Case Study Conclusion: Hormonal Havoc
Gabrielle, who you read about in the beginning of this chapter, has polycystic ovary syndrome (PCOS). PCOS is named for the multiple fluid-filled sacs, or cysts, that are present in the ovaries of women with this syndrome. You can see these cysts in the illustration above, which compares a normal ovary with a polycystic ovary. The cysts result from follicles in the ovary that did not properly produce and release an egg. Mature eggs are normally released from follicles monthly during the process of ovulation, but in PCOS this often does not occur. Ovarian cysts can be common and do not necessarily mean that a woman has PCOS, but the presence of multiple ovarian cysts plus other telltale signs and symptoms may cause her physician suspect PCOS.
Gabrielle’s symptoms of PCOS included irregular menstrual periods, weight gain, acne, and excess facial hair. There are many other symptoms of PCOS that women can experience, such as male-pattern baldness, pelvic pain, and depression, among others. As you may recall, Gabrielle also had some abnormal blood test results, such as high levels of androgens and blood glucose. These can also be indications of PCOS.
As you have learned, androgens are a term for male sex hormones, but females also normally produce androgens, albeit to a lesser extent than males. In women with PCOS, the level of androgens is abnormally high. These androgens include testosterone, which is produced by the ovaries, and DHEA, which is produced by the adrenal glands. This increase in androgens can have a “masculinizing” effect on women, including an increase in facial and body hair, male-pattern baldness, and interference with the menstrual cycle by preventing ovulation. Androgens also can cause weight gain and acne — two of the other common symptoms of PCOS.
In addition to hypersecretion of androgens, PCOS often causes high blood glucose as a result of insulin resistance. As you have learned, insulin is a hormone secreted by the pancreas that works in conjunction with other pancreatic hormones (such as glucagon) to regulate the level of blood glucose. What is another disease involving insulin resistance? If you answered type 2 diabetes, you are correct! In fact, women with PCOS are at a high risk of developing type 2 diabetes because of their resistance to insulin. More than 50 percent of women with PCOS will develop diabetes or pre-diabetes before they are 40 years old.
Besides diabetes, women with PCOS have a higher chance of developing fertility problems, heart disease, sleep apnea (briefly stopping breathing during sleep), and uterine cancer, among other diseases and disorders. There is hope, however. Lifestyle modifications and medicines not only can help women cope with the symptoms of PCOS, but may also reduce the risk of some of the possible long-term consequences by lowering blood sugar and androgen levels. For instance, eating a healthy diet and exercising regularly can help women with PCOS lose weight. This can help lower blood glucose levels, improve insulin functioning, and can even make the menstrual cycle more regular. Medications such as birth control pills and anti-androgens can help restore a more regular menstrual cycle and reduce facial and body hair and acne. The diabetes medication metformin can be used to treat several of the symptoms of PCOS, and even may prevent type 2 diabetes, by improving insulin functioning and lowering testosterone. Finally, women with PCOS who are trying to conceive may be helped with fertility medications that stimulate ovulation.
The underlying cause of PCOS is not definitively known, although it is thought that both genetic and environmental factors play a role. PCOS tends to run in families, and women with a sister with PCOS are twice as likely to also have it. Researchers think that the insulin resistance seen in PCOS may cause an increase in androgens, illustrating how hormonal systems can influence each other.
As you have seen throughout this chapter, endocrine hormones can have a wide variety of effects on the body, including the regulation of metabolism, reproductive functions, homeostasis of different ions and molecules, and mediating responses to stressful situations. Different hormones have different effects, but even a single hormone can have multiple effects. Hormones travel throughout the bloodstream and affect any cells that have the appropriate receptors for them, known as target cells. Many hormones have target cells in multiple types of organs and tissues, or they regulate molecules, such as blood glucose, that affect many organ systems. These are some of the reasons why changes in the normal level of an endocrine hormone — either hypersecretion or hyposecretion — can result in a wide variety of symptoms, such as is seen in Cushing’s syndrome, diabetes, and PCOS. By understanding what goes wrong in these disorders, you can better appreciate how important the endocrine system is for regulating the many diverse functions of the human body.
In this chapter, you learned about the glands and hormones of the endocrine system, their functions, how they are regulated, and some diseases and disorders of the endocrine system. Specifically, you learned that:
- The endocrine system is a system of glands that release chemical messenger molecules called hormones into the bloodstream. Other glands, called exocrine glands, release substances onto nearby body surfaces through ducts.
