Lesson Explainer: The Thyroid and Parathyroid Glands Biology

In this explainer, we will learn how to describe the function of the thyroid and parathyroid glands in the human body and the symptoms of diseases associated with these glands.

Every minute, there are millions of chemical reactions that take place in our bodies, enabling our organ systems to function normally and supporting us through our active day-to-day lives. The body’s endocrine system is full of chemical messengers called hormones. Hormones work in coordination, affecting growth, metabolism, emotions, reproductive cycles, and so much more!

Definition: Hormones

Hormones are chemical messengers that travel throughout an organism’s body, usually in the blood or another transport medium.

Glands are organs in the body that secrete substances that perform different roles in the body. For example, milk is produced by mammary glands, and saliva is produced by salivary glands. Glands may be endocrine, exocrine, or mixed depending on how they release their products. Hormones, however, are only produced by endocrine glands. Endocrine glands are a set of glands that secrete hormones directly into the bloodstream, often working in coordination with one another. In this explainer, we will be learning about the structures, functions, and importance of the thyroid and parathyroid glands, which are endocrine glands. In Figure 1, you can see the locations of the various different endocrine glands in the human body.

Key Term: Endocrine Gland

An endocrine gland is an organ consisting of a group of specialized cells that secrete hormones directly into the blood.

Let’s begin by taking a look at the structure of the thyroid gland.

The thyroid gland is a two-lobed gland, located at the base of the front of the neck just below the larynx or the Adam’s apple, with one lobe on either side of the trachea, as you can see in Figure 2. The two lobes are enclosed in connective tissue and joined by a band of tissue called the isthmus. The shape of the thyroid gland is often compared to a butterfly or a bow tie. Interestingly, in 1656, Thomas Wharton coined the name of the gland that arose from the Greek word thyreos, which means “shield,” because the shape of this gland also resembles a type of ancient Greek shield!

Key Term: Thyroid Gland

The thyroid gland is a butterfly-shaped endocrine gland located at the base of the front of the neck. It secretes thyroid hormones that control functions like metabolism, growth, and development.

Each lobule in the thyroid gland is made of many structural and functional spherical units called thyroid follicles. The wall of each follicle is made up of rectangular-prism-shaped cells called follicular cells that enclose a cavity, as you can see in Figure 3. The follicular cells secrete a thyroid hormone, which circulates in the blood mainly in the form of thyroxine and small quantities of triiodothyronine.

Key Term: Thyroxine

Thyroxine is a hormone secreted by the thyroid gland that affects functions like metabolism, growth, and development in the body.

Key Term: Triiodothyronine

Triiodothyronine is a hormone that is produced by the conversion of thyroxine into a more active form.

The thyroid gland receives a rich network of blood vessels that provides the important nutrients, such as iodine, and carries the hormones produced by the gland via the bloodstream to target cells elsewhere in the body.

The follicular cells have pumps that help them take up iodine, which is crucial for the production of these thyroid hormones. The chemical structure of a thyroxine molecule contains four atoms of iodine. This is why thyroxine is sometimes called T4, or tetraiodothyronine. A large proportion of the T4 produced in the thyroid gland is converted to a more active form called T3 (triiodothyronine) by organs like the liver and other tissues. It is very important for the thyroid gland, therefore, that one receives enough iodine in their diet. Approximately, 90% of the hormones produced by the thyroid gland are T4 and 10% T3.

Before we begin learning about the functions of the thyroid hormones, we need to understand how the thyroid gland is stimulated. As we mentioned before, the glands of the endocrine system work in harmony, sharing chemical signals and maintaining stability within the body. You might have heard the pituitary gland, which is located in the brain, being referred to as the “master gland,” because it controls so many of the other glands’ functions, including that of the thyroid gland. The anterior pituitary gland releases an important hormone called the thyroid-stimulating hormone, or TSH, which travels in the bloodstream to the thyroid gland, triggering it to secrete thyroid hormones. This is represented in Figure 4.

Key Term: Pituitary Gland

The pituitary gland is an endocrine gland connected to the brain that regulates many important body functions and controls other endocrine glands.

Key Term: Thyroid-Stimulating Hormone

Thyroid-stimulating hormone is a hormone released by the anterior pituitary gland that triggers the production of thyroid hormones in the thyroid gland.

