Module 2 The Endocrine System

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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 and 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 and presents differently in babies/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 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.

Glossary

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

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

central cavity: also called the lumen, is surrounded by follicle cells and stores the precursor for thyroxine formation
follicle cells: as they relate to the thyroid gland, they are the cells that surround the central cavity and are responsible for storing iodine and synthesizing thyroxine from the precursor

C cells: also called parafollicular cells, they are located between the follicles of the thyroid gland and are responsible for synthesizing calcitonin

calcitonin: 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

thyroxine: 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

precursor: an inactive form of a molecule that can easily be changed into the active form; the precursor thyroglobin can easily be changed into the active thyroxine by the addition of iodine atoms

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 over-stimulation

hypothyroidism: disorder of the thyroid gland indicated by a lack of thyroxine, which may be caused by a ack 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: form of hypothyroidism in infants and young children

Grave’s Disease: an autoimmune disorder caused by an abnormal protein which stimulates the thyroid gland to produce and secrete excess thyroxine; most common cause of hyperthyroidism