Hormones are the chemical messengers that help body replace and repairs body cells as they wear out or are damaged. These chemical messengers are created and controlled by your endocrine system.
Your endocrine system controls metabolism, growth and reproduction and helps you adapt to stress. Your endocrine system also regulates the concentrations of substances in your blood, like glucose, calcium, sodium, potassium and water. The hormones responsible for female characteristics like breast development are called oestrogens while the hormone responsible for male characteristics like beard growth and voice deepening are called androgens. Oestrogen sex hormones tell your body to follow a female pattern and androgen sex hormones tell your body to follow a male pattern.
There are different kinds of hormones in your body and they are all produced by glands. The glands that produce most of the androgen sex hormone are called testes and the glands that produce most of the oestrogen sex hormone are called ovaries. The adrenal glands (above the kidneys) produce a small amount of androgens by both males and females.
These sex hormones (androgens or oestrogens) also travel to many different areas throughout your body via your blood stream. They regulate growth and development of breast tissue, hair follicles and fat tissue under your skin, activate your reproductive organs, play a role in sexual functioning, regulate the reaction of your body to emergencies and influence your moods.
If you take a different sex hormone to that which is naturally produced by your body, the new hormone will slowly block the message from your own glands and introduce a new message.
So for example, a man identified female at birth who takes the male androgen testosterone will over time naturally develop male secondary-sex characteristics in his physical body. Physical development occurs slowly, similar to that of natural puberty.