Thyroid Hormone ABC’s – Let’s Start at the Beginning

A nice, normal, healthy thyroid gland has a big role to play in the body. This tiny gland located near your Adam’s apple in your neck is a powerhouse that creates homeostasis through its release and conversion of thyroid hormones. In a complex interaction between the brain, the body, the thyroid gland, and specific enzymes, a perfect system is maintained. This normal state is called “euthyroid”.

The system exists in a feedback loop, with no start or end point. Each action and interaction creates another action and interaction. The body needs to maintain adequate levels of thyroid hormones to function properly. There are two main hormones it uses to do this: T4 and T3. T4’s more complicated name is thyroxine and T3’s is triiodothyronine. There are both made up of iodine, and T4 has – you guessed it – 4 atoms of iodine and T3 has – yep, 3 atoms of iodine.

More about T3 and T4

Now hormones like to make things complicated, and each person’s body likes a different level of these hormones. More than that, actually, each person’s body needs a different level of these hormones. That makes thyroid gland loss a very convoluted issue, and replacement of thyroid hormones by way of medication much more challenging than it was previously believed to be.

The thyroid gland produces and releases mostly T4, and it also produces and releases a small amount of T3. The biologically active hormone, which means it works in the cells of the body to simply make everything work, is T3. Your liver, your heart, your muscles, gut…you get the picture. The only reason you can poop – thyroid hormones. The only reason you can breathe – thyroid hormones. Without them your organs would not work. Nor would your muscles or your brain. So why the heck would the thyroid gland produce mostly T4 if T3 is the “big cheese” of thyroid hormones?

Because the system is perfect. The gland releases a lot of T4, kind of like when you buy a bunch of groceries but you don’t plan to eat them all at once. Why do you stockpile them? So you can use them when you need them, of course. So the body has all this T4 in the blood, and the organs and tissues of the body can change this T4 into the T3 it needs by magically removing one of the iodine atoms and now it’s T3! How does it do that, you ask?

A Little Help from Some Enzymes

We have enzymes in our bodies called deiodinase enzymes. They take away one of those iodine atoms and turn the T4 into T3. They do this on demand when it’s needed. Kind of like an on demand hot water heater. You have all this T4 in your body (water in your pipes), and when you need active thyroid hormone, the deiodinase enzymes turn it into T3 (like when you turn on the hot water tap and the heating element provides you instant hot water). You wouldn’t want all your water to be hot all the time. It’s much the same in the body.

So why does the thyroid gland produce any T3 at all? Why wouldn’t it just produce all T4 and the “on demand system” could do the rest? That’s a great question that we just don’t have all the answers to, of course. Our body system is built of years upon years of adaptation and for some reason, those deiodinase enzymes, as incredible as they are, just can’t do the whole job themselves. They are incapable of producing the right amount of T3 from T4 on their own, so the thyroid gland itself makes a contribution.

Where the heck does that leave us? The thyroidless, the people without a thyroid gland?? Until now, I don’t think anyone gave it much thought. The times they are a’changing! It was believed that providing T4 to a person without a thyroid was enough, and that the body would make the T3 it needed. Yes, but…and the big but, of course, is the inability of these enzymes to do the complete job. Hence, combination thyroid therapy with both T4 and T3.

So now what? My body has all this T4 swimming around, and then it’s converting it into T3, and how does the system know when enough is enough? As I said, you wouldn’t want your system to be working in overdrive (having the hot water on full blast). The hypothalamus in the brain is capable of sensing the levels of thyroid hormones in the body and this monitoring of those levels causes the hypothalamus to request an increase in those hormones when they run low. The hypothalamus does this by releasing another hormone, TRH (thyroid releasing hormone), and instead of sending this directly to the thyroid gland, for some reason, it sends it to another part of the brain called the pituitary gland.

TSH – How Does it Fit In?

Why doesn’t the hypothalamus send the message directly to the thyroid gland and by-pass the “middle man”? We don’t know. Maybe the hypothalamus doesn’t speak the language of the thyroid and the pituitary does. Nobody knows. Anyway, it sends off its message to increase thyroid hormone and the pituitary talks to the thyroid gland by using yet another hormone, TSH (thyroid stimulating hormone) to communicate an increase is needed. When there is a higher amount of TSH in the body, it is stimulating the thyroid gland (talking loudly) because there are not enough thyroid hormones in the system. Once the thyroid has done its job of releasing thyroid hormones, the TSH lowers (quiets its voice).

The measurement of TSH is used in thyroid replacement therapy as a marker, or measurement of sufficiency. It is frequently used as the only measurement. That can be problematic for patients; particularly those of us without a gland because we don’t respond to the “feed forward effect” of TSH. We are not stimulated to produce or convert thyroid hormone in response to TSH. The system is designed for perfection, and there is an assumption on the part of the hypothalamus and the pituitary that everything happening downstream is as it should be. That all systems are a “go”. Those guys don’t even know their buddy, the thyroid gland, isn’t in attendance!

Sometimes, actually all too frequently for us, the thyroidless, the TSH doesn’t tell the story of what’s happening in our cells. We are usually treated with T4 only thyroid hormone (Synthroid, Eltroxin, etc) and the body is left to do whatever conversion it can from T4 to T3 without that “top up” of T3 directly from the thyroid gland. Remember I told you the deiodinases couldn’t do the whole job on their own? Well, they can’t, but the system has plenty of T4 in it and since the hypothalamus and pituitary don’t know their friend has flown the coop, they think all is well so the TSH lowers and you’re left with not enough T3 in your body. Some patients have more trouble with that than others and nobody knows why. That’s how me and Kathy got here. We are two of those patients.

I think you get it – in a perfectly functional system, this whole thing is pretty amazing. I was going to say “kicks ass” and then I thought, “should I really say that?”, so I censored myself and then added it anyway. The system kicks ass when it’s working well. Unfortunately for me, and likely you, something in the system has changed because we’ve lost our glands to the knife or to RAI, or to atrophy from any origin.

Stay tuned for more ABC’s of Thyroid to come.