1. Module 2

1.32. Page 6

Lesson 4

Module 2—The Endocrine System

Lesson Summary

 

This lesson explored the following focusing question:

  • How do the thyroid and parathyroid glands contribute to homeostasis?

The thyroid gland, a butterfly-shaped gland located just below the larynx, has embedded in it the four pea-sized parathyroid glands. These glands produce two antagonistic hormones, calcitonin and PTH, that work together to maintain blood calcium homeostasis. Through mechanisms such as increasing absorption of calcium from food or urine and by stimulating decomposition of bone tissue, PTH increases blood calcium levels. Calcitonin decreases blood calcium levels by stimulating uptake of calcium by the bones and inhibiting decomposition of bone tissue. Both are regulated by negative feedback mechanisms.

When stimulated by TSH from the anterior pituitary, the thyroid gland also produces thyroxine, which increases metabolism (cellular respiration) in every cell of your body—especially in cardiac cells, muscle cells, and cells of the kidney and liver. It stimulates the liver to convert glycogen to glucose and increases body temperature. Thyroxine also stimulates proper development and function of the nervous system. Iodine is required to synthesize thyroxine. Lack of thyroxine is called hypothyroidism, which presents differently in babies or children and adults. Overproduction is called hyperthyroidism. Goitre, myxedema, and cretinism are disorders caused by imbalances in thyroxine. Thyroxine levels are regulated by negative feedback of the tropic hormone TSH, which is produced by the anterior pituitary. Surgical removal of parts of the thyroid, death of the most active follicle cells initiated by radioactive iodine, and consumption of thyroid pills are medical technologies that can treat thyroxine imbalances.

 

Following is a summary of conditions related to the thyroid and parathyroid glands:

  • goitre: sometimes called endemic goitre, is an enlargement of the thyroid gland caused by a lack of iodine, which is necessary for the production of thyroxine
  • exothalmic goitre: an enlargement of the thyroid gland caused by the overactivity of the thyroid gland due to overstimulation
  • hypothyroidism: a disorder of the thyroid gland indicated by a lack of thyroxine, which may be caused by a lack of dietary iodine or a lack of TSH
  • hyperthyroidism: overactivity of the thyroid gland that leads to high levels of thyroxine and subsequent speeding up of bodily functions
  • myxedema: form of hypothyroidism in adults
  • cretinism: a form of hypothyroidism in infants and young children
  • Grave’s disease: an autoimmune disorder caused by an abnormal protein that stimulates the thyroid gland to produce and secrete excess thyroxine; the most common cause of hyperthyroidism
Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

calcitonin: a hormone produced by the C cells (parafollicular cells) in the thyroid gland that decreases blood calcium levels by increasing uptake of calcium into the skeleton and inhibiting decomposition of bone when the blood levels of calcium are too high

 

C cells (parafollicular cells): cells located between the follicles of the thyroid gland that are responsible for synthesizing calcitonin

 

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 the form of ATP and waste products, such as water and carbon dioxide

 

follicle: as it relates to the thyroid gland, a microscopic structure consisting of a circle of cells called follicle cells surrounding a central cavity

 

The thyroid gland consists of many follicles.

 

metabolism: all of the chemical reactions that occur in living organisms to support and sustain life processes

 

One type involves the buildup or synthesis of complex compounds from simple ones. A second type consists of the breakdown or decomposition of complex compounds into simple ones. Cellular respiration is one of the major metabolic processes.

 

precursor: an inactive form of a molecule that can easily be changed into the active form

 

thyroxine: a hormone produced by the follicle cells of the thyroid gland; mainly controls the rate at which the body metabolizes carbohydrates, fats, and proteins for energy and stimulates the proper development of the nervous system

 

The precursor thyroglobin can easily be changed into the active thyroxine by the addition of iodine atoms.