Menstrual Cycle: 28 Days Female Cycle

00:00 So let's walk through a traditional 28-day cycle breaking it into its ovarian phases, which are the follicular phase and the luteal phase. If we start off by looking at LH and FSH, you can see a very traditional peak associated right between these two phases. This is called the LH surge and the LH surge is going to be associated with ovulation. FSH increases right about the same time but to a lesser extent. What are the changes for estradiol? Estradiol increases just before the LH surge, drops off a little bit and then it stays at a pretty high level throughout the luteal phase. Progesterone is really only increasing once you start to get to the luteal phase and it stays elevated for a good deal of that time only tailing off in the last few days before menses. This LH surge needs to be explained to a greater degree. What causes it is an increase in sensitivity to gonadotropin-releasing hormone. So normally gonadotropin-releasing hormone, if given, causes a very small LH surge. In the late follicular phase right before ovulation, it causes a big response. So a certain dose of gonadotropin-releasing hormone causes a larger increase in luteinizing hormone. How does this work? If you look at how LH and FSH are released, it works on a principle in which gonadotropin-releasing hormone stimulates the docking and fusing of LH and FSH to be released for these gonadotropes but there are a number of other factors. One of which I will highlight here and that is estrogen. Both estrogen and activin upregulate this response. So you remember that estrogen was starting to peak just before that LH surge? That is sensitizing these gonadotropes to release even more luteinizing hormone when gonadotropin-releasing hormone is present.