Diagnosis – hyperthyroidism, hypothyroidism.
We haven’t gotten into great detail, specifically
of Graves’ disease or Hashimoto to give
you an example of hyper/hypothyroidism, but
what we have done in this discussion is talk
about TBG excess and TBG deficiency.
At this point, I need you to focus upon the
bottom two differentials.
In addition, the topic is differential diagnosis
in summary of results of T3-resin uptake.
I’ve shown you resin.
What is it going to bind to?
Do not confuse the resin with your TBG.
Where’s thyroglobulin bind to?
What you’re going to be using, as a clinician,
is the T3-resin uptake to give you further
confirmation for your diagnosis.
Is that understood?
None of this would make any sense to you,
unless you’ve understood the discussions
that we had prior.
In TBG excess, think about a common example
– pregnancy and estrogen.
TBG deficiency – drugs that displace the
T4 from your TBG or heparin that I showed
you earlier that increases lipoprotein lipase
That increasing free fatty acid inhibits the
binding of T4 to TBG.
Those are your differentials.
Let’s move on to next column.
If it is pure hyperthyroidism, we know that
the serum free T4 index is increased.
Example – Graves’s disease.
Hypothyroidism – as a differential –Hashimoto.
We know that the free T4 index is decreased.
The topic for this entire section was what?
It wasn’t hyper or hypo.
It was euthyroid.
When you say euthyroid, what does that mean?
Take a look.
Either TBG excess or TBG deficiency showing
you what with free T4?
What’s the clinical representation of normal?
So, how can you possibly tell what’s causing
the normal, but yet resulting in hyperthyroxinemia
Let’s take a look at it further.
Serum total T4 is my third column.
What’s total mean to you?
Bound versus free.
In hyperthyroidism, we can for sure expect
there to be more T4, thus, there will be more
bound; therefore, total is increased.
I want you to skip down to TBG excess.
In TBG excess, we talked about estrogen.
We talked about increased thyroglobulin.
What is it doing?
It’s binding to more T4.
If your bound is increased, what about the
Increased serum total T4.
How can you confirm and how can you differentiate
between the two?
Give me a second.
Take a look at the hypothyroidism with decreased
T4, obviously decreased bound.
You’ll have decrease in total T4, whereas
if you have TBG deficiency, you have less
of your T4 that is bound, because you have
less TBG and you have decreased total T4.
This now brings us to our last and final column
that I’ve highlighted in red for you.
What this is showing you is that resin and
this is how you’re going to interpret and
differentiate one type of pathology from another.
So, that’s what the resin is binding to.
And this is then referred to as your thyroid
hormone binding resin.
If it’s hyperthyroidism, with all that increased
T3 and T4 you can expect there to be more
T3 bound to my resin.
It is increased.
Skip down to TBG excess.
Now, this is where you want to pay attention
and this is where you’re going to take all
that information that I’m giving you and
put it all together.
That TBG excess, give me your differential.
Number one differential.
Oh, pregnancy or contraceptive pill, what
There’s something going on with the liver
that you have increase in TBG.
Let’s move a little bit faster now.
That TBG excess is bound to whom? T4.
T4 What happens to your total?
Take a look at hyperthyroidism.
The first row and the third row.
Both of those are increased for total.
How can you differentiate between the two?
In hyperthyroidism, that T3 will be bound
to resin in increased amounts, whereas why
is it that it’s decreased with TBG excess?
Because all that T4 is now then bound to my
TBG and now, at this point, you’ll have
less T3 that is now available to bind to your
So, if you have T3 resin uptake, that’s
decreased in TBG excess.
Let’s take a look at T3-resin with hypothyroidism
and TBG deficiency.
In hypothyroidism, think of Hashimoto, you
have less T3 bound to the resin.
In TBG deficiency, you have less T4 that is
then bound to TBG.
You have more T3 that is available to then
bind to my resin.
At this juncture, with that fourth and final
column, it would behove you to go back to
those previous illustrations that I showed
you, where you can clearly see with TBG deficiency
more T3 bound to the resin, whereas if it’s
TBG excess, you have less T3 available to
bind to your resin.
How important is this?
This is going to work for you on every single
board exam that you can think of and as you
walk into your residency program, you are
interpreting this for the attending who will
never pimp you ever again.