What are hormones?

Some hormones uniquely contribute to female health – here we go into great detail about female hormones from childhood through to the menopause.

Hormones, part of the endocrine system, are incredibly complex molecules that play a major role in growth, repair and reproduction in the body.

Like other systems in the body, the endocrine system regulates itself through a mechanism called homeostasis (staying the same). To maintain homeostasis, the body uses negative or positive feedback mechanisms. It is important to be aware of feedback, as this is critical in hormone control, with the release of some hormones being able to stimulate or prevent the production and/or secretion of other hormones.

Negative feedback is the most common type of feedback in the body.

For example: Yellow gland releases red hormone. Red hormone stimulates cells to release blue hormone. When levels of blue hormone are too high, the body tells yellow gland to stop releasing red hormone (which stops the secretion of blue hormone).

Positive feedback has the opposite effect, and can increase or amplify the effect.

For example: If green hormone produces more of pink hormone, this causes the production of more green hormone.

Problems with glands, hormone production or secretion in the body usually causes problems in feedback mechanisms, such as: if yellow gland does not produce enough red hormone, then red hormone is unable to stimulate enough cells to release blue hormone, leading to a shortage of blue hormone in the body.

Breakdown of female hormones

Although many hormones carry out the same function in males and females, a number of hormones do contribute uniquely to female health. Some of the major female hormones are:

  • Estrogen
  • Progesterone
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin
  • Oxytocin

Perhaps the easiest way to provide an overview of female hormones is from early childhood, through puberty, menstruation and pregnancy, to the menopause.

Female hormones in childhood

Although some female hormones are in the body from the time of birth, they are at low levels.

Around the age of seven or eight, hormones called adrenal androgens (for example DHEA) become active and are responsible for the growth of pubic and armpit hair. These also increase secretion of LH and FSH, stimulating the secretion of estrogen from the ovaries.

Estrogen, along with progesterone, allow breasts, the uterus, vagina and associated tissues to develop. Once a female has the correct amount of body fat, the first menstrual cycle will occur, which is roughly 12 years of age.

Where does the hormone command chain start?

The hypothalamus in the brain releases gonadotropin releasing hormone (GnRH). GnRH is like a master hormone, as it controls the production and release of other key female hormones.

GnRH can stimulate the release of FSH and LH from the pituitary gland.

FSH stimulates estrogen secretion by growing follicles in the ovaries. LH helps the ovarian follicles develop further, causing further increases in estrogen secretion. This higher level of estrogen means ovulation can occur and that production of other hormones such as progesterone and relaxin can begin.

FSH

FSH ‘recruits’ a number of ovarian follicles that have been growing for some months in the ovaries.

Reduced FSH and LH levels cause the recruits to die, with the exception of a dominant follicle. This dominant follicle becomes a bulge on the surface of the ovary. When a surge in LH occurs, the follicle ruptures, releasing an oocyte (an immature egg) into the pelvic cavity. This is ovulation. 

When the oocyte/egg is released from the follicle, it begins to move from the ovary to the uterus, via the fallopian tube. If the oocyte is fertilized by a sperm cell in the fallopian tube, the egg will continue its journey until it is able to adhere to the wall of the uterus. If two follicles ovulate and both eggs are fertilized, fraternal twins can occur.

LH

Not only does luteinizing hormone help ovarian follicles to develop and release an egg and increase estrogen secretion, it also assists with the corpus luteum. This ‘yellow body’ is what holds the remains of a mature follicle that has moved through ovulation.

After ovulation, while the oocyte is moving along the fallopian tubes and into the uterus, the follicle remains collapsed into the folded and temporary structure called the corpus luteum. This produces a number of hormones, including estrogen, progesterone, inhibin and relaxin. Cleverly, this increase in hormone levels also signals to the pituitary gland (through negative feedback) that it should not let another follicle develop.

If fertilization does not occur, the corpus luteum stops producing hormones and instead becomes a ‘white body’ or corpus albicans, seen as whitish scar tissue in the ovaries. At this point, the decrease of hormone levels promotes shedding of the womb lining (menstruation) and the release of GnRH – the master hormone – that triggers the start of a new cycle.

