A hormone is a chemical "messenger" which is made and secreted (released) by an endocrine gland.

There are up to 40 different hormones in your blood at any one time. A hormone often affects different parts of the body in different ways. For example, the male sex hormone testosterone is not only responsible for sexual drive, but it also increases muscle size and encourages the growth of pubic, facial and body hair. Generally, only small quantities of hormone are needed for a specific part of your body to respond.

Take a look at Figure 2. A hormone is secreted by one of your endocrine glands, typically into the bloodstream. Once in your blood, the hormone circulates around your body until it hits a specific "target" or "targets". These targets can either be other endocrine glands or other parts of your body.

Hormone action.

Normally, there is a fine balance of hormones. Too much or too little of one hormone could have an impact on the release of other hormones. If this situation arises, some of your body's systems do not work properly because of the hormonal imbalance. In order to bring the hormones back to their normal levels, your body has a way of keeping track of them and responding to any changes. It does this by means of a complex but highly efficient feedback system.

The feedback system links some endocrine glands with others. It is perhaps easier to use the simple analogy of the thermostat, a temperature-controlling device used in your home central heating system. A thermostat keeps the temperature of a room constant for as long as the heating is switched on. The hormone feedback system works in a similar way, except that it is not room temperature, but rather the level of a hormone which is monitored. Too little or too much of a hormone is corrected by "feeding" some of it (and sometimes other hormones) from the target back to the original endocrine gland. This "tells" the endocrine gland to release more or less of the hormone and the balance is restored.

Kallmann's syndrome is the result of an hormonal imbalance, caused by the failure of your hypothalamus to secrete one particular hormone. This has rendered the feedback mechanism ineffective, resulting in your abnormally low hormone levels.