Let’s now investigate the different effects that thyroid hormones can have on the body and understand why this hormone is so crucial.

Thyroid hormones are important for growth and development of the physical body and in particular, the brain. The presence of thyroid hormones (thyroxine and triiodothyronine) stimulates and regulates the production of several important growth factors, which promote normal cell growth, division, and differentiation. Growth factors have hormonelike activity on cells, but they are distinct from the pituitary growth hormone in that they are produced by many cell types in the body that are not necessarily endocrine glands. For example, epidermal growth factor (EGF) can be secreted by the cells of the skin to heal wounds.

Thyroid hormones (thyroxine and triiodothyronine) are also crucial for the development of the brain in the embryonic stage, by promoting the proliferation and differentiation of neurons, enhancing the formation of synapses and myelination. These hormones are also crucial for the brain to function normally throughout life.

Further, thyroid hormones (thyroxine and triiodothyronine) play a role in metabolism. Metabolism involves chemical reactions within the body that support and maintain life. Let’s trace the effect of thyroid hormones on metabolism right from the cellular level and understand the effects this might have.

Key Term: Metabolism

Metabolism is a word that describes all chemical reactions that occur within living organisms to maintain life.

You might recall learning that, in every cell, the mitochondria are primarily responsible for cellular respiration. Cellular respiration breaks down molecules like glucose to release chemical energy, which is stored in the form of ATP.

Thyroid hormones stimulate and enhance energy release in the mitochondria. This happens in all cells, which means that the cells begin to take up more glucose to liberate more energy. This increases the basal metabolic rate, which is the rate at which your body uses energy, when you are at rest, to perform crucial activities like respiration, circulation, digestion, and the normal functioning of your brain, kidneys, liver, and other essential organs. Since the cells take up more glucose, thyroid hormones also help increase the absorption of glucose, which is a monosaccharide, in the intestine. These functions of thyroid hormones (thyroxine and triiodothyronine) help to maintain balanced and stable metabolism within the body.

Key Term: Basal Metabolic Rate

The basal metabolic rate is the rate at which the body uses energy, while at rest, to perform crucial body functions like respiration and circulation.

Can you think about the other effects that increased metabolism might have? We know that the heart is responsible for pumping blood, containing oxygen and nutrients, to the tissues of the body. If the cells are respiring at a higher rate, the heart will pump faster in order to efficiently supply the cells with the glucose and oxygen they need. Another effect of thyroid hormones, therefore, is maintaining heart rate and blood pressure, keeping them stable.

By increasing the amount of energy available to the body, thyroid hormones also play a role in maintaining body temperature, working with the hypothalamus in the brain. Thyroid hormones also help maintain healthy skin and hair. Low levels of thyroid hormones can cause skin to be dry and flaky and hair to be brittle and prone to breakage. Figure 5 shows a summary of some important functions of the thyroid gland.

Another important hormone released by the thyroid gland is calcitonin. As shown in Figure 3, there are cells beside the follicular cells called parafollicular cells. These cells are also named C cells because they release calcitonin. Calcitonin works to reduce the level of calcium in the blood by preventing its release from bones to maintain a normal, healthy calcium concentration. This hormone is part of a calcium-regulating system that works in balance with the parathyroid glands, as we will learn further on in this explainer.

Key Term: Calcitonin

Calcitonin is a hormone produced by the thyroid gland that is involved in calcium homeostasis.

Example 1: Identifying the Functions of Thyroxine in the Human Body

Which of the following is not one of the primary functions of thyroxine in the human body?

  1. Stimulating mental and physical growth
  2. Maintaining normal body temperature
  3. Increasing the amount of calcium in the bloodstream
  4. Maintaining a normal heart rate
  5. Regulating the basal metabolic rate

Answer

Thyroxine is the main hormone secreted by the thyroid gland in the human body. Thyroxine has several important functions regulating the activity of organ systems in the body to maintain steady, balanced internal conditions in the body. In this question, we are being asked to identify the incorrect function of thyroxine from the list provided. Let’s work through the options to find the right answer.

Thyroxine does play a crucial role in stimulating mental and physical growth. This hormone is important for brain development in the embryonic stages and controls the production and release of several growth factors involved in physical growth. This function is therefore correct, so we can eliminate this option. By regulating the amount of energy available in the body, thyroxine helps maintain normal body temperature, working with the hypothalamus in the brain. We can eliminate this option too.

