The name hormones is given to a number of chemicals produced in the body which are secreted into the bloodstream and affect the functioning of many body systems and organs. The word hormone is taken from a Greek root meaning to excite, since the action of the hormone is most often to stimulate activity in an organ. A characteristic of hormones is that they influence bodily functions at a distance from the point of origin of the hormone. For this reason they are often referred to as 'chemical messengers'.

The group of glands which produces hormones is known as the endocrine system, and is responsible for regulating growth, metabolism, reproduction, our response to stress and the levels of various vital nutrients in the bloodstream. None of these glands operates in isolation from the others; there are very complex and finely-balanced relationships between them, and the whole is regulated by the pituitary gland. This is situated just below the brain, which is itself influenced by the hypothalamus, which acts as an interface between the brain, the nervous system and the endocrine system.

The pituitary gland itself, as well as regulating the activity of the others, governs growth. The thyroid gland is involved with growth and metabolism. The parathyroids regulate the levels of calcium in the blood. The adrenal glands are involved in a variety of body functions, including the metabolism of starches, and our response to stress; they also influence the testes and ovaries and are involved in producing secondary sexual characteristics. The Islets of Langerhans (specialised areas within the pancreas) produce the hormone insulin, which is involved in the maintenance of levels of sugar in the blood. The ovaries and testes produce the male and female hormones, oestrogen, progesterone, testosterone and others, and regulate the reproductive cycle, lactation and secondary sexual characteristics (facial hair, breast development, etc.). Many essential oils have an effect on the endocrine system, and they may do so in one of two ways:

Some oils contain plant hormones, known as phytohormones, which are similar in action to our own, and act within the body in a similar manner, reinforcing or replacing the effects of the corresponding human hormones. For example, oil of Fennel contains a plant oestrogen, and is beneficial in menopausal problems and pre-menstrual tension, and it stimulates the production of breast milk. Phytohormones can be used in a similar way to some of the synthetic or animal-derived hormones used in orthodox medicine, but without the dangers and moral objec­tions associated with them.

Other essential oils influence hormone secretion of the various glands, acting as a trigger or balancer of hormone production. For example, Garlic and Onion help to balance thyroid secretion, and are particularly valuable when the thyroid gland is under-active. Basil, Geranium and Rosemary stimulate the adrenal cortex, though Geranium is also a balancer of hormone production in general. Eucalyptus and Juniper help to reduce excessive blood-sugar levels, and here again Geranium acts as a balancer. Many oils, but particularly Rose, including Camomile, Clary Sage, Cypress, Fennel, Jasmine and Lavender, affect the reproductive system and are very valuable in disturbances of the menstrual cycle and menopausal problems.

The influence of essential oils on the endocrine system is one of the most important ways in which they act on the human body, and there is still a lot that is not known about plant hormones. It is very probable that many more essential oils which influence the body do so through subtle hormonal effects that are not yet understood.