The hypothalamus and pituitary gland are joined by many other glands in using hormones to signal and regulate physiology. From anxiety about public speaking, to the changes in sleep patterns after flying to a new time zone, to the changes in metabolism that accompany aging, endocrine glands throughout the body are responsible for detecting and responding to signals reflecting an organism’s internal and external environments. Here we explore the signals of some of the most important of these other endocrine glands (FIGURE 24-9).

Figure 24-9Functions of the glands of the endocrine system: pineal, thyroid, parathyroids, adrenals, pancreas, and gonads.

Adrenal Glands Regulating an organism’s response to stress is largely a function of the secretions of the two adrenal glands, which sit just above the kidneys and secrete the hormones cortisol and adrenaline (also called epinephrine), among others. Simply the sight of a predator is enough to initiate the “fight-or-flight” response—the secretions of adrenaline and cortisol that prepare the body for action (FIGURE 24-10). Within seconds, these secretions—in a case of positive feedback—can cause goose bumps, an increased heart rate, an increased rate of glycogen breakdown in the liver and skeletal muscles, the release of stored fatty acids, and a dilation of bronchioles in the lungs that enables greater absorption of oxygen, for delivery to needy tissues.

Figure 24-10Who will win in this conflict? Adrenal glands influence the fight-or-flight response in animals.

The stress pathways are modulated by negative feedback loops. As an animal takes action in response to a stressful situation and the source of the stress is then removed, the secretions of cortisol and adrenaline are reduced. This is how the stress response usually works in nature. But when there is no outlet by which an organism can deal with the stress, long-term consequences of a chronic stress response include ulcers, cardiovascular problems, decreased immune function, and illness. With an increased understanding of the stress response and its function as a short-term physiological state that helps organisms quickly and effectively respond to stressful situations, researchers are gaining insights into how to better treat anxiety.

Pineal Gland The 17th century philosopher René Descartes believed that the pineal gland was where the soul connected with the body. He believed this largely because the pea-sized gland is located near the center of the brain, is singular (that is, there’s just one, not one on the left side of the brain and another on the right), and is, so he thought, unique to humans. This view about the role of the pineal gland has been abandoned by scientists (and we now know that the pineal gland is present in all vertebrates), but there’s still considerable scientific interest in this gland. It has neuron connections with the retina of the eye, and it controls secretion of the hormone melatonin, which is derived from the amino acid tryptophan and affects diurnal-nocturnal wake and sleep patterns, called circadian cycles.

Although the exact mechanism by which melatonin influences circadian cycles is not understood, it has been shown to have some benefits in synchronizing individuals’ sleep and wake cycles to the environment and in treating some types of insomnia, and melatonin is now sold throughout the United States.

Thyroid Gland One of the largest endocrine glands in humans is the thyroid gland, located in the neck, just below the Adam’s apple. It secretes hormones—including thyroxine—that influence the speed and efficiency with which body cells break down macromolecules in our diet and use the energy released to produce proteins. In short, it controls most of what we think of as metabolism. As a consequence, poor thyroid function is believed to be at the root of many metabolic disorders, with underactive thyroid responsible for fatigue and weight gain, and overactive thyroid responsible for jitteriness, rapid heartbeat, weight loss, and irritability.

Figure 24-11An enlargement of the thyroid gland in the neck is called a goiter.

Figure 24-12The body’s use of calcium is regulated by the thyroid and parathyroid glands.

Figure 24-13Even after a heavy meal, a healthy person can maintain blood glucose levels within a narrow range.

Goiter is a common health problem caused by an enlargement of the thyroid gland (FIGURE 24-11). There are several causes of goiters, and they are particularly common in areas with low iodine consumption. When iodine intake is low, the thyroid is unable to produce thyroxine (which contains iodine). This causes thyroxine levels in the body to drop. In the absence of the normal negative feedback telling the body to slow its production of thyroxine, the hypothalamus and anterior pituitary produce increasing amounts, respectively, of thyroxine-releasing hormone and thyroxine-stimulating hormone. These cause the thyroid to swell into a visible lump—a goiter—as it tries unsuccessfully to make thyroxine. In the United States, the widespread use of table salt fortified with iodine prevents most iodine deficiencies, but they are common in Asia, Central Africa, and parts of South America.

The thyroid also regulates levels of calcium in the blood. Calcium is necessary for building and maintaining bones and teeth, and it influences the functioning of nerves and muscles. When there is too much calcium in the blood, the thyroid increases its release of calcitonin, a hormone particularly important in babies and children, which causes bones to take up the excess calcium. Additionally, embedded in the surface of the thyroid are the four small parathyroid glands. They produce parathyroid hormone, which plays a central role in regulating calcium levels in adults (FIGURE 24-12). Throughout life, bone is continually broken down and remade as minerals are lost and added. Parathyroid hormone is important in stimulating much of this continued turnover of bone, including reabsorption of old bone and production of new bone. Parathyroid hormone further reduces calcium loss in the urine, regulates the release of calcium from bone, and in conjunction with vitamin D (from the diet and produced in response to sun exposure), helps increase the body’s ability to utilize the calcium supplied in the diet.

Pancreas Located next to the stomach and connected to the small intestine via a short duct, the pancreas is an endocrine gland that is most important in controlling the levels of blood glucose. As we saw in Section 22-18, the pancreas maintains blood glucose within a narrow range— a typical blood glucose concentration in humans is 90 mg/100 mL—through the coordinated secretions of insulin and glucagon (FIGURE 24-13).

Following a meal (particularly one rich in carbohydrates), the concentration of blood glucose rises. This stimulates release of insulin by the pancreas. Insulin in the bloodstream causes the liver and other tissues—primarily muscle—to take up glucose, which reduces the blood glucose level. As blood glucose levels fall, there is a reduction in insulin secretion.

Conversely, after a few hours of fasting, the blood glucose level gradually drops. The reduced blood glucose triggers release of glucagon by the pancreas. Glucagon has the reverse effect of insulin, causing the liver to convert stored glycogen into glucose, which is released into the bloodstream. Rising blood glucose concentration then causes the pancreas to reduce its glucagon secretion, maintaining homeostasis through negative feedback.

Gonads The sex steroids, including testosterone, estrogen, and progesterone, are produced largely by the gonads—the testes in males and the ovaries in females. These hormones are responsible for numerous physical, behavioral, and emotional characteristics, including much sexual behavior and growth, sexual development (embryonically, in puberty, and continuing into adulthood), and maintenance of gamete production throughout an organism’s reproductive life.

A summary of the major animal hormones and their actions is in FIGURE 24-14.

Figure 24-14Summary of endocrine glands, hormones, and their primary functions.

In the next few sections, we investigate some of the gonadal hormones and see how closely linked they are to athletic performance and the attributes necessary to excel physically. We’ll also see a dark side to hormones, in the dramatic physical changes and improvements in athletic performance resulting from illegal use of hormone supplements. We also note the extreme health consequences that accompany such hormone abuse.