Lesson 1: What is Hyperthyroidism?
In this lesson, students will become acquainted with the thyroid gland and its function. They'll learn to identify the two major thryoid hormones, and they'll learn how the pituitary hormone thyrotropin (TSH) helps to regulate thryoid hormone levels. The focus of this chapter is an understanding of what happens when the thyroid gland begins to produce excess thyroid hormone.
Topics include:
The endocrine system
The thyroid gland and its function
What is thryoid hormone?
The hypothalmic-thyroid-pituitary axis
The role of TSH
Topics for discussion
Introduction
In this lesson, students will learn about normal thyroid function, and they'll learn what happens when the thyroid's normal function is disrupted. Course participants will be introduced to the endocrine system, and they'll be shown how the thyroid gland, which is the largest of the endocrine glands, affects the function of all of the body's tissues and cells. This lesson primarily covers the normal anatomy and physiology of the thyroid gland, the hormones produced by the thyroid gland, the regulation of thyroid hormone levels, and the effects of hyperthyroidism.
Hormones are chemical messengers that travel from the gland that produces them directly to the body's cells and tissues. When the hormone reacts with a specific type of cell, for instance the cardiac cells that make up the heart, the cardiac muscle receives a specific order, such as the command to beat faster or more forcefully. Without adequate thyroid hormone, the heart's function is sluggish. In hypothyroidism, a condition caused by insufficient thyroid hormone, the heartbeat is slow and the blood vessels move blood slowly, causing congestion. In hyperthyroidism the heartbeat is rapid, and blood pressure often rises. The heart, like all of the body's organs, needs adequate thyroid hormone to do its job properly.
Too little or too much of any hormone can adversely affect our health. In this lesson we'll see how the ability of glands to function properly is dependent on other bodily systems and their relationships with one another. In order to understand these complex relationships that exist between the body's endocrine and other bodily systems, students will also be introduced to homeostasis, which is a natural mechanism by which all of the body's systems work together in an effort to maintain health.
In order to show how the endocrine and other bodily systems influence one another, students are introduced to the science of psychoneuroimmunology, commonly known as the mind-body connection. Specifically, psychoneuroimmunology or PNI describes the many ways in which the endocrine, nervous and immune systems influence each another. For instance, emotional distress causes measurable changes in both immune system chemicals and the chemical hormones secreted by the endocrine glands. From an acquaintance with these topics, students are able to understand why stress is considered the major trigger for autoimmune hyperthyroidism, which is Graves' disease.
Students will also learn how thyroid hormone levels are under the control of both the thyroid gland and the pituitary gland. This regulatory mechanism, a key player in homeostasis, is known as the hypothalamic-pituitary-thyroidal axis. This system, described by Hippocrates hundreds of years ago, ensures that under normal circumstances, the body will have adequate thyroid hormone for its needs.
In this axis, the hypothalamus in the brain monitors the body's thyroid hormone levels, vigilantly watching the slightest changes in these levels. As soon as the hypothalamus detects even the slightest change, it takes action to prevent thyroid hormone levels from falling too high or too low.
In response to these changes, the hypothalamus secretes or stops secreting a hormone known as thyrotropin releasing hormone or TRH. The hormone TRH signals the pituitary gland to secrete or hold off on secreting the hormone thyrotropin. Thyrotropin is also known as thyroid stimulating hormone or TSH. TSH, in turn, orders thyroid cells to produce and secrete thyroid hormone.
From this discussion, students can understand how blood levels of the pituitary hormone TSH can be used to evaluate thyroid function. In hyperthyroidism, the pituitary stops secreting TSH, and TSH levels are low, typically less than 0.1 mu/L. In hypothyroidism, the pituitary gland secretes more TSH in an effort to raise thyroid hormone levels, and TSH levels are elevated.
This concept of regulation is used to show how under normal circumstances the thyroid gland will produce essentially the same amount of thyroid hormone. When temperatures fall and extra thyroid hormone is needed to keep the body warm, the hypothalamic-pituitary-thyroid axis causes the thyroid gland to secrete more hormone into the blood circulation. To accomplish this, the pituitary gland secretes more TSH. Thus, TSH levels will show some variation as the pituitary works to provide stable thyroid hormone levels and alter its function to accommodate the changes caused by stress, climate, diet, exercise and illnesses.
By the end of this section, students will understand that the thyroid gland is an endocrine gland. As an endocrine gland, the thyroid is able to produce hormones and secrete these hormones directly into the blood circulation.
In this section, students will come to understand the critical need for adequate thyroid hormone. They'll understand how all of the body's organs and tissues depend on adequate thyroid hormone in order to function properly. Students will be shown how deficiencies or excesses of thyroid hormone can cause physiological changes that result in disease.
