Video Transcript
In this video, we will learn about
the anatomy and function of the thyroid and parathyroid glands, what hormones they
release, and what effect these have on the body.
The thyroid gland impacts all the
tissues in our body, setting up its pace, metabolism, growth, and development. Such an important role means that
even small thyroid dysfunctions have various impacts on the entire body, causing a
butterfly effect, where a small change can result in large consequences. So let’s shed some light on these
very special glands.
The thyroid and parathyroid glands
sit together at the front base of our neck, looking a little like a bow tie. They belong to the human endocrine
system, which is a series of endocrine glands that secrete chemical messengers
called hormones. Various hormones are produced by
the various cells that are found in endocrine glands. These hormones are then secreted
into the bloodstream. The blood then transports them
throughout the entire body to reach their target cells or organs. Once hormones reach their target,
they regulate the functions of cells in order to adapt our body to our needs or to
changes in our environment, to maintain the normal functioning of the body. In other words, hormones contribute
to maintain homeostasis, which is the fine balance that our body maintains to ensure
that all our body functions work in harmony.
Let’s begin by taking a look at the
structure of the thyroid gland. The thyroid gland is a two-lobed
gland located just below the larynx, which is sometimes called the Adam’s apple. Each lobe attaches on either side
of the trachea. The two lobes are joined by a band
of tissue called the isthmus. The name thyroid was originally
coined from the Greek word “thyreos,” which means shield, because the shape of this
gland is said to resemble a type of ancient Greek shield. Each lobule of the thyroid gland is
made up of many spherical units called thyroid follicles. The wall of each follicle is made
up of many rectangular cells called follicular cells. These cells enclose a cavity filled
with a fluid called colloid. This is the place of important
reactions during the production of the thyroid hormones. The thyroid gland receives a rich
network of blood vessels that play critical functions. Let’s take a look at some of these
functions.
The pituitary gland is located just
under the brain, and it’s responsible for releasing many different hormones. It’s often nicknamed the master
gland as it controls the activity of many other endocrine glands, including the
thyroid gland. The anterior lobe of the pituitary
gland secretes a hormone called TSH into the bloodstream. TSH is short for
thyroid-stimulating hormone, otherwise known as thyrotropin. The blood vessels associated with
the thyroid gland deliver TSH to its target cells. These are the follicular cells of
the thyroid gland, and they’re activated to produce thyroid hormones. Another important function of these
blood vessels is to carry nutrients to the thyroid gland. These nutrients, like the amino
acid tyrosine, come from our food and are absorbed into the blood by the digestive
system.
Another nutrient that plays a vital
role in thyroid function is an essential mineral called iodine, which is mostly
found in our diet in fish, dairy products, and eggs. Most table salt is supplemented in
iodine too. And if you’re lucky enough to live
near the sea, studies have shown that breathing sea air can contribute to our iodine
intake. The blood can transport these
nutrients to the thyroid gland, so they can move into the follicular cells and then
into the colloid. The colloid is where enzymes are
used to combine iodine atoms with the amino acid tyrosine. This mostly produces a thyroid
hormone called thyroxine, also known as T4, which has four iodine atoms. Another hormone which only contains
three iodine atoms is produced in smaller quantities and is called T3 or
triiodothyronine.
These hormones are stored in the
colloid. They are then secreted into the
blood vessels if the follicular cells are instructed by TSH to release them. Through the blood vessels, they can
then reach their other target cells in the body. A lot of T4 hormones are converted
in the liver into T3, which is a more active form. Aside from the rectangular
follicular cells, there are also round cells called parafollicular cells in the
thyroid gland. These cells are sometimes called C
cells as they release a hormone called calcitonin. Calcitonin works to reduce the
level of calcium in the blood. This hormone works in balance with
the parathyroid hormones, as we will learn later on in the video. Let’s now investigate the different
effects that these thyroid hormones can have on the body.
