FSH (Follicle Stimulating Hormone)
You start generating FSH during your period in response to a signal from the Pituitary Gland. It prompts follicles in both ovaries to begin maturing eggs.
High levels of FSH in the early part of the cycle are often taken to be an indicator that egg reserves are running low, but this is not necessarily the case - there could merely be an overall imbalance of hormones.
Low levels of FSH can be an indicator of Polycystic Ovary Syndrome.