Thyroid Hormone Physiology Primer

Many of you report to my office with a complaint of being hypothyroid only to have been told by your primary care provider that you are normal on testing. Guess what? They are correct!! But, so are you. However, they have not gone far enough to document what you know to be true. Your thyroid gland most of the time is producing adequate amounts of thyroid. However, after it leaves the gland, the brain takes over and it responds to environmental cues, sleep, stress, exercise, etc. This response changes the activity of enzymes that control the conversion of your thyroid hormone to its more active form known as T3. This has to be assessed to adequately determine your true thyroid activity. I have included a feedback chart below of this physiology. The following is a review of the thyroid axis: HYPOTHALAMUS-> TRH(thyrotropin releasing hormone-this is decreased in times of stress) -->PITUITARY ---------> TSH(thyroid stimulating hormone - also decreased under times of stress) -->THYROID (the gland produces 4 types of thyroid hormone, T1-4- 90% of output as T4, 10% of output as T3. T2 andT1 are < 1% of output). T3 is 5 times more potent than T4. As such 50-80% of T4 is converted to its more potent counterpart, T3. The conversion occurs via the activity of enzymes called DEIODINASES. They are known as the D1-3 enzymes. This is how the brain controls thyroid activity and why in times of stress, the activity of the thyroid is reduced. D1 and D2 control the conversion in muscle and nerve. When stress occurs, however, the D3 enzyme is activated converting T4--> rT3- reverseT3. Reverse T3 is not able to bind the thyroid receptor and acts as a "thyroid brake" limiting the metabolic effects of thyroid. The primary stress hormone that controls all of this is CRH(corticotropin releasing hormone). This is also released by the hypothalamus in the brain. It stimulates the release of ACTH(adrenocorticotropin hormone). It stimulates the adrenal gland to release adrenaline and cortisone, the primary stress hormones. When CRHis released, TRH and subsequently TSH are reduced. Think of it this way. The thyroid's main function is to set the metabolic rate and oxygen consumption in the body. When the thyroid is reduced, fatigue, body temperature decline, weight management problems, constipation, hair loss, etc. ensue. This happens under stress because the brain thinks it must prioritize fueling to supply the stress response that may entail running or fighting to escape danger. Reducing metabolic rate by reducing thyroid activity will help the body maintain adequate fuel for the stress response/escape. When your primary care providers assess your thyroid, they generally only measure TSH and perhaps T4. They do not look at the rest of the chemistry. So, when they tell you your thyroid is producing normally, technically they are correct, but they are not accounting for the environmental influences that we all endure these days. Not to mention, there are micronutrient issues/deficiencies in our food supply that deny us adequate micronutrient support that supports the normal conversion of thyroid. Without assessing T3 and rT3 levels, there is no acknowledgment of environment / nutritional issues and the source of hypothyroid symptoms may be missed. But, here is the KEY POINT! If your brain is not allowing conversion to T3 and shunting to rT3 instead, it is doing so as compensation to your lifestyle and stressors in an effort to protect you. In many cases, trying to supply T3 to compensate for altered conversion will not solve the problem as the brain will see this as going against its programming. In my experience, most of the time, some improvement in this chemistry can be obtained. However, you may not be able to achieve the complete rebalancing without some homage being paid to your stressors. Sorry, this is just the reality of how this works. This is a very complex chemistry that demands a more thorough evaluation when symptoms of low activity present. More and more physicians are starting to look at this more in depth. Hopefully, this primer will educate you a bit on what you should be looking for and asking your doctor about. You may not get a favorable response to your inquiry, unfortunately. Take heart, there are more of us out there that will listen. Hope this helps..... My name is Courtney Ridley, MD FACOG. I am a board-certified obstetrician gynecologist who also incorporates functional medicine into my practice. Functional medicine approaches are starting to make inroads into mainstream practice, but, there is still work to be done. This is the reason for this and subsequent submissions. My website is: Article Source:

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