Learning About...the Thyroid Hormones: The Basics
What Are the Thyroid Hormones
Anyway? 
There are two chief thyroid
hormones, 
and they are T-4 (thyroxine) and T-3 (triiodothyronine). They are
tyrosine-based hormones produced by the thyroid when stimulated by TSH (Thyroid
Stimulating Hormone). As indicated by the chemical name of T-3, an important
component in the creation of these hormones is iodine.
In normal body function, once the
hormones are within the cells of the body, enzymes called deiodinases convert
T-4 to T-3, the more active of the two hormones. Decarboxlyation and
deiodination further process the T-3 into iodothyronamine (T1a) and thyronamine
(T0a).
"Round, round, get
around..." 
The thyroid hormone gets circulated
throughout the bloodstream by being bound to transport proteins. 
Therefore,
very little of T-4 or T-3 is free or unbound (around .03 to .3%), and thus the
measurement of free T-3/T-4 hormone is of large diagnostic worth to the medical
community. 
For this reason, measuring
total thyroxine in the blood can be, and often is, misleading, though the
diagnostic tool of detecting TSH is critical for checking hormone levels.
There are three different proteins
to which the thyroid hormone is usually bound. 
First there is thyroxine-binding
globulin, the protein with the most receptors for the hormone. The hormone is
usually bound to this protein at a 70% ratio. Then there is transthyretin, the
second-strongest binder for T-4/T-3, and it is present in the cerebro-spinal
fluid. The carrying percentage for transthyretin to hold T-4 is around 10-15%.
Lastly, there is albumin, the most common and present plasma protein in
mammalian bodies. It has the fewest receptors for thyroid hormone, therefore it
is the weakest bond, though its carrying percentage for T-4 is at 15-20%.
T-3 and T-4 then cross the cell membranes, via amino acid importins. They then function via a set of thyroid hormone receptors in the cell nucleus. T1a and T0a, the product of the processing of T-3 and T-4, are positively charged and cannot cross the cell membrane. They are thought to function via an amine-associated receptor called TAAR1, also known as a G-protein-coupled receptor in the cell membrane.
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That's all for this week...I'll be back next week with more adventures from the Thyroid Zone! 
But I also understand that some people might prefer medical jargon, simply because that's what they're used to.
Blessings and Good Health,
Kat ^.^