- Endocrine hormones travel more slowly than nerve impulses, which are the body’s other way of sending messages. However, the effects of endocrine hormones may be much longer lasting.
- The pituitary gland is the master gland of the endocrine system. Most of the hormones it produces control other endocrine glands. These glands include the thyroid gland, parathyroid glands, pineal gland, pancreas, adrenal glands, gonads (testes and ovaries), and thymus gland.
- Diseases of the endocrine system are relatively common. An endocrine disease usually involves hypersecretion or hyposecretion of a hormone. Hypersecretion is frequently caused by a tumor. Hyposecretion is often caused by the destruction of hormone-secreting cells by the body’s own immune system.
- Endocrine hormones travel throughout the body but affect only certain cells, called target cells, which have receptors specific to particular hormones.
- Steroid hormones such as estrogen are endocrine hormones made of lipids that cross plasma membranes and bind to receptors inside target cells. The hormone-receptor complexes then move into the nucleus where they influence gene expression.
- Non-steroid hormones such as insulin are endocrine hormones made of amino acids that bind to receptors on the surface of target cells. This activates an enzyme in the plasma membrane, and the enzyme controls a second messenger molecule, which influences cell processes.
- Most endocrine hormones are controlled by negative feedback loops in which rising levels of hormone feedback to stop its own production — and vice-versa. For example, a negative feedback loop controls the production of thyroid hormones. The loop includes the hypothalamus, pituitary gland, and thyroid gland.
- Only a few endocrine hormones are controlled by positive feedback loops in which rising levels of hormone feedback to stimulate continued production of the hormone. Prolactin, the pituitary hormone that stimulates milk production by mammary glands, is controlled by a positive feedback loop. The loop includes the nipples, hypothalamus, pituitary gland, and mammary glands.
- The pituitary gland is at the base of the brain, where it is connected to the hypothalamus by nerves and capillaries. It has an anterior (front) lobe that synthesizes and secretes pituitary hormones and a posterior (back) lobe that stores and secretes hormones from the hypothalamus.
- Hormones synthesized and secreted by the anterior pituitary include growth hormone, which stimulates cell growth throughout the body, and thyroid stimulating hormone (TSH), which stimulates the thyroid gland to secrete its hormones.
- Hypothalamic hormones stored and secreted by the posterior pituitary include vasopressin, which helps maintain homeostasis in body water; and oxytocin, which stimulates uterine contractions during birth and the letdown of milk during lactation.
- The thyroid gland is a large endocrine gland in the front of the neck. It is composed mainly of clusters of cells called follicles, which are specialized to absorb iodine and use it to make thyroid hormones. Parafollicular cells among the follicles synthesize the hormone calcitonin.
- The thyroid hormones thyroxine (T4) and triiodothyronine (T3) cross cell membranes and regulate gene expression to control the rate of metabolism in cells body-wide, among other functions. The production of T4 and T3 is regulated by thyroid stimulating hormone (TSH) from the pituitary, which is regulated, in turn, by thyrotropin-releasing hormone (TRH) from the hypothalamus.
- Calcitonin helps regulate blood calcium levels by stimulating the movement of calcium into bone. It works in conjunction with parathyroid hormone to maintain calcium homeostasis.
- Abnormal secretion of thyroid hormones may occur for a variety of reasons and may lead to the development of a goiter. The most common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder. Iodine deficiency is a common cause of hypothyroidism worldwide. In the United States, the most common cause of hypothyroidism is Hashimoto’s thyroiditis, another autoimmune disorder. Hypothyroidism in pregnant women may cause permanent cognitive deficits in children.
- The adrenal glands are endocrine glands that produce a variety of hormones. The two adrenal glands are located on both sides of the body, just above the kidneys. Each gland has two layers: an outer layer called the adrenal cortex and an inner layer called the adrenal medulla.
- The adrenal cortex produces steroid hormones called by the general term corticosteroids, of which there are three types: mineralocorticoids such as aldosterone, which helps control electrolyte balance; glucocorticoids such as cortisol, which helps control the rate of metabolism and suppresses the immune system; and androgens such as DHEA, which is converted to sex hormones in the gonads.
- The adrenal medulla produces non-steroid catecholamine hormones including adrenaline and noradrenaline. These hormones stimulate the fight-or-flight response.
- Disorders of the adrenal glands generally include either hypersecretion or hyposecretion of adrenal hormones. The cause may be a problem with the adrenal glands or with the pituitary gland, which controls adrenal cortex hormone production. Examples include Cushing’s syndrome, in which there is hypersecretion of cortisol; and Addison’s disease, in which there is hyposecretion of cortisol and mineralocorticoids.
