The Female Endocrine System
Hormones in the Menopausal TransitionHormones
They literally impact every single event happening in your body!
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:
- Breathing
- Changing food into energy
- Reproduction
- Movement
- Sexual development
- Growth
- Emotions
- Much more!!
The Female Endocrine System
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.
Following Figure 1 from top to bottom, the endocrine glands are as follows:
| Gland | Hormones Produced | Effects |
|---|---|---|
| Hypothalamus | • Gonadotropin-releasing hormone (GnRH) • Prolactin-releasing hormone • Relaxin • Growth hormone | • Body temperature • Hunger • Moods • Thirst • Sleep • Directs the release of hormones from other glands. |
| Pineal gland | • Melatonin | • Sleep |
| Pituitary gland (aka the ‘master control gland’) • Has two parts: anterior and posterior; • Connects to the hypothalamus by a stalk made of blood vessels and nerve fibers | • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH) • Prolactin • Growth hormone • Adrenocorticotrophic hormone (ACTH) • Oxytocin | • Regulates most other endocrine glands (including ovaries) • Controls function of some organs • Growth • Milk production • Development of breast tissue • Autonomic nervous system (including heart rate, body temperature, and urination) • Cortisol production (maintains blood pressure and blood sugar levels) • Progresses labor |
| Thyroid gland | Uses iodine from food to make • Triiodothyronine (T3) • Thyroxine (T4) | • Metabolism • Breathing • Heart rate • Cholesterol levels |
| Parathyroid glands | • Parathyroid hormone (PTH) | • Regulates the amount of calcium, phosphorus and vitamin D in the body |
| Thymus gland • Only active till puberty • Also part of the immune system | • Thymosin | • Production and maturation of T-lymphocytes or T cells (note that all T cells in the body are produced by puberty) |
| Adrenal glands | • Cortisol • Aldosterone • Androgenic steroids (converted to estrogens in the ovaries) • Epinephrine (Adrenaline) Norepinephrine (Noradrenaline) | • Response to stress • Metabolism • Immune system • Blood pressure • Heart rate |
| Pancreas | • Insulin | • Maintains blood sugar levels |
| Ovaries | • Progesterone • Estradiolstrone • Estriol | • Development of female sex characteristics • Menstrual cycle • Reproductive system |
How Hormones Work
At its most basic, the endocrine system works like this:
Gland → hormone → bloodstream → receiving/target cell → action
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.
- The first loop (shown with the blue arrow) happens when FSH and LH have been secreted by the anterior pituitary and trigger the development of the follicle. Before ovulation, FSH secretion increases causing the follicle to secrete estrogen, which loops back up to the anterior pituitary signaling to decrease production of FSH.
- The second loop (shown with the red arrow) occurs when estrogen levels rise at the mid-point of the menstrual cycle. The rise in estrogen triggers a positive feedback loop to increase secretion of LH from the anterior pituitary triggering ovulation.
- The last loop (shown in purple) happens when the follicle has released the egg during ovulation and it changes into the corpus luteum. The corpus luteum secretes progesterone and some estrogen which supports continued formation of the lining of the uterus (to prepare for the egg implantation). The progesterone and estrogen also send a signal to the anterior pituitary to decrease the secretion of FSH and LH. The drop in these hormones causes the corpus luteum to dissolve and menstruation to start.
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.
The Most Relevant Sex Hormones for Women in the Menopausal Transition
This table shows the most important hormones during the menopause transition, what the hormones are, where in the body they are produced and what they do in the body.
| Hormone Name | What is it? | Where is it made? | What does it do? |
|---|---|---|---|
| Progesterone | • In the corpus luteum in the uterus (which is formed from a ruptured follicle that just released an egg) | • Causes the lining of the uterus to thicken in preparation for pregnancy after an egg is released • If there is no pregnancy, the corpus luteum (where the progesterone is formed) breaks down, dropping progesterone levels and triggering menstruation |
|
| Estradiol (Estrogen) aka: E2G Oestradiol 17β-estradiol | • Strongest of the three types of estrogen • A steroid hormone made from cholesterol | • Mostly in the ovaries • Smaller amounts in other tissues such as the brain, fat tissue, and blood vessel walls | • Primary activity is with the reproductive system • Maintains and controls the menstrual cycle • Triggers breast tissue development • Increases bone and cartilage density • Acts on multiple centers in the brain |
| Estrone (Estrogen) aka: E1G Oestrone | • Weaker form of estrogen • The major type of estrogen produced post-menopause | • Mostly in the ovaries • Some from the adrenal gland • Smaller amounts from fat tissue | • Specifics are poorly understood • As an estrogen it is involved in the female reproductive system. |
| Estriol (Estrogen) aka: E3G Oestriol | • Exists in very low levels in non-pregnant women | • High amounts produced by the placenta • Triggered by a chemical produced in the fetus’ adrenal gland | • Involved in uterine growth • Helps prepare the body for childbirth |
| Testosterone aka: 4-androsten-17β-ol-3-one | • Member (and best known) of a group called androgens | • Mostly in the adrenal gland • Small amounts in the ovaries | • Stimulates development of male characteristics • Enhances libido • Regulates the secretion of LH and FSH |
| Follicle-stimulating Hormone (FSH) | • Member of a group of hormones called gonadotropins | • The anterior pituitary gland (from cells called gonadotrophs) • Its release is regulated by the hypothalamus | • Essential for development at puberty • Triggers the release of estrogen • Triggers egg development |
| Luteinizing hormone (LH) | • Member of a group of hormones called gonadotropins | • The anterior pituitary gland (from cells called gonadotrophs) • Release is regulated by the hypothalamus | • Triggers ovulation (the release of an egg) • Triggers estrogen and progesterone production from the corpus luteum |
| Anti-Mullerian Hormone (AMH) | • Follicles in the ovaries | • Presence in the fetus will trigger development of a male • Important in the development of the follicles • The more ovarian follicles a woman has, the more AMH her ovaries will produce • Used as a measure of the ‘ovarian reserve’ or how many eggs are remaining. |
|
| Androstenedione | • Member of a group called androgens | • Cortex of the adrenal glands • The ovaries | • Once secreted it gets quickly converted into estrone and testosterone • Produces almost all the estrone in the body |
Compiled References
[1] You and Your Hormones (The Society for Endocrinology) https://www.yourhormones.info/hormones/follicle-stimulating-hormone/
[2]https://www.endocrineweb.com/endocrinology/introduction-endocrinology-endocrine-surgery
[3] Melmed, S., Polonsky, K. S., Larsen, P. R. & Kronenberg, H. M. (2016) Williams Textbook of Endocrinology. Philadelphia, Pa: Elsevier.