First, very early in our life,
these hormones already play an important role in our development. During the development of an
embryo, thyroid hormones T3 and T4 promote the proliferation and differentiation of
neurons, making them build connections called synapses that are going to be very
important for learning and memory. As a consequence, a lack of thyroid
hormones or a diet poor in iodine during pregnancy that means less thyroid hormones
can be produced can have long-lasting effects on the fetus. This can sometimes cause various
delays in development and intellectual disabilities in a syndrome called congenital
iodine deficiency syndrome. These hormones remain important
throughout our lives to our brain function.
Furthermore, T3 and T4 play an
important role in our growth and also in our metabolism. Metabolism involves all the
chemical reactions that take place in our body to support and maintain life. Let’s try to understand first the
effect of thyroid hormones on metabolism at the cellular level. In almost all the cells of our
body, the mitochondria are responsible for cellular respiration. This process breaks down glucose to
release energy in the form of ATP. Firstly, the thyroid hormones T3
and T4 help with the absorption of glucose in the intestines from the food that we
eat. They then stimulate almost all our
cells to take up glucose, and they also promote the activity of the mitochondria to
release more energy. This means that at the body level
now, there is more energy to fuel all of the biological activities, such as our
heart rate, blood pressure, body temperature, or digestion.
Now, we can begin to understand the
effects of too little or too much of these thyroid hormones in the body. Insufficient levels of these
thyroid hormones will cause a condition called hypothyroidism. In adults, this is typically caused
by a lack of iodine in the diet or an autoimmune reaction. This means that someone’s own
antibodies will attack the thyroid gland, preventing the gland from producing
hormones. This causes the thyroid gland to
accumulate precursors of the thyroid hormones in the follicular cavities. This makes the thyroid gland appear
swollen and sometimes lumpy. This symptom is called a simple
goiter.
Myxedema is a condition associated
with severe hypothyroidism. As the brain functions are altered,
the symptoms are slowness, depression, difficulty concentrating, and fatigue. In children, delays in growth and
puberty are often observed. Myxedema also results in the
reduction of a person’s metabolism. This means that their heart rate
decreases while blood pressure increases and digestion is slow. People experiencing myxedema will
often struggle with feeling particularly cold, gaining weight, constipation, and
thinning hair. Usually, hypothyroidism can be
treated with replacement hormones.
Now, let’s ask ourselves what
happens when the problem is the opposite, a thyroid gland that produces too many
hormones. Too many thyroid hormones leads to
hyperthyroidism. This can be caused by thyroid
cancer or more commonly another autoimmune disease that makes the thyroid produce
more thyroid hormones than required. This causes the gland to become
swollen and form a goiter. In this disorder, the immune system
also mistakenly attacks the tissues behind and around the eyes, which become
inflamed and swollen. The type of goiter that forms is
called an exophthalmic goiter, where ex- means out and “ophthalmos” refers to the
eyes because of the protruding aspects of the eyes in these patients.
The symptoms of hyperthyroidism are
diverse. By overstimulating brain functions,
hyperthyroidism can cause mood swings, nervousness, irritability, and difficulty
sleeping. The changes in metabolism increase
a person’s heart rate, cause high blood pressure, a higher body temperature, and
they often lose weight. It can be treated with surgery or
radioactive iodine.
Now, let’s take a look at the
parathyroid glands, starting with their structure. These four glands are about the
size of peas. And we have two located behind each
lobe of the thyroid gland. The parathyroid glands release a
hormone called PTH, which stands for parathyroid hormone, sometimes called
parathormone. This hormone works in balance 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. So, how is the calcium blood level
controlled by PTH and calcitonin? Let’s find out by looking at the
parathyroid glands’ function.
When the levels of calcium in the
blood are low, PTH is secreted, which activates bone cells to release calcium. The calcium that’s released can
then reenter the bloodstream, raising blood calcium levels back to a normal
level. If calcium levels in the blood
become too high, however, then the parafollicular cells of the thyroid gland release
calcitonin, which activates the absorption of calcium into the bones. This reduces the level of calcium
in the blood, returning it back to a normal healthy level. Calcium is very important to build
our bones, but also in a vast number of functions, including nerve transmission and
muscle contraction. Therefore, it’s very important to
have blood levels of calcium well balanced.
