- [Voiceover] The thyroid and parathyroid glands
are both located in the neck.
The thyroid itself is located just below the larynx.
So you can think of it as the lower half of the neck.
The parathyroid glands, well they're embedded
into the sides and back of the thyroid.
They're small little spherical structures
embedded right with the thyroid.
Both thyroid and parathyroid,
they deal with calcium balance,
but the thyroid itself is also dealing with metabolism.
So that means the thyroid
is both calcium balance and metabolism,
while the parathyroid is just dealing with calcium balance.
So look at the thyroid to start.
The thyroid, well its secretion, is controlled through
that hypothalamus-pituitary control again.
You have two main steroid-based hormones, T3 and T4.
Now both T3 and T4 are going to help control
the production of ATP from glucose.
Well we know glucose is our sugar,
that's the main sugar in our bodies.
ATP is the energy source.
That's what our cells use for energy.
So converting glucose to ATP is a major effect
of the metabolic rate.
Calcitonin, also from the thyroid glands.
Calcitonin is going to
help to lower your blood calcium levels.
It lowers the blood calcium levels
by causing a decreased rate of bone resorption.
That means the bones are not pulling as much calcium out.
So instead of as much calcium leaving the bones,
calcitonin stimulates the bones to take up calcium.
So calcitonin lowers the blood calcium levels
by causing the bones themselves to pull more calcium in.
All right, so here we go.
We have this thyroxine concentration.
Remember thyroxine is a thyroid hormone.
If it's decreased, if it's low,
the hypothalamus will pick it up.
It can release the hormone
and now all of a sudden we're at the anterior pituitary.
Anterior pituitary releases TSH,
thyroid stimulating hormone,
which means you now stimulate the thyroid.
Releases thyroxine.
Thyroxine is going to move it back to a normal set point.
Keep it balanced.
So we have the hypothalamus in there,
though we also have the anterior pituitary.
Both are potential control mechanisms.
You release TSH, thyroid stimulator hormone.
The thyroid gland is stimulated, produces more thyroxine.
The parathyroid hormone
is only responding to lowered blood calcium levels.
So unlike the thyroid,
which deal with controlling mechanisms
from the pituitary gland and hypothalamus,
the parathyroid really only looks at
how much calcium is in your blood.
If you have lowered blood calcium levels,
it'll actually pull calcium and phosphate from your bones,
causing this calcium to leave the bones
and go into the blood.
But at the same time it's going to cause the digestive tract
to increase calcium absorption.
So any calcium that's found in the digestive tract,
it'll try to pull it out, pull it into the body.
Well the kidneys themselves?
The kidneys are not going to excrete as much calcium,
going to try to keep every last molecule of calcium
that it can.
And the parathyroid hormone, PTH,
is the major regulator of blood calcium.
So even though calcitonin also deals with calcium levels,
PTH, parathyroid hormone, is the main function of the two.
Here's a little chart trying to help illustrate
how the thyroid and parathyroid
react to a drop in blood calcium.
First off, the parathyroid gland and the thyroid gland
are both going to respond to a drop in blood calcium.
Firstly the thyroid gland will decrease
its amount of calcitonin.
Less calcitonin might be all that's necessary
to allow the bones to release a little more calcium.
It might be, which would then put the blood calcium levels
back to normal.
But that's not always enough,
which means sometimes the parathyroid hormone,
from the parathyroid gland, is going to also
stimulate the bones to release more calcium.
At the same time, good chance it's going to
activate the kidneys to save more calcium.
If it's activating the kidneys to save more calcium,
well the kidneys are probably going to be having
more vitamin D stored
to help with more absorption of calcium in the intestines.
If you're saying, "Hold on.
"What does vitamin D have to do with calcium?"
Well if you take any calcium supplements,
odds are if you look at that bottle
you going to see calcium supplement,
or calcium pill, or calcium whatever, with vitamin D.
Your body cannot absorb calcium without vitamin D.
So it's important to make sure
you have an increase in vitamin D
if you're trying to have an increase in calcium absorption.
But the exact opposite occurs
if the blood calcium levels are too high.
Thyroid and parathyroid again both respond.
The parathyroid gland
will decrease the amount of parathyroid hormone.
That means the kidneys themselves
will actually end up excreting more calcium.
But by excreting more calcium they then have less vitamin D.
Less vitamin D is going to mean
less absorption of calcium in the intestines.
Well those two alone a lot of times
can return the calcium levels back to normal.
Well usually the thyroid,
you're going to increase calcitonin levels.
More calcitonin means that bones are taking up calcium more.
By absorbing more calcium in the bones
you're resetting the blood calcium levels
back to the normal set point.
Disorders.
Goiter is one of the most common ones you hear about.
Goiter is simply an iodine deficiency.
An iodine deficiency then
will cause a thyroid gland to grow and grow and grow.
What it's trying to do is make more cells
to make more hormone.
The problem is it doesn't matter how many cells it has,
if there's not enough iodine, it can't work.