Do you know the hormones involved in the menstrual cycle?

What are hormones?

A hormone is a chemical that is made by special cells in the body, usually within an endocrine gland, and is released into the bloodstream to send a message to another part of the body. These chemical messages (think of them like keys) move around the body and act on target cells with very specific receptors (think of receptors like locks). So only certain keys can attach to certain locks to enable specific effects.

What is the menstrual cycle?

The menstrual cycle doesn't just mean your period or days you bleed. The menstrual cycle starts the first day of bleeding and ends the day before your next bleed, it refers to a series of changes within the body controlled by hormones that cause follicles to grow, an egg to be released (ovulation) and a regular bleed to occur (period).

There are multiple stages that both the ovaries and the lining of the uterus go through each month, all in conjunction with one another, working together to achieve the same goal! Ovulation!! Ovulation is the main event that enables an egg to be released in the hope that it will meet a sperm to fertilise and create a baby. Our body wants this to occur every month, whether we want this to occur or not. She doesn't give up and is relentless in achieving her goal!

The important thing to note is that so many things take place over our menstrual cycle because of communication between our brains, ovaries, the lining of our uterus and the hormones that allow these messages to travel around the body. Really our bodies are just multi-tasking queens! It's no wonder we are so good at it, our bodies do it each and every day!

Endocrine glands and reproductive organs involved in the menstrual cycle

Hypothalamus: The hypothalamus is an endocrine gland, its job is to keep the human body stable. It's the command centre responding to a variety of signals from the internal and external environment, including body temperature, hunger, feelings of fullness after eating, blood pressure, and hormone levels. Its role in the menstrual cycle is to release a hormone (gonadotrophin releasing hormone) that acts on the anterior pituitary gland, also within the brain.

Pituitary Gland: The pituitary gland is often referred to as the 'master gland' since it produces hormones that control many processes within the body. It secretes a range of hormones into the bloodstream that are used as messengers to communicate information from the pituitary gland to distant cells around the body. During the menstrual cycle, a couple of these hormones (follicle stimulating hormone and luteinising hormone) are responsible for communicating with the ovaries.

Ovaries: The ovaries are endocrine glands that are a part of the female reproductive system. They are around the size of an almond and lie on either side of the uterus (womb) via ligaments holding them in place. The ovaries go through the repetitive process of follicle growth (growth of fluid-filled sacs that contain an egg), ovulation (release of an egg), corpus luteum formation (a temporary endocrine gland that helps maintain the uterine environment in hope of pregnancy) and regression (corpus luteum can only survive for a set number of days). The ovaries are responsible for producing steroid hormones oestrogen and progesterone which communicate to both the brain and the uterus

The hypothalamus, pituitary gland and ovaries make up what's called the HPO axis and the endocrine communication that enables follicle growth and ovulation to occur. This communication pathway and the messages sent around the body depend on what phase of the menstrual cycle a woman or person who menstruates is in. This communication starts at puberty and continues until the menopause, where there are reduced amounts of follicles (fluid-filled sacs that contain an egg) within the ovary and the follicles no longer respond to these communication pathways to enable the growth and release of an egg.

Uterus: Or womb is an organ also a part of the female reproductive system. It is shaped like a pear and although quite small, measuring around 8cm long, 6cm wide and 4cm deep, it can grow to the size of a watermelon when pregnant and supporting a growing baby.

Even though the uterus doesn't produce hormones, it responds to hormones produced by the ovaries (oestrogen and progesterone) in order to thicken and prepare itself for implantation (embryo attaching to the lining). If no pregnancy occurs, two-thirds of the lining sheds from the uterus to flow out the vagina during menstruation/period. This enables new lining growth each menstrual cycle.

Hormones involved in the menstrual cycle

Gonadotrophin-releasing hormone (GnRH): GnRH is not talked about as much as the other hormones but is needed to start the whole cascade of events that enable puberty, the menstrual cycle and ovaries to grow a follicle. GnRH is produced by the hypothalamus within the brain and acts on the pituitary gland in order to produce gonadotropic hormones FSH and LH.

Follicle-stimulating hormone (FSH): FSH is a gonadotrophic hormone that is released from the anterior pituitary gland. FSH acts on the follicles in the ovaries to stimulate their growth and development. As the follicle grows, they produce increasing amounts of oestrogen, which is needed for ovulation to occur. Low levels of oestrogen, enable communication (negative feedback) back to the hypothalamus and pituitary gland in the brain to reduce GnRH and in turn, FSH. This prevents too many follicles from growing each month. Usually, only one or two follicles will become dominant and destined to ovulate (egg released from the follicle) each menstrual cycle.

