First is the pituitary gland which produces hormones that regulate other glands. By increasing or decreasing the amount of these regulatory hormones, the pituitary can control the function of the glands. For example, thyroid stimulating hormone (TSH) increases the activity of the thyroid gland, causing it to manufacture and release more thyroid hormone. More TSH will stimulate the thyroid gland into greater activity, while small amounts of TSH will slow the thyroid down.
The pituitary gland itself is under the regulation of another gland - the hypothalamus. In the case of the thyroid, the hypothalamus manufactures a substance called thyrotropin realeasing hormone (TRH), which regulates the production of TSH by the pituitary. So TRH stimulates the pituitary, which releases TSH, which in turn stimulates the thyroid gland.
There are similar regulating substances that the hypothalamus uses to control pituitary function with respect to other glands. In the case of estrogen and progesterone (and testosterone in men), this substance is called gonadotropin releasing hormone (GnRH).
The hypothalamus is under the direct control of the brain. In fact it is a part of the brain that specializes in endocrine function. Thus, the brain and the nervous system are ultimately in control of hormone release and production. This is why your mental and emotional state can have a strong effect on hormone balance and metabolism.
In addition to the brain, each hormone has an ability to regulate its own production. This mechanism is known as negative feedback. Again, I will use the thyroid hormone as an example. As the thyroid hormone is released into the blood, it circulates throughout the body, eventually finding its way to the pituitary and the hypothalamus. Like all the other cells in the body, the cells of these glands have thyroid receptors. As these receptors become more activated, they convey the message that there are increasingly greater amounts of the thyroid hormone present in the circulation. This results in the decreased production of both the TRH and TSH, which leads to lower activity of the thyroid gland.
If the pituitary and hypothalamic receptors do not 'see' enough thyroid hormone, a message is sent to produce more TRH and TSH. A similar feedback loop exists for most hormones in the body.
The glands and hormones are programmed to work harmoniously with one another to make sure the body functions properly. In some cases, the hormones or gland do not perform correctly. Thankfully, the medical community has developed therapies and drug treatments to assist with many of these deficiencies and help improve performance of the glands and hormones.