If fertilization does occur, another hormone called human chorionic gonadotropin (HCG) stops the corpus luteum from breaking down, allowing it to continue releasing its hormones to support the early pregnancy.

Estrogen

Estrogen is a group of hormones that includes estrone (E1), estradiol (E2) and estriol (E3).

It helps develop and maintain female sex organs and secondary sex characteristics such as widening hips and changing body fat distribution.

It’s also important in controlling fluid and salt levels in the body, increasing protein metabolism and lowering blood cholesterol levels. This is possibly why younger women have a lower risk of coronary artery disease than men do.

In developing babies, estrogen assists in the development of organs and regulating bone density.

While FSH and LH are organizing follicle maturation and oocyte movement, estrogen starts the formation of a new layer of endometrium cells in the uterus (womb lining) and stimulates crypts in the cervix to produce cervical mucus. This can alter the acidity of the vagina, allowing sperm to survive.

When estrogen levels reach their highest, they cause the surge in LH that triggers ovulation.

Progesterone

Progesterone is another key female hormone that works alongside estrogen to prepare the womb for the implantation of a fertilized egg. It also prepares the breasts for milk secretion and helps to soften joints in the body, which can cause aches in the hips during late pregnancy.

High levels of progesterone are able to stop GnRH and LH secretion, meaning progesterone can also stop ovulation (if an egg is fertilized).

If progesterone levels fall, it causes the corpus luteum and the womb lining to break down and menstruation to begin.

Relaxin

This hormone is able to relax the uterus by inhibiting contractions, probably to allow easy implantation of a fertilized egg.

Relaxin is also produced by the placenta during pregnancy. It is thought to assist in dilation at the time of birth helping the pelvis joints to stretch and the cervix to soften.

Human chorionic gonadotropin (HCG)

Produced by the placenta around eight days after fertilization, HCG mimics LH, allowing continued secretion of progesterone and estrogen. Most early pregnancy tests detect low levels of HCG in urine.

Oxytocin

Oxytocin not only triggers uterine contractions at the time of delivery but also allows the breasts to release milk when stimulated by suckling, or even the sound or sight of a stimulus such as a baby crying.

Human placental lactogen (HPL)

Human placental lactogen (HPL), also known as human chorionic somatomammotropin (HCS), helps the breasts prepare for breastfeeding and allows the body to produce colostrum, the rich pre-milk, in the early days after delivery.

HPL also adjusts maternal metabolism, so that more glucose is available for feeding. High levels of HPL may contribute to gestational diabetes.

Prolactin (PRL)

Prolactin is also essential for breastfeeding. Although the body secretes higher amounts of prolactin as the pregnancy progresses, progesterone stops lactation from occurring until after delivery. At this point the mother’s levels of progesterone and estrogen decrease.

Inhibin, activin and other pregnancy hormones

Inhibin is secreted in the ovary and inhibits secretion of FSH, counteracting the effect of activin, which increases the production and secretion of FSH. Both inhibin and activin have other roles in the body, some of which are still under investigation.

There are many more hormones involved in pregnancy, some of which are critical in normal bodily functions as well in reproduction:

  • Adrenocorticotropic hormone (ACTH) can stimulate and regulate the release of other hormones, including the ‘stress hormone’ cortisol. During pregnancy, ACTH can contribute to hormone disorders, stretch marks and swelling.
  • Thyroxine is a key hormone involved in heart and muscle function, brain development, bone maintenance and digestion. In pregnancy, it is linked to adjusting oxygen metabolism.
  • Insulin controls blood glucose (sugar) levels during pregnancy and in everyday life.

The menopause

Over the 5-10 years that lead to the last period (peri-menopause), levels of estrogen and other female hormones slowly fall.

This causes the ovaries to stop producing eggs and, because the womb lining is no longer thickening, periods also stop. An altered level of estrogen and other hormones are the main causes of menopause symptoms such as hot flushes, mood swings and night sweats.

Conclusion

Hormones are essential for the daily functioning of everyone, whether male or female, young or old. But they are particularly important for female health and wellbeing.

The imbalances that can occur in female hormone production or secretion cause more health problems than it is possible to list here.

References

 

Last reviewed 04/Sep/2017

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