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Module 2 The Endocrine System

Lesson 1.2.4

1.2.4 page 3

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The Thyroid Gland and Metabolism

 

Do you know someone who seems to be able to consume great amounts of food and still stay thin? Do you know other people who seem to put on weight just by smelling or looking at food? Thin people often excuse themselves by explaining that they have a high metabolism. Metabolic rate is regulated by the hypothalamus/pituitary complex, which produces tropic hormones that stimulate the thyroid gland to release increased amounts of thyroxine. Thyroxine increases the number of mitochondria in cells and generally increases cellular respiration, a major set of decomposition chemical reactions that make up metabolism. You may wish to review these concepts from Biology 20.

 

metabolism: all of the chemical reactions which occur in living organisms that support and sustain its life processes; one type involves the build-up or synthesis of complex compounds from simple ones, and a second type consists of the breakdown or decomposition of complex compounds into simple ones; cellular respiration is one of the major metabolic processes

 

cellular respiration: a process occurring in the mitochondria of cells of all living organisms in which energy rich compounds such as carbohydrates (glucose), proteins, and fats are burned with oxygen to release energy in form of ATP, and waste products such as water and carbon dioxide

 

To gain an understanding of the composition of thyroxine, how it is regulated, what functions it performs, and what happens when it is over-secreted or under-secreted, answer the following questions in Try This #2. The completed answers to these questions can become your notes for this lesson so it is important to answer in complete sentences. To help you answer the questions, you may use library sources, the internet, pp. 446 – 448 in your textbook, or the video segments which are listed below.

 

Negative Feedback Loop: Thyroxine

 

 

Try This

 

TR 2.

 

After completing the questions in this exercise, check your answers and file your work in your course folder. If you do not understand any of the concepts, consult with your instructor for clarification.

 

Use the following information to answer the questions that follow.

 

Usually people who have FMEN1 do not develop tumors in the thyroid gland, and Emily did not have a thyroid tumor. However, in Lesson 2 and 3, you learned that Emily had tiny tumors in her anterior pituitary which affected the tropic regulatory pathway for ACTH by causing the over-secretion of this hormone. Those tumors have also affected the secretion of thyroid stimulating hormone (TSH) by causing an under-secretion of this regulatory hormone.

  1. Draw a feedback loop that would illustrate the proper negative feedback regulation on the production of thyroxine.
  2. Explain how Emily’s feedback loop would be different from the one you drew in Question #1.
  3. Identify the element that is necessary for the synthesis of thyroxine. Although we may live in a zone where this element is in short supply in the soil and water, we are able to produce a fully functioning thyroxine. Explain why.
  4. Outline the functions of thyroxine in both children and adults.
  5. Emily’s feedback mechanism was not working and she could not produce adequate amounts of thyroxine. Identify Emily’s resulting thyroid disorder.
  6. Describe the symptoms that Emily would exhibit as a result of a lack of thyroxine. Outline a treatment that is possible for Emily.
  7. Define goiter. Left untreated, do you think Emily could have developed a goiter? Explain why or why not.
  8. Outline the results of under-secretion of thyroxine in newborn babies. Identify this disorder.

  9. © Carolina K. Smith, M.D./shutterstock
    Due to the anterior pituitary tumor, Emily could have just as easily over-secreted thyroxine as she had under-secreted it. Describe the symptoms of over-secretion of thyroxine. Identify the disorder associated with the over-secretion of thyroxine and outline a possible treatment.
  1. Is hyperthyroidism different from Grave’s Disease? Explain.
Check your work.
Self-Check Answers

 

TR 2.

  1. The negative feedback loop regulating thyroxine in the blood should resemble the one shown below. High levels of thyroxine: (1) inhibit the hypothalamus from secreting the releasing hormone (which is sometimes abbreviated as TRH – thyroid releasing hormone) and thus do not allow stimulation of the anterior pituitary, (2) inhibit the anterior pituitary directly by limiting TSH production. Boxes may be placed around the words “hypothalamus”, “anterior pituitary”, and “thyroid gland” but you do not have to do that. The loop may be drawn horizontally rather than vertically.

    Negative Feedback Regulation of Thyroxine

  2. Because of the tumor in Emily’s anterior pituitary, she does not secrete the proper amount of TSH. Thus her thyroid gland does not produce enough thyroxine, and the low levels of thyroxine constantly stimulate both the hypothalamus and the anterior pituitary. Since the tumors prevent the secretion of TSH, the stimulation is relentless.
  3. Iodine is the element necessary for the production of thyroxine. To ensure adequate iodine in our diet, it is put into table salt as potassium iodide (K2I). Also the udders of dairy cows are disinfected with an iodine solution before milking and a small amount of iodine is present in dairy products.
  4. Thyroxine influences almost every tissue in the body in order to increase the rate at which the cells metabolize fats, proteins, and carbohydrates for energy purposes. It especially stimulates the heart, skeletal muscles, liver and kidneys to increase their rate of cellular respiration. Therefore, in adults, thyroxine affects heart rate, respiratory rate, the rate at which calories are burned, skin maintenance, heat production, and it promotes normal adult nervous system function. In children and infants, thyroxine is particularly important in stimulating growth and development of the skeleton, and development of the nervous system.
  5. Lack of thyroxine in the body is called hypothyroidism. A very severe case in adults may be referred to as myxedema.
  6. Emily may show any of the following symptoms of hypothyroidism:

    • general slowing of body functions
    • fatigue
    • slow heart rate, decreased blood pressure and weak pulse
    • puffy skin
    • hair loss
    • weight gain even though she eats little
    • slow speech
    • inability to tolerate cold
    • slow mental function

    Treatment usually involves taking thyroxine in pill form.
  7. Goitre is an enlargement of the thyroid gland. The enlargement is brought about by the accumulation of the precursor for thyroxine stored in the central cavity. Hypothyroidism (myxedema), resulting from lack of iodine in the diet, usually results in goiter. The anterior pituitary produces TSH which relentlessly stimulates the thyroid gland to produce thyroxine, but the only result is the accumulation of more and more precursor in the central cavity of the follicles.

    Emily would probably not develop a goiter because her diet contained adequate amounts of iodine (iodized salt). Further, she is lacking stimulation of the thyroid gland because she under-secretes TSH, and therefore her thyroid gland should remain at a normal size.
  8. Under-secretion of thyroxine in babies, called cretinism, results in stunted growth of the skeleton making the child short in stature, as well as in poor development of the nervous system resulting in poor mental development.
  9. Over-secretion of thyroxine is called hyperthyroidism, and generally causes a speeding up of body functions. Most people will have an enlarged thyroid gland (exothalmic goiter). Most of the other symptoms reflect the speeding up of body functions including:

    • increased heart rate, increased blood pressure and strong pulse
    • excessive sweating
    • intolerance to heat
    • nervousness and anxiety
    • difficulty sleeping
    • weight loss in spite of increased appetite
    • retention of fluid in eyes causing bulging eyes
    Treatment involves the surgical removal of certain active parts of the thyroid gland or the administration of radioactive iodine which selectively destroys the most active follicle cells. This is possible because of the fact that iodine is automatically taken up by the thyroid gland and does not spread to other body organs.
  10. Grave’s Disease is one type of hyperthyroidism. It is an autoimmune disorder where the thyroid gland is stimulated to produce excess thyroxine by proteins that mimic TSH. An enlarged thyroid gland and protruding eyeballs are both common symptoms of Grave’s Disease.