Hormones play an essential role in sustaining the body’s communications system. Without them, boys would not develop their masculine characteristics and women would be incapable of reproducing.
Hormones are chemicals which are produced by the endocrine glands. They play a major part in influencing our tissues and organs and in controlling and linking up different parts of our bodies. Because of this they are often called the body’s messengers.
Men and women have the same endocrine glands apart from the reproductive glands, or gonads, which in a man are the testes and, in a woman, the ovaries. In both sexes the gonads secrete male and female hormones, but in different quantities. Men produce extra quantities of the male sex hormone, testosterone, and women produce extra quantities of the female sex hormone, oestrogen.
The hypothalamus – a tiny area of the brain – is ultimately responsible in both sexes for controlling the output of sex and other hormones. It exercises this control via the pituitary gland.
The pituitary, the most complex of the endocrine glands, lies just below the hypothalamus and is connected to it by a stalk. The hypothalamus sends chemical messages to the pituitary gland to release hormone-stimulating substances, which regulate the manufacture and release of hormones in the other endocrine glands.
Changes at puberty
At puberty, the level of sex hormones in boys and girl rises dramatically. The pituitary gland releases gonadotrophin (follicle-stimulating hormone and luteinising hormone) which cause the sex glands to develop. In boys, testosterone makes the penis and testes grow, and is responsible for the production of sperm and seminal fluid. It is also responsible for the development of a man’s masculine characteristics.
The menstrual cycle
At puberty, a girl starts menstruating and will soon be able to have children.
The first few menstrual cycles tend to be anovulatory – that is, the ovaries are not yet releasing ripened eggs to be fertilised.
Before the first period, the pituitary gland releases the follicle-stimulating hormone (FSH) which encourages growth of the egg-producing follicles and production of oestrogen in the ovaries.
Oestrogen is the hormone responsible for a woman’s feminine characteristics – her rounded figure, full breasts, smooth skin and higher-pitched voice. It also causes the uterus to grow and the vagina to enlarge.
The quantity of oestrogen in a woman’s body remains high until the menopause, but varies according to which part of the menstrual cycle she is in.
At the start of the menstrual cycle – the first day of every period – the level of oestrogen in the body is comparatively low. Over the next two weeks it slowly rises as some of the follicles in one of the ovaries are stimulated to make egg cells which produce large amounts of oestrogen.
Once oestrogen is at a certain level, less FSH is released. The pituitary gland begins to produce luteinising hormone (LH), which stimulates the growth of one follicle.
Midway in the cycle, this follicle releases the egg which bursts out of the ovary and goes to the nearest Fallopian tube for possible fertilisation. This is ovulation, and it is now that a woman’s oestrogen level is very high.
The ruptured egg follicle, the corpus luteum, carries on producing oestrogen in decreasing amounts, then it starts to produce the hormone, progesterone. These two hormones help make the lining of the uterus, or endometrium, thick and spongy for the possible arrival of the egg.
If the egg is unfertilised, it fragments and is dispersed. The corpus luteum shrinks away, the oestrogen and progesterone levels drop until the output of progesterone stops altogether and the oestrogen in the blood is at its lowest.
Without these two hormones to sustain it, the lining of the uterus begins to disintegrate, and menstruation (the ‘period’) begins. The pituitary gland then increases its output of FSH to stimulate a new crop of follicles, and the cycle begins again.
Progesterone and PMS
Pre-menstrual syndrome (PMS) affects many women just before their periods. It may be caused by insufficient levels of progesterone in the body just before menstruation, and women who take extra progesterone at this time sometimes find that their PMS symptoms are alleviated or will disappear.
Although taking the contraceptive Pill often has the welcome side-effect of relieving painful periods, it will not dispel the symptoms of PMS, which are caused by a low progesterone level. This is because the normal combined Pill also contains progestogen, a synthetic hormone which reduces the body’s natural hormone, progesterone. The Pill can therefore intensify PMS.
A pregnancy is established when the fertilised egg implants itself in the lining of the uterus, and hormone production in the menstrual cycle is interrupted.
Part of the fertilised egg now begins to produce the hormone chorionic gonadotrophin (HCG). This causes the corpus luteum in the ovary to enlarge and to go on producing large amounts of the hormone progesterone.
This level of progesterone prevents ovulation and menstruation, and maintains the lining of the uterus in a suitable condition to nourish the developing embryo while the placenta forms.
During the early weeks of pregnancy, ovaries continue to secrete oestrogen, but output remains low until the 12th to 14th week, when it rises significantly.
The placenta is now able to make its own supplies of HCG, oestrogen and progesterone to sustain the foetus. At this stage the corpus luteum starts to reduce its hormone output and degenerate.
During pregnancy, the pituitary gland doubles in size as it makes increasing amounts of the milk-producing hormone, prolactin. Within the first weeks, a woman’s breasts start to swell as the milk-producing glands become larger.
Levels of oestrogen and progesterone continue to rise until the baby is ready to be born, when the amount of oestrogen in the blood suddenly drops.
This alerts the pituitary to release the hormone, oxytocin, which causes the womb to contract during labour so that the baby can be born.
The last major hormonal change in a woman’s life occurs at the menopause.
The menopause is usually gradual. As the ovaries age, the egg follicles in them begin to degenerate, and become less responsive to FSH and LH. As production of these hormones declines, the pituitary increases its output of FSH and LH in an attempt to stimulate the follicles into releasing their eggs. This causes a hormone imbalance which is why so many unwelcome symptoms, such as hot flushes and headaches, can accompany the menopause.
Lack of oestrogen causes the uterus and ovaries to shrink, and periods to become irregular and eventually stop.
Some women sail through the menopause, while others may be given a treatment known as hormone replacement therapy (HRT) to alleviate such distressing symptoms.