What are hormones?

Besides producing sex cells, another function of the gonads is to secrete hormones. Hormones are chemical molecules found in all multi-cellular organisms. They are secreted by the body’s endocrine glands directly into the blood, which carries them around the body so they can perform their tasks. Hormones are best thought of as messengers that carry instructions to our tissues and organs: an internal communication system between different parts of the body, making its cells do certain things. There are many kinds of hormone, affecting such things as our growth and puberty, sleep, cognition, metabolism, and sexual reproduction. The endocrine system therefore is a conversation in the body between glands, hormones and cells.

The endocrine system is made of various glands scattered around the body, the major ones besides the gonads being:

- Brain: hypothalamus, pituitary gland, and pineal gland
- Neck: thyroid and parathyroid glands
- Between the lungs: thymus
- Above the kidneys: adrenal glands
- Behind the stomach: the pancreas

The glands produce the hormones, the hormones travel around the body in the bloodstream (usually), and the hormones bind to receptors on the target cells. This hormone/receptor union triggers processes in the cell. By exposing millions of cells to regulated quantities of different hormones, the endocrine system can orchestrate changes in the body.

Hormonal change can also provoke mood changes, because they can affect the quantities of chemicals like seratonin that influence how we feel.

Human sex hormones are steroid hormones, which mean they can pass through the fatty outer membrane of their target cells, enter the nucleus, and bind to its DNA, and can therefore influence gene expression. (You may also have heard of anabolic steroid hormones, which are artificial molecules that mimic the actions of testerone.)

Males and females produce the same hormones: the difference lies in the relative quantities of each.

Male

The testes secrete a group of sex hormones known as androgens, which play a role in producing male sexual traits. They are produced by women’s ovaries too, but in much lower quantities.

The major androgen is testosterone (produced primarily by the testicles but a bit is also produced by the adrenal cortex). Things governed by testosterone include:

• Growth of male reproductive organs
• Heavier skeleton and musculature
• Growth and distribution of body hair e.g. beard
• Larger larynx and thus deeper voice
• Sex drive
• Sperm production

The production of testosterone begins while the foetus is developing in the womb, and continues for a short time after birth. During childhood it virtually stops until resuming at puberty, the point where boys really open up the physical gap with girls. Then it starts to decline after the age of 30 or so. As there is no male menopause, there is no sudden fall in testosterone production equivalent to the fall in women.

Female

As well as producing eggs (oocytes), the ovaries secrete the hormones oestrogen and progesterone, both of which govern female sex characteristics amongst other things. Men’s testes (and adrenal gland) produce them too, but in much lower quantities.

Oestrogen is technically a group of hormones, the three major ones being estradiol, estrone, and estriol. It helps develop and maintain both the reproductive system and female sex characteristics, such as the breasts, but also affects more general health such as the bones and metabolism.

Progesterone is particularly associated with preparing the uterus for pregnancy, but both hormones play a role in pregnancy and the menstrual cycle (there are also other hormones that contribute).

• Growth of female reproductive organs (uterus, vagina)
• Growth of breasts
• Widening of the pelvis
• Distribution of body fat
• Sex drive

Production of oestrogen and progesterone increases during puberty. During the menopause, i.e. when a woman stops being able to give birth to children and her periods cease, the levels of these hormones fall fast.

Sex hormones

Contrary to common misconception, there are no exclusively ‘male’ or ‘female’ sex hormones. Both sexes produce testosterone, oestrogen and progesterone, just in differing quantities.