On the posterior surface of the thyroid gland, there are four parathyroid glands, which secrete parathyroid hormone (PTH). Parathyroid hormone, not thyroxine, is responsible for increasing the amount of calcium in the bloodstream when it is too low to maintain the calcium level in blood, which means that this function is incorrect.

The basal metabolic rate is the rate at which the body uses energy while at rest to perform crucial body functions. By enhancing the rate of cellular respiration and uptake of glucose in cells, thyroxine increases the basal metabolic rate. If glucose needs to be delivered to cells faster, the rate of the heart beat that pumps blood containing glucose to the body cells will also need to be maintained. Thyroxine therefore not only regulates basal metabolic rate, but also maintains a normal heart rate.

The incorrect function of thyroxine is, therefore, increasing the amount of calcium in the bloodstream.

Now that we have learned about the normal functions of the thyroid gland, we can begin to understand the effects of too little or too much thyroid hormone in the body.

Hypothyroidism is a condition in which the thyroid gland produces insufficient quantities of thyroid hormones. The word hypo means “low.” As we mentioned earlier, iodine is essential for the production of thyroxine and triiodothyronine. Hypothyroidism, therefore, can be caused by a deficiency of iodine in a diet. Hypothyroidism may also be caused by an autoimmune disease called Hashimoto’s disease. An autoimmune disease is a disease in which the body’s immune system mistakenly attacks its own tissue; in this case, it attacks the thyroid gland.

Key Term: Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland produces insufficient thyroid hormones.

When the thyroid gland produces insufficient thyroid hormones such as thyroxine and triiodothyronine, this causes the gland to swell, which is called a goiter, such as the simple goiter that is shown in Figure 6. Goiters have different causes depending on their type. One of the reasons why a goiter might form is due to a buildup of thyroid hormones in the follicular cells of the thyroid gland. It may also occur due to a lack of iodine, which means that the precursor thyroid hormones cannot be converted into active thyroid hormones, so the thyroid enlarges to try and compensate and capture more iodine.

Key Term: Simple Goiter

A simple goiter is the enlargement of the thyroid gland that can be caused by iodine deficiency.

Hypothyroidism can be treated with iodine supplements, but if left untreated, a deficiency of thyroid hormones can have several adverse effects.

In infants and children, congenital hypothyroidism (which means present from birth) can cause problems with physical growth and development, leading to short stature and delayed development of secondary sexual characteristics. Since thyroid hormones are crucial to the development of the brain, hypothyroidism in infants can lead to intellectual disabilities caused by impaired neurological development. Congenital hypothyroidism in infants may be caused if a mother does not have enough iodine in her diet during pregnancy, causing hypothyroidism. Her baby could then be born with a syndrome named “congenital iodine deficiency syndrome” (previously known as “cretinism”).

In adults, severe hypothyroidism can cause a condition called myxedema. We know that thyroid hormones play important roles in maintaining metabolism, heart rate, body temperature, and healthy skin and hair. Since myxedema is caused by too little thyroid hormones such as thyroxine and triiodothyronine, it is characterized by pale dry skin, hair loss, weight gain, decreased heart rate, high blood pressure, and decreased basal metabolic rate. Patients with myxedema struggle to tolerate cold weather and are frequently fatigued. This condition can also cause decreased mental capabilities including slowness and difficulty concentrating. Iodine deficiency is now usually prevented by consuming iodized salt, and hypothyroidism can be medically treated by a manufactured form of thyroid hormones.

Key Term: Myxedema

Myxedema is the condition brought about by severe hypothyroidism in adults.

While low levels of thyroid hormones can be dangerous, an excess can also cause several health problems. Increased thyroid hormone secretion, beyond the required level, is called hyperthyroidism. The word hyper means “high.”

Key Term: Hyperthyroidism

Hyperthyroidism is a condition in which the thyroid gland produces excess thyroid hormones (thyroxine and triiodothyronine).

Hyperthyroidism can be caused by another autoimmune disease called Graves’ disease, in which the immune system attacks the thyroid, making it produce more thyroid hormones than required. Other causes of hyperthyroidism include nodules in the gland and thyroid cancer.

