Thyroid 101

Thyroid masses, thyroid enlargement, and disorders of function are topics I address every day in the office. There are many sources of information available on thyroid; most of what I will address will be topics I address with patients. Normal thyroid function will be addressed here.

The thyroid is an endocrine gland, meaning it makes hormones that are first stored within the gland, then released to circulate to the body tissues by way of the bloodstream. It sits in the neck, wrapped around the windpipe, and crossing in front just below the cricoid cartilage, better known as Adam’s apple. It is usually about the size and approximate shape of the Honda H symbol on a car. When normal, it is soft and fairly flat, not easy to feel. Normal thyroid hormone production is first stimulated in the brain, which then signals the Pituitary gland (also in the skull right under the brain) to make Thyroid Stimulating Hormone or TSH. TSH then travels to the thyroid gland to stimulate the production of Thyroid hormones. TSH also feeds back to the brain to signal for the production of less (if high) or more (if low) TSH. TSH is the most common blood test done when screening for thyroid disorders.

There are 2 main hormone molecules that together constitute Thyroid Hormone. They are produced by the thyroid gland itself. (These aren’t the only ones but the others are for another post). Tyrosine, an amino acid, and Iodine, a mineral, are the main ingredients that are first made into Thyroxine, or T4. We call it T4 because there are 4 iodine molecules attached to the tyrosine. The molecules of T4 are made while attached to a long chain of protein molecules called thyroglobulin. These protein chains with T4 are organized and stored in the thyroid gland in structures called Follicles. A certain amount of T4 is stored by the thyroid gland; in case of a period when no iodine is available for new manufacture, it can keep releasing T4 for many weeks before running out. So we call T4 or Thyroxine the storage form of the hormone. Most of the hormone found in the bloodstream is still bound by carrier proteins. We can measure T4 in the blood in 2 ways – Total T4, which is a mix of bound and unbound T4, or FreeT4 which is only the non-protein bound T4. Synthetic T4 is also the most common thyroid prescription medication that is used to treat disorders of inadequate thyroid function.

In order to be used by the body, T4 has to be activated; one iodine molecule is removed by an enzyme called 5 prime deiodinase. 5 prime refers to which particular iodine atom is removed in order to make Triiodothyronine or T3. Most of the T3 is actually made within the body tissues though some of it is made in the thyroid gland itself. T3 acts within the cells of the body tissue to activate metabolism. This means it is involved in cells having the energy to move (in the case of muscle cells) or secrete or grow or transmit signals or whatever the function of that particular tissue is. Too much or too little, and the organism (or the person) sickens and eventually dies if the imbalance is too much.

As may be imagined, there are complex feedback loops that try to make sure that there is never too much or too little or to mitigate the effect of such imbalances on the body. For example, if the amount of T4 in the bloodstream gets too high, then the brain slows down signaling to make more TSH, and the amount of both TSH and T4 will fall. Another interesting thing is that too much T4 in the body can also lower the amount of the 5 prime deiodinase enzyme so that less T3 (the active hormone) is made. Excess T4 can also be diverted to make ReverseT3, which is like T3 but with the wrong iodine molecule removed. Instead of activating cells, Reverse T3 actually blocks the action of T3. There are several physiologic states which depend on this action to keep us alive, the most important being Starvation. In this instance, slowing down the metabolism in the face of no available food can literally keep you from starving to death. In the case of extreme illness where slowing or shutting down metabolism gives a better chance at natural recovery. Extreme mental stress can have a similar though less dramatic effect. Of course, this system is even more complex than I am making it seem here and though better understood than it once was, is still not well understood by many doctors who order Thyroid blood tests and prescribe Thyroid medications. 

Some of the body systems and functions controlled or influenced by the thyroid include:

• Body temperature

• Pulse and blood pressure

• Skin elasticity and tone

• Fluid balance 

• Sweat secretion

• Hair growth

• Energy and metabolic function

• Sleep Quality

• Digestion and bowel function

• Sex hormone secretion and balance, ability to conceive and  maintain pregnancy

• Mood and sense of well being
• Immune system function 

So that's it for now.  I'm Dana Gibbs MD. I'm an integrated physician in North Texas. I help people address thyroid and other hormone imbalances. And if you're a thyroid or chronic fatigue suffer in North Texas and you want a caring doctor to help you resolve your exhaustion, joint and muscle aches, poor sleep issues and weight gain, come sign up for a new patient evaluation with me now.

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