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Thyroid Adventures..Learning about the Thyroid--.Part I: The Basics
7 years ago
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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.


That's all for this week...I'll be back next week with more adventures from the Thyroid Zone!


7 years ago
For the purist: T4 is also known as tetraiodothyronine..
Re: T4
7 years ago
John, thank you for helps to know the proper names of things sometimes...though I am attempting to post these ideas in ways that non-medical people can understand them.

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 ^.^