$6M question

[Guest guest]

Mo,

You wrote:

>

> ...So the pituitary responds to what is in the bloodstream rather than

> what is getting into the cells?

> This is the $6,000,000 question....

The pituitary has no way to know what is going on in distant cells. It

responds to thyroxins, mainly FT3, in the blood. Check or money order

would be fine.

Here is an additional 2 cents worth:

Technically, " getting into the cells " is a misnomer, since transport

across the cell membrane is virtually guaranteed for the free fractions

of T4 and T3. The enzyme bound T4 and T3 cannot get in, so that is the

state for " storage " in the blood. If you do not have a binding or

conversion problem, then the T3 in the blood should get into the cells.

However, there are two things that can go wrong inside the cells. One is

called thyroid resistance. It means there are fewer than normal T3

receptors. This is a rare condition for which the treatment is to simply

provide extra hormone, usually T3, thus suppressing the TSH.

The other is that too much of the T4 is converted into RT3 instead of

T3. RT3 is also transported readily into the cells, where it blocks

those same receptors, preventing T3 from getting in and triggering the

desired metabolic effects. This is also pretty rare, but there is a

reliable blood test for RT3, if this condition is suspected. The

treatment is to supply more T3, sometimes nothing but T3, which also

suppresses the TSH.

Chuck