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Today we’re going to look at hyperthyroidism vs hypothyroidism.
We’re not only going to walk through the symptoms,
but we’re also going to explain why the symptoms occur.
We’re also going to review thyroid hormone and how it works in the body,
which will then help you better understand the symptoms of hyper and hypothyroidism.
We’ll go back to this chart at the end, so make sure to watch the entire video.
You can find all of the information from the video linked down below in the lecture on our website.
There is also a really good cheat sheet summarizing all the information,
so make sure to get your copy of that.
That’ll be linked down below as well.
All of the videos come with lectures and study guides,
so make sure to hit that subscribe button to not miss out on those.
Let’s quickly review the pathway in which thyroid hormone is produced and regulated.
The pathway starts with the hypothalamus,
which is located in the center of the brain below the thalamus.
The hypothalamus releases a hormone called thyrotropin-releasing hormone, also known as TRH.
Located below the hypothalamus is the pituitary gland.
TRH acts on the pituitary gland and stimulates
the anterior pituitary gland to release another hormone.
This hormone is called thyroid-stimulating hormone or TSH.
TSH is released from the anterior pituitary gland into the bloodstream.
TSH then binds to another gland called the thyroid gland.
The thyroid gland is a butterfly shaped structure located at the base of the anterior neck.
TSH activates the thyroid gland to synthesize or produce thyroid hormone.
Thyroid hormone is then released from the thyroid gland into the bloodstream
where it acts on many tissues and organs in the body, which we’ll talk about shortly.
The main thyroid hormones are triiodothyronine which is T3 and thyroxine which is T4.
T3 and T4 will then have a negative feedback loop on the hypothalamus and anterior pituitary gland.
The negative feedback loop is to help regulate thyroid hormone levels in the blood
and maintain thyroid homeostasis in the body.
For example, if thyroid hormone levels are high in the blood,
then it will inhibit the hypothalamus and anterior
pituitary gland from releasing of TRH and TSH respectively.
The opposite will be true when thyroid hormone levels are low.
There will decreased inhibition from the negative feedback loop
and there will be increased TRH and TSH release in order to produce more thyroid hormone.
This pathway that regulates thyroid hormone production and release is known as the
hypothalamic-pituitary-thyroid axis or HPT for short.
The easy way to remember the order of of the structures involved in this pathway
is to go in alphabetical order, H-P-T.
This will help you remember the H for hypothalamus is first,
followed by the P for pituitary gland, followed by the T for thyroid gland.
It can also be easy to mix up TRH and TSH.
You can also remember these by going in alphabetical order.
Use the R in TRH and the S and TSH.
The R comes before the S which will help you remember the releasing hormone or TRH
comes first, followed by the stimulating hormone or TSH.
Now that we understand the pathway, let’s look at the effects thyroid hormone has on the body.
Thyroid hormone has many different target tissues and effects on the body.
We’re going to focus on the main functions of thyroid hormone because once you understand these
effects, then figuring out the symptoms of hyper and hypothyroidism is simple.
When thinking of the overall effect thyroid hormone has on the body,
think of it as ramping and speeding things up.
So if we look at the cellular level first, the overall effect thyroid hormone has on most cells
is it increases protein synthesis and increases metabolic activity within the cell.
The increased metabolic activity within cells will increase the overall metabolism in the body.
Increased metabolism also generates body heat, which is known as thermogenesis.
Let’s now look at what this increased metabolic activity means for different organs.
Starting with the heart, thyroid hormone will act
on the heart to increase the number and sensitivity of beta-1 receptors.
We know from our video on beta receptors that the beta-1 adrenergic receptors are
found in the heart, and they bind epinephrine and norepinephrine,
which are involved in the fight or flight sympathetic response.
Increased beta-1 receptors in the pacemaker cells of the heart will increase heart rate.
And the increased beta-1 receptors on contractile cells will increase cardiac contraction.
If we increase cardiac contraction, then we increase stroke volume.
And if we increase both heart rate and stroke volume, this will increase cardiac output,
which will increase blood pressure.
So thyroid hormone can increase heart rate, stroke volume, cardiac output, and blood pressure.
Thyroid hormone can also act on the GI system.
Thyroid hormone increases the metabolic activity within the GI tract,
which will increase GI motility and secretions.
