meandtheguys2--
Here it is. This is from a "top" endocrinologist who moderates our thyroid cancer listserv. There are other doctors (endos) that agree with this. So, I am a little nervous about doing this. I've removed the doctor's names in the text as I did not request permission to post.
PERIPHERAL CONVERSION OF T4 TO T3
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Yes, the thyroid gland makes T4 and T3.
But the bulk of T3 in the body comes from the T4 secreted by the thyroid
gland that is transformed into T3 outside of the thyroid - mostly in
Liver and kidneys, probably in humans some in the muscle as well - by a
process called Peripheral Conversion.
This Peripheral Conversion of T4 to T3 is a process that is regulated
very tightly, increasing or decreasing the out-of-thyroid production of T3
according to your overall health and nutrition. If you are fasting, the
amount of T3 produced in the Peripheral Conversion goes down rapidly because
the body is trying to cut down oxygen consumption in non vital organs.
In that way you save energy.
The same thing happens if you have a heart attack, an automobile
accident, an infection, or any severe form of stress.
The peripheral production of T3 goes down to decrease oxygen consumption
and to save energy in situations of crisis.
This may be a life saving manuever in some cases.
If you take T3 by mouth, the you lose this fine regulation of T3 production
and when the time comes that you are in a situation of crisis, you will not
be able to decrease the production of T3, to cut down on your oxygen
consumption and you will not be able to save energy.
Is that clear?
If you take T4 alone, then the body (Mostly liver and kidneys) convert the
T4 in T3 exactly in the amounts that are right for you at that
particular time.
This was shown many years ago in persons with total thyroidectomies (No
production of T3 in the thyroid because there was no thyroid). Even though
they were taking T4, the levels of T3 were in exactly the normal range.
This in general for both persons who are hypothyroid and persons with Thyroid cancer.
Thyroid cancer persons are in a different situation. They need more than the
usual replacement; they need to suppress their TSH to prevent any further
growth of thyroid tissue.
Dr ***'s last post is quite right emphasizing that T4 gives a much
longer suppression of TSH than T3 may. The effect of T3 lasts only hours, it
does not suppress TSH for the full 24 hours. This can create problems. Imagine
that you take T3 fasting in the morning. It may last in circulation - let's
say for 6 hours - and will suppress TSH during that time. You come to see
your doctor at 9 AM and he/she checks your TSH. It will be suppressed
because you still have the T3 that you took earlier and is still
circulating. By the evening, you run out of T3 and TSH may go up higher
than it should if you are not taking T4 or if you are taking a small dose of
T4.
For that reason and to make it very clear and safe to patients and other
doctors, most endocrinologists use T4 and only T4.
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also from Dr. ****** (November 1999) ...
Thyroid replacement with T3 (Cytomel) does not make much sense.
Under normal circumstances replacing with T4 will give you exactly the blood levels
of T3 you need for every circumstance. This is done because T4 is converted to T3 in
several tissues (Mostly liver, kidney and central nervous system) by a process called
Peripheral Conversion or Deiodination (One molecule of Iodine is removed from T4
for it to become T3).
Let me explain this a little.
Under normal circumstances the organism regulates how much T3 is produced
from T4 in the process of peripheral conversion. This is done to match your
metabolic needs at all times. Example, during the winter the body makes
more T3 from T4 to increase your body temperature. During the summer, the
opposite occurs: less T3 is made and you stayed "cooler". If you are sick,
you don't want to waste energy in unnecessary functions, so the body cuts
down the peripheral conversion and you get less T3 in circulation but the
important organs make locally all the T3 they need.
If you are starving, the body rapidly decreases the levels of T3 to protect
you from starvation, slow down your weight loss and save as much as 30% of
your daily energy requirements! If you decide to by-pass this unique body
function and you add T3 to a person that is fasting (Let's say because you
WANT to lose weight - a very common situation, including my personal wishes)
Then, with fasting + T3, you stop saving energy, lose more weight
but you do so by losing much more muscle than before. The life saving
mechanism of "protein sparing" disappears and you start wasting proteins
first from your muscles and immediately after that from your vital organs.
This is a big disadvantage.
Also, if you are taking T3 and get sick, you will be sicker because you are
abolishing many metabolic compensations that are beneficial and vital when
you automatically lower the T3 production and the blood levels of this
hormone.
Next, T3 by mouth gives you very high levels in blood immediately and then
they fall rapidly. So, you get a roller coaster even if you take 1/3 of the
dose three times a day. Your body, specially your heart, senses those wide
fluctuations in T3 and goes "crazy" trying to accommodate to the varying
orders from the T3 levels and the metabolic demands. Again, this is not good.
When you are replaced with T4, the levels of T3 are VERY stable and more
physiologic, plus they go up and down with your daily metabolic needs. This
is wonderful and so precise that I could erect a statue to "Peripheral Conversion".
The three T4 products in the market are very good and very reputable. For
all practical purposes and for every dose, Synthroid = Levothroid - Levoxyl.
Very soon there will be fourth one, that will be called - I can say it now -
Euthyrox and will be manufactured by EM Pharma. Euthyrox also will be very good.
The legal problem of the Synthroid company had to do with research done
showing that their pills were identical to the other ones, and the company
tried to hide that information.
But aside of the wait for the RAI scan or treatment in persons with
Thyca, T3 has very little indications. For you to have an idea, I have not
written a prescription for T3 in over 10 years. The other indications are very
questionable, controversial and even dangerous. I'll mention them here for
general information but, I repeat, this is very, very controversial and not
mainstream medicine. T3 has been used in resistant forms of depression. This
is in patients that have not improved after all the known treatments. Under
those circumstances, some of them seem to improve with T3 in large doses.
T3 has also been used intravenously in the recovery of heart by-pass operation.
This is even more controversial, and in general done by surgeons who are not
endocrinologists. I, personally, think that this is even more dangerous.
