too low, production increases.
During the first half of the cycle, rising oestrogen levels from developing follicles gradually suppress FSH production. This prevents too many eggs from maturing at once. However, when oestrogen reaches very high levels just before ovulation, it switches to positive feedback, triggering the LH surge.
Understanding how FSH and LH work requires seeing the bigger picture of all four key hormones working together:
FSH levels rise as the cycle begins. Low oestrogen and progesterone levels (from the previous cycle ending) allow FSH to start stimulating new follicles. Think of this as pressing the reset button on the whole system.
FSH continues to stimulate follicle growth, but one follicle becomes dominant. This follicle produces increasing amounts of oestrogen, which starts to suppress FSH through negative feedback. LH levels begin to rise slowly as oestrogen increases.
The LH surge occurs! High oestrogen levels trigger massive LH release, causing ovulation within 24-48 hours. FSH also surges, but less dramatically than LH.
After ovulation, LH helps maintain the corpus luteum (the empty follicle), which produces progesterone. Both FSH and LH levels drop as progesterone and oestrogen provide negative feedback. If pregnancy doesn't occur, hormone levels crash and the cycle starts again.
Understanding FSH and LH has revolutionised fertility treatments. Women undergoing IVF often receive synthetic FSH injections to stimulate multiple follicles to develop simultaneously - something that wouldn't happen naturally. Then, synthetic LH or hCG (which mimics LH) triggers ovulation at precisely the right time for egg collection. This shows how crucial these hormones are for reproduction and how medical science can work with natural processes to help couples conceive.
Sometimes the FSH and LH system doesn't work perfectly, leading to fertility problems:
In Polycystic Ovary Syndrome (PCOS), LH levels are often too high compared to FSH. This disrupts normal follicle development and can prevent ovulation. Women with PCOS may have irregular periods because their hormone balance is disrupted.
Doctors can measure these hormones through blood tests to diagnose fertility problems or monitor treatments. FSH levels are often tested on day 3 of the cycle to assess ovarian function, while LH can be monitored to predict ovulation.
Ovulation predictor kits work by detecting the LH surge in urine. When the test shows a positive result, ovulation typically occurs within 24-48 hours. This technology puts the power of hormone monitoring into the hands of anyone trying to conceive.
The LH surge is so powerful that it can increase LH levels from about 5-20 units to over 100 units in just 24 hours! This dramatic change ensures that ovulation happens at exactly the right time when the egg is mature and ready for fertilisation.
FSH and LH are essential hormones that control the menstrual cycle through precise timing and feedback mechanisms. FSH starts the process by stimulating follicle growth, while LH delivers the crucial surge that triggers ovulation. Together, they ensure that eggs develop properly and are released at the optimal time for potential fertilisation.
Understanding these hormones helps explain not only how the menstrual cycle works, but also how fertility treatments work, why some women have irregular periods and how the body maintains its remarkable reproductive rhythm month after month.