So far, we have learned that thyroid gland makes hormones T3 and T4, these hormones regulate the metabolism and energy production in all the cells of the body. And ideally, thyroid gland produces just the right amount of these hormones.
The question is, how does the thyroid gland know how many hormones to make? It turns out that there is a very sophisticated system designed to regulate the production and distribution of the thyroid hormones.
First, the thyroid has a direct boss that issues instructions and orders.
This boss is another gland called pituitary. It is located inside the brain. The pituitary gland is the “boss” of quite a few endocrine glands, including adrenal glands, testicles, ovaries, and some others. It regulates them by producing special hormones that stimulate their activity and makes them produce more of their products.
The pituitary gland regulates the thyroid by making a hormone called TSH or Thyroid Stimulating Hormone. TSH is also called thytrophin. In other words, TSH and thytrophin are the same thing. TSH travels from the pituitary to the thyroid gland and stimulates all the phases of the hormone production. TSH makes the thyroid gland produce more T3 and T4. The more TSH, the more thyroid hormones are produced. The less TSH, the fewer thyroid hormones are made.
To complicate matters, it turns out that the pituitary gland is not the ultimate boss of the thyroid. There is another part of the brain called hypothalamus and it produces a hormone that controls the pituitary gland. The name of this hormone is TRH, thytrophin releasing hormone. It increases the production of TSH, which stimulates the thyroid and increases T3 and T4.
So far, pretty simple. Hypothalamus controls the pituitary, which in turn controls the thyroid gland. So ultimately, it’s the brain that decided how much T3 and T4 should be produced.
Then, there is another regulating mechanism that is called negative feedback. The levels of T3 and T4 have an effect on TSH level. If the level of thyroid hormones is high, TSH production goes down. And if the level of T3 and T4 is low, the production of TSH will go up, to stimulate more thyroid activity. In other words, they move in the opposite directions. High TSH level is one of the tools for diagnosis of hypothyroidism.
Ultimately, the thyroid hormone production is controlled by the brain, by TRH and TSH and by the negative feedback mechanism.
Thyroid gland produces T4 and T3 in 4 to 1 ratio, about 80% T4 and 20% T3. The question is, why are there 2 thyroid hormones? The answer is that only one of them is really a hormone. That’s T3. T4 is a precursor from which T3 is made. It’s a ready source for creating T3 when the body needs it.
What happens to the thyroid hormones after they are made by the thyroid gland? Most of them remain inside the thyroid. At any given time, the thyroid gland contains a month-worth or more of the thyroid hormones, ready to be released into the blood when needed.
A small fracture of the hormones is not stored inside the gland. They are released directly into the blood. Almost all the T3 and T4 that go into the blood are attached to special proteins. There are several such proteins, and they can be measured by a blood test. We don’t normally check them, only in some situations. About 99% of T3 and T4 in the blood are attached to proteins, so they are called “bound”. Proteins with the hormones attached are carried by the blood throughout the body.
Only about 1% of the thyroid hormones in the blood is not attached to proteins. They are called “free” hormones, free T4 and free T3. Out of the two, it is free T3 that is active and does all the work.
When the body needs more free T3, it makes it from free T4 with the help of an enzyme called de-iodinase, which removes one of the iodines from T4 and makes it T3.
Only the free hormones can work, only they can go inside the cells, so eventually their level in the blood goes down.
That’s where “bound” hormones come in. When the level of free hormones drops, some of the hormones that are bound to proteins are released and become free. At this point, some hormones that are stored inside the thyroid gland are released to replace those that were given up by the proteins. And new hormones are made by the thyroid to replace those that leave the gland.
To put it another way, thyroid hormones are so important, that the body produces more than it needs at any given moment to make sure that there is enough when more is needed. That’s why there is this sophisticated system of regulation that tightly controls the production, release and storage of thyroid hormones.
The brain produces not one but two different hormones to tell the thyroid what to do and how many thyroid hormones to produce, how many to store inside, how many to release into the blood. The body makes sure that there are free hormones available for Immediate action and there is a large supply of thyroid hormones in the blood, bound to proteins but available to be released as needed.
Plus, there is a negative feedback mechanism, which lowers TSH when there is enough T4 and T3, and raises TSH when thyroid hormone are low.
That’s why, when things are working properly, there are plenty of thyroid hormones in various stages of readiness. But if there are not enough, we run into problems.
In this lecture we have learned how thyroid hormones are regulated, how they are distributed and how they are designed to be plentiful and readily available. We also learned that it’s free T3 that is the actual active hormone, that there are hormones that are bound and those that are free.
In the next lecture we’ll talk about converting T4 into T3, how it is done, what is needed for that to happen, what can improve this process and what can make it worse, which can lead to low thyroid.
If you know someone who might be interested in low thyroid, please refer them to LowThyroidDoctor.com so they can download this ebook. Also, if you have questions or comments, please email to DrTeplisky@gmail.com.
As an additional bonus, you can get a free, no obligation 30 min phone consultation with me to see if I can help you figure out if you have low thyroid. Just book it online or call my office at 718-769-0997.