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- [Voiceover] The thyroid
and parathyroid glands

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are both located in the neck.

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The thyroid itself is located
just below the larynx.

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So you can think of it as
the lower half of the neck.

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The parathyroid glands,
well they're embedded

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into the sides and back of the thyroid.

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They're small little spherical structures

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embedded right with the thyroid.

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Both thyroid and parathyroid,

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they deal with calcium balance,

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but the thyroid itself is
also dealing with metabolism.

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So that means the thyroid

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is both calcium balance and metabolism,

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while the parathyroid is just
dealing with calcium balance.

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So look at the thyroid to start.

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The thyroid, well its
secretion, is controlled through

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that hypothalamus-pituitary control again.

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You have two main steroid-based
hormones, T3 and T4.

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Now both T3 and T4 are
going to help control

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the production of ATP from glucose.

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Well we know glucose is our sugar,

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that's the main sugar in our bodies.

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ATP is the energy source.

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That's what our cells use for energy.

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So converting glucose
to ATP is a major effect

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of the metabolic rate.

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Calcitonin, also from the thyroid glands.

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Calcitonin is going to

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help to lower your blood calcium levels.

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It lowers the blood calcium levels

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by causing a decreased
rate of bone resorption.

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That means the bones are not
pulling as much calcium out.

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So instead of as much
calcium leaving the bones,

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calcitonin stimulates the
bones to take up calcium.

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So calcitonin lowers
the blood calcium levels

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by causing the bones themselves
to pull more calcium in.

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All right, so here we go.

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We have this thyroxine concentration.

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Remember thyroxine is a thyroid hormone.

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If it's decreased, if it's low,

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the hypothalamus will pick it up.

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It can release the hormone

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and now all of a sudden we're
at the anterior pituitary.

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Anterior pituitary releases TSH,

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thyroid stimulating hormone,

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which means you now stimulate the thyroid.

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Releases thyroxine.

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Thyroxine is going to move it
back to a normal set point.

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Keep it balanced.

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So we have the hypothalamus in there,

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though we also have
the anterior pituitary.

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Both are potential control mechanisms.

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You release TSH, thyroid
stimulator hormone.

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The thyroid gland is stimulated,
produces more thyroxine.

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The parathyroid hormone

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is only responding to
lowered blood calcium levels.

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So unlike the thyroid,

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which deal with controlling mechanisms

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from the pituitary gland and hypothalamus,

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the parathyroid really only looks at

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how much calcium is in your blood.

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If you have lowered blood calcium levels,

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it'll actually pull calcium
and phosphate from your bones,

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causing this calcium to leave the bones

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and go into the blood.

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But at the same time it's going
to cause the digestive tract

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to increase calcium absorption.

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So any calcium that's found
in the digestive tract,

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it'll try to pull it out,
pull it into the body.

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Well the kidneys themselves?

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The kidneys are not going
to excrete as much calcium,

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going to try to keep every
last molecule of calcium

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that it can.

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And the parathyroid hormone, PTH,

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is the major regulator of blood calcium.

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So even though calcitonin also
deals with calcium levels,

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PTH, parathyroid hormone, is
the main function of the two.

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Here's a little chart
trying to help illustrate

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how the thyroid and parathyroid

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react to a drop in blood calcium.

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First off, the parathyroid
gland and the thyroid gland

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are both going to respond
to a drop in blood calcium.

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Firstly the thyroid gland will decrease

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its amount of calcitonin.

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Less calcitonin might
be all that's necessary

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to allow the bones to release
a little more calcium.

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It might be, which would then
put the blood calcium levels

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back to normal.

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But that's not always enough,

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which means sometimes
the parathyroid hormone,

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from the parathyroid
gland, is going to also

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stimulate the bones to
release more calcium.

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At the same time, good
chance it's going to

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activate the kidneys to save more calcium.

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If it's activating the
kidneys to save more calcium,

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well the kidneys are
probably going to be having

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more vitamin D stored

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to help with more absorption
of calcium in the intestines.

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If you're saying, "Hold on.

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"What does vitamin D
have to do with calcium?"

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Well if you take any calcium supplements,

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odds are if you look at that bottle

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you going to see calcium supplement,

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or calcium pill, or calcium
whatever, with vitamin D.

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Your body cannot absorb
calcium without vitamin D.

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So it's important to make sure

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you have an increase in vitamin D

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if you're trying to have an
increase in calcium absorption.

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But the exact opposite occurs

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if the blood calcium levels are too high.

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Thyroid and parathyroid
again both respond.

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The parathyroid gland

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will decrease the amount
of parathyroid hormone.

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That means the kidneys themselves

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will actually end up
excreting more calcium.

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But by excreting more calcium
they then have less vitamin D.

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Less vitamin D is going to mean

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less absorption of
calcium in the intestines.

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Well those two alone a lot of times

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can return the calcium
levels back to normal.

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Well usually the thyroid,

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you're going to increase
calcitonin levels.

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More calcitonin means that bones
are taking up calcium more.

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By absorbing more calcium in the bones

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you're resetting the blood calcium levels

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back to the normal set point.

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Disorders.

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Goiter is one of the most
common ones you hear about.

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Goiter is simply an iodine deficiency.

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An iodine deficiency then

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will cause a thyroid gland
to grow and grow and grow.

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What it's trying to do is make more cells

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to make more hormone.

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The problem is it doesn't
matter how many cells it has,

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if there's not enough
iodine, it can't work.