- The pancreas is a gland located in the upper left abdomen behind the stomach that functions as both an endocrine gland and an exocrine gland. As an endocrine gland, the pancreas releases hormones, such as insulin, directly into the bloodstream. As an exocrine gland, the pancreas releases digestive enzymes into ducts that carry them to the gastrointestinal tract.
- Tissues in the pancreas that have an endocrine role exist as clusters of cells called pancreatic islets. The islets consist of four main types of cells, each of which secretes a different endocrine hormone. Alpha (α) cells secrete glucagon, beta (β) cells secrete insulin, delta (δ) cells secrete somatostatin, and gamma (γ) cells secrete pancreatic polypeptide.
- The endocrine hormones secreted by the pancreatic islets all play a role, either directly or indirectly, in glucose metabolism and homeostasis of blood glucose levels. For example, insulin stimulates the uptake of glucose by cells and decreases the level of glucose in the blood, whereas glucagon stimulates the conversion of glycogen to glucose and increases the level of glucose in the blood.
- Disorders of the pancreas include pancreatitis, pancreatic cancer, and diabetes mellitus. Pancreatitis is a painful inflammation of the pancreas that has many possible causes. Pancreatic cancer of the endocrine tissues is rare but increasing in frequency. It is generally discovered too late to cure surgically. Smoking is a major risk factor for pancreatic cancer.
- Diabetes mellitus is the most common type of pancreatic disorder. In diabetes, inadequate activity of insulin results in high blood levels of glucose. Type 1 diabetes is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting beta cells of the pancreas. Type 2 diabetes is usually caused by a combination of insulin resistance and impaired insulin secretion due to a variety of environmental and genetic factors.
Chapter Summary Review
1. The pituitary gland is considered the master gland of the endocrine system because its hormones control other endocrine glands. For each of the glands below, describe one way in which it is controlled by the pituitary gland.
a. The thyroid gland
b. The adrenal gland
c. The gonads (ovaries and testes)
2. What is the name of the main brain structure that secretes hormones that control the pituitary gland?
3. Define hyposecretion and give an example of an endocrine disorder involving hyposecretion. Be sure to include the name of the hormone involved.
4. Define hypersecretion and give an example of an endocrine disorder involving hypersecretion. Be sure to include the name of the hormone involved.
5. Which hormone plays a role in regulating metabolism in some way?
B. Thyroid hormone
D. All of the above
6. Which endocrine gland plays an important role in the fight-or-flight response?
7. True or False. Sex hormones, such as androgens, are only produced by the gonads.
8. True or False. Estrogen can travel to the nucleus of a cell.
9. Explain why non-steroid hormones typically require the activation of second messenger molecules to have their effects, instead of directly affecting intracellular processes themselves.
10. Explain what it means that endocrine hormones are “chemical messengers.”
11. a. If you were a physician, and a patient came to you complaining of excessive thirst and urination, what endocrine disorder might you suspect the patient has?
b. In order to diagnose this disorder, what would you want to check for in the patient’s blood? Explain your answer.
12. Pancreatic islet cells all produce:
C. Endocrine hormones
D. Digestive enzymes
13. Give one example of negative feedback in the endocrine system.
14. Explain the circumstances in which organs and hormones in a negative feedback loop can actually increase the level of a hormone.
15. True or False. The hormone vasopressin is synthesized by the hypothalamus.
16. True or False. Like most other hormones, prolactin is regulated by a negative feedback loop.
17. Identify the gland that secretes each of the following hormones:
b. Growth hormone
c. Thyroid stimulating hormone
18. A goiter is an enlargement of the ________ _________ .
19. Explain why giving iodine can treat some cases of hypothyroidism, but it is not usually helpful when someone has hypothyroidism due to Hashimoto’s thyroiditis.
20. For each disease below, identify the hormone involved and whether the problem involves hyposecretion or hypersecretion of this hormone.
a. Addison’s disease
b. Graves’ disease
c. Cushing’s syndrome
d. Type 1 diabetes
21. What is an example of a disease that is due to hormone resistance?
22. True or False. Adrenaline is an exocrine hormone.
23. Steroid hormones:
A. always increase muscle mass
B. are fat soluble
C. bind to receptors on the plasma membrane
D. include insulin
24. Explain generally how autoimmune disorders can disrupt the endocrine system, and give one example.