The intact thyroid gland makes a bunch of stuff, of which
T4 and T3 have been identified as active hormones.  (Which
means the other things may do something, but no one has
identified what -- so doctors ignore them as not doing
anything, despite the fact that most bio-systems do NOT
make chemicals which are then not used.)

The dance of T4 adn T3 is complex.  T3 is many times more
acitve than T4, and T4 converts to T3 in the liver and
certain other places.  Both circulate in the blood stream.
And both are carried in two forms: free (i.e., active) and
bound (i.e., inactive until freed).  Both are also stored
at various places in the body.  

Confused yet?  But wait, there's more!

You also get multiple time constants -- the time constants
for uptake from the digestive system and for elimination
from the body and for storage and release from storage!
And not everyone agrees on the values for these time
constants.

Next, there are TRH and TSH -- two hormones involved
in regulating thyroid, but neither is made the thyroid gland.
TSH is made by the pituitary, and it is basically the "thyroid
thermostat"  -- high TSH means the pituitary is telling
the thyroid to up the output, low TSH is the pituitary
telling the thyroid to cool it.

Now, here is the real kicker -- when the thyroid system
is broken, the MDs believe only the production of T4 and T3
is broken and that the TRH/TSH regulatory systems and the
external T4 to T3 conversion systems work just fine. (Frankly
that always amazes me, as an engineer, when I know a system
is broken, I don't trust ANY part of it.)

So they measure TSH 'cause that's what the pituitary is telling
the thyroid to do, and they follow it's orders.

1) The range established for "normal" in TSH is under question,
both the methodology used by labs to calibrate, and the original
establishment of "normal" suffer from contamination of undiagnosed
hypo-.  There are some MDs in England who suggest that
normal TSH should go from 0.5 to 2.0 not 0.5 to 5.5 . . .

2) the system is broken, why assume TSH is correct?  The active
hormones are freeT3 and, to a lesser extent, freeT4 -- yet
when people with "normal" TSHs on T4 only supplementation
are measured, usually freeT4 is out of range HIGH and freeT3
is out of range LOW -- and yet, the medical community still
practices and teaches that measuring TSH only to monitor
thyroid replacement is adequate -- who cares where the
thermostat is, if the active hormones are OUT OF RANGE

3) MDs tend to prescribe T4 only.  T3 is harder to work with,
and it converts anyway, so clearly patients don't need T3 and
then I (the doctor) don't have to bother  . . .  $#%^&
a) the intact gland produces both T4 and T3, so why assume T4 only is
enough?
b) DECADES of patients have reported feeling better on combos
than T4 only
c) the system is broken, why believe conversion works properly?
d) finally, the Feb 1999 NEJM article showed clear need for T3
in double blind studies
T4 only is NOT enough, this fallacy should never have been accepted,
it has been demolished scientifically -- and yet the practice of
medicine has NOT changed on this score  (more #$%^&*)

You and 99 & 44/100's of the MDs out there.  It is a complicated
and confusing system, for which certain areas are not well known,
and some are just still unknown.  But, MDs are taught in med. school
that it is simple to fix by adding T4 only and monitoring TSH only.
Even though neither of those is defensible by scientific standards.
<arrgh>

There is lots more, and I don't know all of it -- keep asking.
--
Kevin G. Rhoads, Ph.D. (The Cheshire Cat for official Internet mascot.)
kgrhoads@NO_SPAM.alum.mit.edu