SCN3230-ABED_1: Module 2 Lesson 4 - 2

Lesson 4 — The Thyroid and Parathyroid Glands

Anatomy of the Thyroid Gland

Read pages 446 - 448

The structure of the thyroid gland is different than the structure pf other endocrine glands. Thyroid tissue contains many follicles that produce and store a precursor of thyroxine. Stimulation by thyroid stimulating hormone (TSH), produced by the anterior pituitary, causes the follicles to change the precursor and release it as thyroxine into the many blood capillaries between the follicles.

Look carefully at the drawing of thyroid tissue. Notice that between the follicles are some large cells (C cells or parafollicular cells). These cells secrete calcitonin, another important thyroid hormone that will be addressed in this lesson.

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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?

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.

The production of thyroxine is regulated by a negative feedback loop.

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A low level of thyroxine in blood triggers the hypothalamus to begin secreting the releasing hormone, thyroid releasing hormone (TRH). TRH stimulates the anterior pituitary and the anterior pituitary produces thyroid stimulating hormone (TSH). In response to TSH, the thyroid gland produces thyroxine. High levels of thyroxine inhibit the hypothalamus from secreting the releasing hormone, thus suppressing the production of TSH and thyroxine. In addition to this, high levels of thyroxine inhibit the anterior pituitary from secreting thyroid stimulating hormone (TSH). This process of regulation maintains adequate levels of thyroxine in the body.

Thyroxine influences almost every tissue in the body 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 rates of cellular respiration. Therefore, in adults, thyroxine affects heart rate, respiratory rate, the rate at which calories are burned, skin maintenance, and heat production. It promotes normal adult nervous system function. In children and infants, thyroxine is particularly important in stimulating growth and development of the skeleton as well as development of the nervous system.

Iodine is the element necessary for the synthesis of thyroxine. Much of the world’s population do not have adequate access to iodine in their diet. To ensure adequate iodine in our diet, it is put into table salt as potassium iodide (KI). After adequate amounts of iodine are present in our bodies, the excess is flushed from the system in urine.

Disorders Related to Thyroxine

Hypothyroidism

Lack of thyroxine in the body is called hypothyroidism. Because thyroxine is produced through a negative feedback system, low production of thyroxine constantly stimulates both the hypothalamus and the anterior pituitary.

An adult with hypothyroidism may show any of the following symptoms:

General slowing of body functions
Fatigue
Slow heart rate, decreased blood pressure and weak pulse
Puffy skin
Hair loss
Weight gain
Slow speech
Inability to tolerate cold
Slow mental function

Treatment usually involves taking thyroxine in pill form.

Cretinism

Under-secretion of thyroxine in babies, called cretinism, results in stunted growth of the skeleton making the child short in stature. It leads to poor development of the nervous system resulting in poor mental development.

Goitre

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, resulting from lack of iodine in the diet, usually results in goitre. The anterior pituitary produces TSH that relentlessly stimulates the thyroid gland to produce thyroxine, but the only result is the accumulation of increasing amounts of precursor in the central cavity of the follicles because the precursor cannot be synthesized into thyroxine without iodine.

Hyperthyroidism

Over-secretion of thyroxine is called hyperthyroidism and generally causes increased speed of body functions. Most people affected have an enlarged thyroid gland.

Most of the other symptoms reflect the increased speeding of body functions including the following:

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 that selectively destroys the most active follicle cells. This is possible because iodine is automatically taken up by the thyroid gland and does not spread to other body organs.

Grave's Disease

Grave's Disease is a type of hyperthyroidism. It is an autoimmune disorder in which 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.

Watch This

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, watch the following video segment from Biologix-10 on Thyroxine (18 minutes):