Hormones are long-range chemical messengers of the body, manufactured and controlled by the endocrine system. Doctors that specialise in the hormone system are thus called endocrinologists.

Both male and female foetuses start out life looking the same, with all the same organs and female appearance. During development of the foetus, hormones are released, which trigger different cells to differentiate into either male or female form. Assuming everything works out OK, the baby is born clearly sexed, with a penis, testes, and prostate for boys, and vagina, ovaries, and womb for girl babies.

Hormones continue to play an important part in a persons development, right throughout their life. Even as a fully grown adult, your body is continually replacing cells. Hormones regulate this function, acting as chemical messengers to tell your body what to put where, and in what form.

Sex differentiating hormones are mostly produced by the testes in males, and the ovaries in females. They are distributed to all the cells in your body by your bloodstream. Sex hormones tell the individual cells, as they rebuild and grow, whether to form male or female patterns.

The main hormone responsible for female characteristics is called oestrogen. The corresponding male hormone is called testosterone. Everyone has both oestrogen and testosterone circulating in their systems, but in females there is much more oestrogen than testosterone, and vice-versa for males.

Earlier this century, scientists identified the major sex hormones, and figured out how to extract them from natural sources, or to reproduce them in a laboratory. Thus it became possible to alter the proportions of the main sex hormones, and cause 'males' to develop along female patterns, and 'females' to develop along male patterns.

Unfortunately, after you're born, it isn't possible to change your primary sex characteristics, such as presence or absence of a penis or vagina, by administering opposite sex hormones. Also, after you've undergone puberty, many characteristics become fixed, such as skeletal structure, height, the size of your hands and feet etc.

The actual degree of change experienced by a person undergoing hormone therapy are highly dependent on the following:

• Age at which you start taking hormones. This isn't a simple linear thing. There is a much more dramatic difference between a 20 year old and a 30 year old, than between a 30 year old and a 40 year old. Generally speaking, the closer you are to puberty, the better the results.
• Genetic disposition. Simply put, if you're trying to grow breasts, for example, you haven't much of a hope if your mum and all your sisters are flat chested. Much the same can be said for any characteristics. Look at your mum, or dad, and siblings.