Next, thyroid hormone can cross the blood-brain barrier and has effects
on the central nervous system, especially in a developing brain during the prenatal period.
Thyroid hormone increases neurogenesis, which is the formation of new neurons.
It also increases myelination.
Myelin is a sheath that forms around axons to help increase the conduction
speed of electrical impulses through the neuron.
Thyroid hormone also promotes synaptogenesis, which is the formation of more synapses or
junctions between neurons and this is how neurons communicate with each other.
It also helps increase dendrite formation, which
are the branches on a nerve cell that receive electrical impulses from other nerve cells.
Lastly, thyroid hormone can increase the overall activity of the sympathetic nervous system.
All of these neurological effects play an important role in central
nervous system and brain development, particularly during the prenatal period,
but they also play a role in mood and mental cognition in adults.
Thyroid hormone can also increase cutaneous blood flow and act on the skin, hair, and nails
which helps to maintain skin homeostasis, stimulate the proliferation of different skin
cells in the epidermis and dermis, maintain sweat gland and oily secretions in the skin,
stimulate hair growth, and stimulate nail growth.
Now that we understand the main effects of thyroid hormone,
the symptoms of hyperthyroidism and hypothyroidism will quickly make sense.
Let’s look at hyperthyroidism first.
We can define hyperthyroidism by breaking down the word.
We know from our medical terminology videos that the prefix “hyper”
means above normal, excess, high, or increased.
The root word “thyroid” pertains to the thyroid gland or thyroid hormone.
And the suffix “ism” refers to a medical condition, disease, or process.
So hyperthyroidism is a condition
in which the thyroid gland is overactive and produces too much thyroid hormone.
This means there will be increased levels of thyroid hormone in the blood.
You can also have hyperthyroidism from exogenous thyroid hormone use
but most of the time it’s from an overactive thyroid.
So if we go back to our original pathway,
(Click) hyperthyroidism is when there is too much thyroid hormone in the
blood which will then have an effect on target tissues and cause symptoms.
Let’s look at those symptoms now.
We already know the major effects thyroid hormone has on the body
and how it “ramps or speeds things up”.
So the easiest way to think about the symptoms of hyperthyroidism
is to amplify or “ramp up” those effects even more.
Individuals with hyperthyroidism are in a hypermetabolic state,
which means they will be showing signs of increased metabolic activity.
Remember we said thyroid hormone increases metabolic activity within cells.
If thyroid hormone levels are high then there will be an even larger increase
in metabolic activity within cells and an increase in overall body metabolism.
So hyperthyroidism can lead to weight loss.
This is also why another symptoms is increased appetite
to try to restore the energy being metabolized and used up.
We also said metabolism produces body heat.
The increased metabolism from hyperthyroidism means there is increased body heat production
which can lead to increased body temperature, heat intolerance, or increased sensitivity to heat.
We also said thyroid hormone increases beta-1 receptors in the heart which
can increase heart rate and cardiac contraction.
These effects will be amplified in individuals with hyperthyroidism.
So this can lead to symptoms of tachycardia or a fast heart rate.
Individuals may also experience palpitations or sensations of the heart racing or skipping beats.
Abnormal heart rates and rhythms are called arrhythmias,
which may be present in hyperthyroidism.
The increased heart rate and cardiac contraction will also increase cardiac output,
which can cause hypertension or high blood pressure to be present.
We also said thyroid hormone can act on the GI system to increase GI motility and secretions.
This means individuals with high levels of thyroid hormone
may experience diarrhea from the extreme increase in GI motility and secretions.
We also said thyroid hormone plays an important role in the
development and function of the central nervous system,
particularly in the prenatal period, and it also increases sympathetic activity.
Individuals who have high levels of thyroid hormone
may experience overstimulation of neuronal and sympathetic activity.
This can lead to symptoms of anxiety, nervousness,
irritability, restlessness, insomnia, tremors, and hyperreflexia.
The increased sympathetic activity also contributes to the tachycardia and
hypertension we saw, as well as the increased sweating which we are about to discuss.
We also know thyroid hormone has effects on the skin, hair, and nails.
Hyperthyroidism can increase cutaneous blood flow,
sympathetic activity, and body temperature which can all increase sweating and skin secretions.
Increased cutaneous blood flow and high levels of thyroid hormone
can also cause hair growth and the hair to be fine and soft.
