Hormones start working when a fertilized egg (you!) implants in your mother’s uterine wall and their activity won’t stop until the day you die.
So what are hormones and why do they exert such a powerful influence on our minds and bodies?
Hormones are chemical messengers that are made in one place in our body and released into the bloodstream to carry messages to other parts of the body. That might not sound like a big deal until you realize that hormone messages carry the instructions to do almost everything going on in your body including:
The endocrine system is the name given to the series of glands that produce hormones and secrete them into the bloodstream. The female endocrine system is shown in Figure 1.
At its most basic, the endocrine system works like this:
In other words, a gland produces a hormone that travels via the bloodstream to its target cell where the message is delivered and the cell then follows the hormone’s instructions to do something.
Many hormones work in a more complex fashion, called feedback loops, as shown in Figure 2. A positive feedback loop happens when one hormone sends a signal to increase secretion of another hormone. A negative feedback loop happens when one hormone sends a signal to decrease the secretion of another hormone.
In the case of the menstrual cycle, there are three (3) feedback loops at play. The menstrual cycle is controlled by the hypothalamus, which acts as the conductor for the orchestra made up of the anterior pituitary, the ovaries, and the endometrium (lining of the uterus). The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses every few hours, and these pulses stimulate the anterior pituitary to release FSH and LH.
Note that if the egg implants, then the entire cycle changes as the body enters into pregnancy. These feedback loops occur in the absence of pregnancy hormones.
 You and Your Hormones (The Society for Endocrinology) https://www.yourhormones.info/hormones/follicle-stimulating-hormone/
 Melmed, S., Polonsky, K. S., Larsen, P. R. & Kronenberg, H. M. (2016) Williams Textbook of Endocrinology. Philadelphia, Pa: Elsevier.