The Endocrine System
The body's internal communication network controls blood glucose
The body's internal communications are handled by two complementary but quite different networks: the nervous system and the endocrine system. The nervous system acts like a sort of instant messenger, an electronic communication that immediately makes its point. The endocrine system is more like regular mail, delivering physical messages along the body's roadways. When there's a breakdown in communication, disease can result.
While there are many endocrine diseases, the most common one in the United States is diabetes. That's why a diabetes specialist is called an endocrinologist: a physician who treats diseases of the endocrine system.
The "mail" of the endocrine system consists of hormones. These chemical messages are released by specialized endocrine cells and travel in the blood to different parts of the body to elicit a particular response. While the nervous system is best for business that requires lightning-quick action, like removing a hand from a hot stove, the endocrine system generally concerns itself with bodily processes that take some time, like growth, puberty, pregnancy, lactation, mood changes, and metabolism. Endocrine disorders can result when hormone levels are too high or too low, or when the body fails to respond to hormones.
The body has eight endocrine glands. The cells of the pancreas that make insulin—the islets of Langerhans—are endocrine glands; so are the hypothalamus, ovaries and testes, and thyroid, as well as the adrenal, parathyroid, pineal, and pituitary glands. In addition, endocrine cells in the stomach and intestine release hormones, too.
The hormones that are sent off into the body from the glands and endocrine cells are of two basic varieties: fat soluble and water soluble. Steroid hormones, like estrogen and testosterone, are fat soluble (they are synthesized from cholesterol) and thus can slip through cell membranes to directly cause changes inside the cell. The water-soluble hormones, like the protein insulin, are made from materials that can't bust through membranes. They get their messages to the insides of cells by docking with receptors that speckle the outer surface of cells.
Hormonal checks and balances in the endocrine system govern the body's ability to maintain stable internal conditions, which is called homeostasis. The levels of glucose in the blood are under homeostatic control; several hormones work together to keep the concentration of glucose within a tight range. When this system breaks down, blood glucose can go too high, resulting in diabetes.
Blood glucose stability is maintained using two types of hormones: those that promote energy storage and those that promote energy use. After digesting a meal, glucose from food enters the blood. When the body is functioning properly, the endocrine cells in the pancreas sense an increase in blood glucose and interpret that to mean the body is well fueled. In response, the cells release insulin, a pro-storage hormone that travels around the body telling muscle, fat, and liver cells to absorb and store glucose from the blood.
If, on the other hand, a person hasn't eaten for a while or is exercising really hard and blood glucose starts to dip, the pancreas senses that the body is short on energy and that it's time to tap some energy stores. The pancreas releases glucagon, a hormone that has almost the opposite effect of insulin. Glucagon travels to the liver, where it triggers the release of glucose stores into the blood and promotes the construction of additional glucose from other energy sources, like protein, to maintain homeostasis.
A shortage of insulin leads to diabetes because blood glucose levels rise unchecked. This is why the discovery of insulin led to the first effective treatment for diabetes, since insulin in the right doses can bring the body back to blood glucose homeostasis. It is still an indispensable medicine for people with type 1 diabetes and for some people with type 2. Medications for type 2 other than insulin work either by getting more insulin out of the pancreas or by making the body more sensitive to the effects of insulin. These treatments have saved the lives of many people with diabetes. However, one side effect of some diabetes medications, including insulin, is overshooting the amount of insulin needed by the body to maintain homeostasis, causing blood glucose to go too low (hypoglycemia).
Other hormones, all part of the endocrine system, can affect blood glucose levels as well. Cortisol is a steroid hormone that is released under physical or psychological stress; it increases blood glucose levels to provide extra energy. (Use of steroid medications can also raise blood glucose levels.) Incretins, hormones that are released from the gut in response to eating, start the flow of insulin from the pancreas even before blood glucose levels rise, helping prevent glucose levels from getting too high after a meal. People with type 2 diabetes have low levels of incretins, which is why some diabetes medications, such as exenatide (Byetta) and sitagliptin (Januvia), attempt to regulate blood glucose by mimicking or assisting the body's incretins.
For people with diabetes, it's typically the hormones that regulate blood glucose homeostasis that are out of balance, but there may be links between diabetes and other types of hormones as well. For example, women with polycystic ovary syndrome (PCOS) and men with hypogonadism—endocrine disorders associated with sex hormone imbalances—are more likely to have diabetes.
Getting a piece of mail quickly to the correct location is, of course, not easy. And the endocrine system has the daunting task of orchestrating the delivery of countless hormonal messages to specific cells in the body. Diabetes occurs when these lines of communication are cut by either the inability to produce insulin or a failure to respond to insulin's message to lower blood glucose. Fortunately, carefully selected medications can restore the dialogue that the endocrine system makes possible and bring the body back into balance.