Other symptoms include nail growth which can lead to onycholysis,
which is separation of the nail from the nail bed.
So in summary we can see how hyperthyroidism will ramp up the different organ systems even more.
Hyperthyroidism can also cause other general symptoms such as shortness of breath, fatigue,
muscle weakness, osteoporosis, among others but the ones shown are some of the main ones.
Hypothyroidism now becomes easy.
It’s going to be the opposite of everything we talked about.
We can define hypothyroidism by breaking down the word again.
We know from our medical terminology videos that the prefix “hypo”
means below normal, deficient, low, or decreased.
The root word “thyroid” pertains to the thyroid gland or thyroid hormone.
And the suffix “ism” refers to a medical condition, disease, or process.
So hypothyroidism is a condition in which the thyroid gland is underactive
and does not produce enough thyroid hormone.
This means there will be decreased levels of thyroid hormone in the blood.
So if we go back to our original pathway again,
hypothyroidism is when there is deficient levels of thyroid hormone in the blood
which will then have an effect on target tissues and cause symptoms.
Let’s look at those symptoms now.
Again we already know the major effect of thyroid hormone is to “ramp or speed things up”.
Hypothyroidism will have an opposite effect,
which is why the symptoms are essentially the opposite of hyperthyroidism.
So the easiest way to think about the symptoms of hypothyroidism is it “slows things down”.
Individuals with hypothyroidism are in a hypometabolic state,
which means they will be showing signs of decreased metabolic activity.
So now there will be decreased metabolic activity in cells
and an overall decrease in body metabolism.
This is why one of the symptoms of hypothyroidism is weight gain.
This is also why another symptoms is decreased appetite
because less energy is being used up and there is less of a need to replace the energy.
The decreased metabolism from hypothyroidism will decrease body heat production
which can lead to decreased body temperature, cold intolerance, and increased sensitivity to cold.
Low levels of thyroid hormone also means there will be a decrease in
beta-1 receptor expression and sensitivity.
This can lead to bradycardia or a decreased heart rate.
However, hypothyroidism can still cause hypertension just like hyperthyroidism did.
So why is this?
In addition to increasing beta receptors, thyroid hormone can also decrease alpha receptors.
So in hypothyroidism, the opposite occurs where beta receptors decrease
and alpha receptors increase, particularly alpha-1 receptors on blood vessels.
We know from our video on alpha receptors that alpha-1 receptors are located on blood vessels
and cause vasoconstriction.
So in hypothyroidism, there is increased vasoconstriction and
peripheral vascular resistance present, which can increase blood pressure.
The decreased body temperature in hypothyroidism can also
cause vasoconstriction and increased peripheral vascular resistance as well.
So with hypothyroidism you can see bradycardia and hypertension,
whereas hyperthyroidism you can see tachycardia and hypertension.
We also said thyroid hormone can act on the GI system to increase GI motility and secretions.
This means in a low thyroid state there will be decreased GI motility and secretions
which can lead to constipation.
Low levels of thyroid can also affect the central nervous system,
which can lead to a decrease in mood, mental cognition, and sympathetic activity.
This can lead to symptoms of fatigue, depression, impaired memory, impaired concentration,
and what many people describe as a “mental fog”.
They don’t feel quite as alert.
There can also be delayed deep tendon reflexes, where we saw hyperreflexia with hyperthyroidism.
The decreased sympathetic activity also plays a role in the bradycardia we saw,
as well as the dry skin which we will discuss next.
We mentioned that thyroid hormone increases cutaneous blood flow and
has effects on the skin, hair, and nails.
The decreased cutaneous blood flow,
sympathetic activity, and body temperature in hypothyroidism can all cause dry, pale skin.
It can also lead to hair loss, hair that is coarse and brittle,
and nails that are thin and brittle.
So in summary we can see how hypothyroidism will slow down the different organ systems.
Hypothyroidism can also cause other general symptoms such as voice changes, hoarse voice,
muscle cramps, joint pain, among others but the ones we went through are some of the main ones.
Here’s a table I put together that summarizes hyperthyroidism vs hypothyroidism.
Again we can see how hyperthyroidism generally increases and speeds things
up, while hypothyroidism decreases and slows things down.
You can find this table in the lecture notes and study guide linked down below.
Hopefully this gave you better understanding of hyper vs hypothyroidism.
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