Graves’ disease results in the overstimulation of the thyroid gland that causes it to swell. In the case of this kind of hyperthyroidism, the goiter is called an exophthalmic goiter. In this disorder, the immune system also mistakenly attacks the tissues behind and around the eyes, which become inflamed and swollen. This autoimmune disorder of the eyes is known as ophthalmopathy. This is why, along with a swollen thyroid gland, this type of goiter is also accompanied by bulging, protruding eyes, as you can see in Figure 7. This bulging of the eyes is called exophthalmos, where “ex” means “out” and “ophthalmos” refers to the eyes.

Key Term: Exophthalmic Goiter

An exophthalmic goiter can be caused by excessive thyroid hormones (thyroxine and triiodothyronine) and is usually accompanied by the bulging of the eyes.

Let’s think about the symptoms of hyperthyroidism, using the normal functioning of the thyroid hormones as a reference. Since thyroid hormones are responsible for regulating metabolism, the overproduction of this hormone leads to a very high basal metabolic rate, weight loss, an increased heart rate, and high blood pressure. Hyperthyroidism can also cause irritability and anxiety.

Hyperthyroidism is usually treated with surgery to remove the affected portion of the gland. Thyroid cancer can, in some cases, be treated using radioactive iodine. Earlier on in this explainer, we learned that iodine is essential for the production of thyroxine and triiodothyronine. The thyroid follicular cells have special pumps that allow them to take up iodine. When radioactive iodine is introduced, it is taken up into the thyroid follicular cells, which are then killed because of the radioactivity of the iodine compound, destroying the suspected cancerous cells.

Example 2: Matching Thyroid Diseases to Their Symptoms

For the following, state what disease is associated with the thyroid gland being described.

  1. Caused by a decrease in thyroid hormones in infants, this condition leads to problems with growth, often resulting in short stature and impaired development of secondary sexual characteristics.
    1. Myxedema
    2. Exophthalmic goiter (Graves’ disease)
    3. Cretinism
    4. Hyperglycemia
  2. Caused by a decrease in thyroxine secretions in adults, this condition can cause hair loss, weight gain due to a decrease in basal metabolic rate, and a decrease in mental capabilities.
    1. Hyperglycemia
    2. Myxedema
    3. Exophthalmic goiter (Graves’ disease)
    4. Cretinism
  3. Caused by an increase in thyroxine secretion, this condition leads to enlargement of the thyroid and neck and bulging of the eyeballs.
    1. Myxedema
    2. Exophthalmic goiter (Graves’ disease)
    3. Hyperglycemia
    4. Cretinism

Answer

Part 1

Thyroxine plays an important role in regulating the production and release of growth hormones in the body, which cause normal cell growth, division, and differentiation. Hypothyroidism, or a deficiency of thyroxine, can lead to problems with growth. When hypothyroidism occurs in infants and children, it can cause a condition called cretinism, leading to short stature and delayed development of secondary sexual characteristics.

The correct option is, therefore, cretinism.

Part 2

In adults, hypothyroidism, or low levels of thyroxine, can cause a condition called myxedema. Normal levels of thyroxine help maintain healthy skin and hair, normal brain function, and a stable basal metabolic rate. Myxedema, therefore, is characterized by hair loss, weight gain due to decreased metabolism, and a decrease in mental capabilities, as stated in the question.

The correct option is, therefore, myxedema.

Part 3

While normal levels of thyroxine help maintain stable internal conditions, excess thyroxine can cause health problems. This condition is called hyperthyroidism and can be caused by an autoimmune condition called Graves’ disease, in which the body’s immune system mistakenly attacks the thyroid gland, causing it to produce excess thyroxine. Hyperthyroidism causes the thyroid gland to swell, forming a goiter, and the eyeballs to bulge, which is called exophthalmos. This type of goiter is called an exophthalmic goiter.

The correct option is, therefore, exophthalmic goiter (Graves’ disease).

Let’s now take a look at the parathyroid glands. There are four parathyroid glands in the body. They are tiny pea-sized structures, found on the posterior (or back) surface of the thyroid gland. As you can see in Figure 8, there are two parathyroid glands behind each side of the thyroid gland.

Key Term: Parathyroid Glands

The parathyroid glands are four pea-sized glands found on the posterior surface of the thyroid gland. The parathyroid glands produce parathyroid hormone.

The parathyroid glands secrete parathyroid hormone, sometimes called parathormone. This hormone works in combination with calcitonin, which we learned about earlier. Together, these two hormones play a very important role in regulating the level of calcium in the body.