In hypoparathyroidism, too little
PTH is released by the parathyroid glands. This means that the calcium levels
remain too low in the blood. This is associated with muscle
cramps, tiredness, and memory problems. In hyperparathyroidism, too much
calcium is taken from the bones. This can make them more prone to
fractures. Because more calcium is circulating
in the blood, this causes abnormal muscle and heart contractions and abnormal
functioning of the nervous system, which left untreated may even lead to a coma. These dysfunctions of the
parathyroid glands can be treated by medication or surgery.
Let’s see how much we’ve learned
about the thyroid and parathyroid glands and their respective hormones by applying
our knowledge to a practice question.
Which of the following is not one
of the primary functions of thyroxine in the human body? (A) Stimulating mental and physical
growth. (B) Maintaining normal body
temperature. (C) Increasing the amount of
calcium in the bloodstream. (D) Maintaining a normal heart
rate. Or (E) regulating the basal
metabolic rate.
Thyroxine, which is sometimes known
as T4, is the main hormone that’s secreted from the thyroid gland in the human
body. And it plays many important
functions in the body. In this question, we are being
asked to identify which function that’s listed below is not one of the functions of
thyroxine. Let’s work through the options to
find the correct answer.
Thyroxine does play a crucial role
in stimulating brain development and brain function. It’s responsible for developing the
brain in the embryonic stage and also for physical growth. We’ve therefore deduced that
stimulating mental and physical growth is one of the main functions of
thyroxine. So we can eliminate this
option.
Thyroxine also plays an important
role in our metabolism. Metabolism describes all the
chemical reactions that occur within the body to maintain life, including those that
release energy for our cells to use. By regulating the amount of energy
available in the body, thyroxine helps to maintain a normal body temperature. So we can eliminate this option
too. Through its regulation of
metabolism, thyroxine also helps to regulate the heart rate. So this option cannot be our
correct answer.
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 the uptake of glucose in cells, thyroxine can increase the basal
metabolic rate. If glucose needs to be delivered to
the cells faster, the rate of the heartbeat that pumps blood containing glucose to
all the body cells will also need to be maintained. This is why thyroxine does not only
regulate the basal metabolic rate, but it also regulates the heart rate. So we’ve deduced that this option
will also be incorrect.
On the posterior surface of the
thyroid gland, there are four glands called parathyroid glands, which secrete a
hormone called parathyroid hormone, or PTH for short. Parathyroid hormone, not thyroxine,
is responsible for increasing the amount of calcium in the bloodstream to maintain
the level of calcium in the blood. This means that this is not a
function of the thyroid gland, but of the parathyroid gland hormones. And so the incorrect function of
thyroxine is therefore increasing the amount of calcium in the bloodstream.
Let’s summarize what we’ve learned
in this video about the thyroid and parathyroid glands. The thyroid gland is a
butterfly-shaped endocrine gland located at the base of the neck below the Adam’s
apple. The thyroid gland produces and
releases the hormones thyroxine and triiodothyronine. These hormones are important for
physical and mental growth and our metabolism. The thyroid gland also secretes a
hormone called calcitonin that decreases the levels of calcium in the blood.
Hypothyroidism or insufficient
thyroid hormones can cause growth delays, mental disabilities, and slowed
metabolism. Hyperthyroidism or excess thyroid
hormones can cause weight loss, irritability, and increased metabolism. The four parathyroid glands are
found on the posterior surface of the thyroid gland. They secrete parathyroid
hormone. Parathyroid hormone increases the
level of calcium in the blood. Hypoparathyroidism can cause
painful muscle cramps and memory problems, while hyperparathyroidism can cause
fragile bones and abnormal muscle and heart contractions.