Luteinising Hormone (LH): LH is another gonadotrophic hormone released from the anterior pituitary gland, that helps with follicle growth. The increasing levels of oestrogen released from the growing follicle cause an increase in LH (LH surge). This helps the egg to mature (able to be fertilised by sperm), the follicle to rupture and the egg to be released (ovulation). After ovulation, the follicle that released an egg forms a "corpus luteum". The corpus luteum is supported by LH in order to release the hormones progesterone and oestrogen, which support the lining during the second half (post-ovulation) of the menstrual cycle.

Oestrogen: Oestrogen is a steroid hormone released into the bloodstream from the growing follicles. There are three types of oestrogen hormones, oestradiol, oestrone and oestriol. Oestradiol is the main oestrogen found in women or people that menstruate during their reproductive years. Oestrogen is made from cholesterol and needs FSH and LH to attach to the cells of the follicle to be produced. When oestrogen reaches its peak level during the follicular phase, it causes a surge in LH to enable ovulation (follicle releasing an egg). Oestrogen is also important during the first half of the menstrual cycle (pre-ovulation) to thicken the lining of the uterus and restore what was lost during menstruation. As well as support the lining of the uterus during the second half (post-ovulation) of the menstrual cycle along with progesterone. Oestrogen is important for a lot of different aspects of your body, including your mood, cervical mucus production, sex drive, sleep, bone, brain and heart health.

Progesterone: Progesterone is a steroid hormone released into the bloodstream by the corpus luteum (temporary organ formed from the leftover follicle after ovulation), which helps stimulate the growth of the uterus lining in the second half of the menstrual cycle. Progesterone prepares the body for pregnancy in the event that the released egg is fertilised with sperm, creating the perfect environment for an embryo (egg fertilised by sperm) to attach to the lining. The corpus luteum survives for a set period of time, on average between 12-16 days. In order for a pregnancy to occur the corpus luteum needs to survive and continue to produce progesterone for at least 10 days to enable enough time to enable an embryo to attach to the lining of the uterus. If the egg is not fertilised during this time frame, the corpus luteum breaks down, the progesterone levels fall and a new menstrual cycle begins.

Inhibin: Inhibin is a protein hormone produced by the granulosa cells within the follicles. Inhibin B is the form predominately secreted in the follicular phase of the menstrual cycle and plays a role in follicular development and FSH regulation. It is produced by the growing follicles and in larger quantities by the selected dominant follicle (follicle destined to ovulate). It works with oestrogen to reduce FSH secretion and initiates cell death (atresia) of any other developing follicles.

How do they all work together?

• The hypothalamus in the brain releases the hormone GnRH.

• GnRH acts on the anterior pituitary gland (also in the brain) to release gonadotropin hormones, FSH and LH.

• FSH and LH act on the ovaries to stimulate the growth of follicles (fluid-filled sacs that contain an egg), as the follicle grows it releases the steroid hormone oestrogen.

• At low levels, oestrogen has a negative effect, keeping FSH and LH low. Along with inhibin produced by the granulosa cells within the follicles also reducing FSH production, both these hormones ensure only one follicle is selected to continue to grow. As the follicle gets bigger it stimulates the release of more oestrogen. During this time oestrogen also stimulates the lining of the uterus to develop after being lost during menstruation.

• Once the peak level of oestrogen is reached, oestrogen then causes positive feedback, triggering the LH surge (rise) which stimulates the release of the egg from the follicle (ovulation).

• The leftover follicle (corpus luteum) releases progesterone, further stimulating the lining to develop. Progesterone and oestradiol block GnRH secretion by the hypothalamus and FSH and LH secretion at the anterior pituitary. Which prevents the growth of another follicle during the second half of the menstrual cycle.

• If no pregnancy was to occur, the corpus luteum breaks down as it can only be maintained for a set number of days, and progesterone levels decrease, initiating important lining events that result in a period and allows GnRH to increase and a new cycle to begin.

• If pregnancy does occur, the pregnancy hormone, human chorionic gonadotrophin (hCG), helps to maintain the corpus luteum and the production of progesterone until the placenta is well established.

References

1. The Society for Endocrinology, You and your hormones [Internet] Available from: https://www.yourhormones.info/

2. Taylor HS, Fritz MA, Pal L & Seli E. Speroff’s Clinical Gynaecologic Endocrinology and Infertility. 9th ed. Philadelphia, USA : Lippincott williams & Wilkins, 2011.

3. Jane Knight. The Complete Guide to Fertility Awareness. London: Routledge, 2017.

4. Oliver R, Pillarisetty LS. Anatomy, Abdomen and Pelvis, Ovary Corpus Luteum. [Updated 2021 Oct 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539704/