Key Term: Parathyroid Hormone

Parathyroid hormone is the hormone produced by the parathyroid glands and is involved in calcium homeostasis.

While calcitonin decreases the calcium levels in the blood by preventing its release from bones, parathyroid hormone has the opposite effect. When the level of calcium in the blood is low, parathyroid hormone stimulates bone resorption, which is the breakdown of the bone to release calcium into the bloodstream.

Key Term: Bone Resorption

Bone resorption is the process of the breakdown of bone tissue to release calcium into the bloodstream.

Calcium is the primary mineral in the bones, contributing bone mass and sturdiness. However, calcium ions are also needed in the bloodstream, as they help in blood clotting, muscle contraction, and the transmission of signals in the nervous system. The two hormones, calcitonin and parathyroid hormone, act in opposition to one another in order to carefully control the levels of calcium in the blood and in the bones, as represented in Figure 9.

Just like thyroid hormones, excess or insufficient production of parathyroid hormone can also be dangerous. Hypoparathyroidism, or low levels of parathyroid hormone, can be caused by injury or damage to the parathyroid glands. Without parathyroid hormone, the level of calcium in the blood will be low, causing overexcitation of the nervous system. This can cause seizures and muscle spasms.

Key Term: Hypoparathyroidism

Hypoparathyroidism is a condition in which the parathyroid glands produce insufficient parathyroid hormone.

Hyperparathyroidism, on the other hand, is the overproduction of parathyroid hormone due to the overactivity of one or more of the parathyroid glands. This can be caused by a tumor or growth in the gland. When the body has too much parathyroid hormone, this causes increased bone resorption leading to increased levels of calcium in the blood, which is called hypercalcemia. Because of this, the bones lose mass, becoming fragile and susceptible to bending or fractures. The high levels of calcium in the body can also cause kidney stones, as calcium is deposited in the kidney tubules.

Key Term: Hyperparathyroidism

Hyperparathyroidism is a condition in which the parathyroid glands produce excess parathyroid hormone.

Example 3: Describing the Functions of the Parathyroid Glands

What is the primary function of the parathyroid gland?

  1. To regulate the calcium levels in the blood
  2. To regulate body temperature
  3. To regulate the basal metabolic rate of the body
  4. To control the rate of unconscious activities, for instance, heart rate and breathing rate
  5. To stimulate the uptake of glucose by the liver

Answer

The parathyroid glands are four pea-sized glands found on the posterior (back) surface of the thyroid gland, with two in each thyroid lobe. The parathyroid glands secrete parathyroid hormone, sometimes called parathormone.

In order to regulate calcium levels in the blood when the level of calcium is low, parathyroid hormone increases it by promoting bone resorption, which is the destruction of bone tissue to release calcium into the bloodstream. This hormone works alongside another hormone, calcitonin, forming a calcium-regulating system in the body. Calcitonin has the opposite function to that of parathyroid hormone: when the level of calcium in the blood is high, calcitonin prevents the release of calcium from bones, reducing the quantity in the bloodstream.

The correct function of the parathyroid gland is, therefore, to regulate the calcium levels in the blood.

Let’s summarize everything we have learned in this explainer about the thyroid and parathyroid glands.

Key Points

  • The thyroid gland is a butterfly-shaped endocrine gland located at the base of the neck, below the larynx or Adam’s apple.
  • The thyroid gland produces thyroid hormones (thyroxine and triiodothyronine), which are important for physical and mental growth and metabolism.
  • Calcitonin is a hormone produced by the thyroid gland that is involved in calcium homeostasis.
  • Hypothyroidism, or insufficient thyroid hormones, can cause growth impairments, neurological disorders, and slowed metabolism. It is associated with simple goiter.
  • Hyperthyroidism, or excess thyroid hormones, can cause weight loss, increased metabolism, and nervous irritability. It is associated with exophthalmic goiter.
  • The parathyroid glands are four pea-sized glands on the posterior surface of the thyroid gland that secrete parathyroid hormone.
  • Parathyroid hormone works with calcitonin, secreted by the thyroid gland, to regulate the level of calcium in the body.
  • Hypoparathyroidism can cause painful muscle spasms and seizures, while hyperparathyroidism can cause bones to become fragile and prone to fractures.

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