 Once T3 and T4 are dumped into the blood, they can go off and have an effect on target cells. T3 is the active version of the thyroid hormone. This is the hormone that does something. However, only 7% of thyroid hormone that's produced is T3. 93% is T4. So there's a couple of things that happen. First of all, T4 naturally degrades over time slowly into T3. Secondly, sometimes target tissues have an enzyme that will break off an iodine and make a T4 into a T3. So the question is, what exactly happens once T3 or 4 gets made into T3? What happens when it gets into your cell? Well, the mechanism of a thyroid hormone is, number one, it diffuses in on its own to the cell, into a target cell. Then it binds to a receptor, that says receptor, on DNA. And if it binds to a receptor on DNA, what do you think is going to happen? Increase protein synthesis. That says protein synthesis. If you increase protein synthesis, what is that going to do? Well, usually that results in increased metabolism and growth. And that works for you, doesn't it? If we're making new proteins, we're going to be doing something. And it's probably going to cost energy to do that. So metabolism, the more thyroid hormone you have, the higher your metabolic rate, the more you grow, the more active you are. And so you can imagine that low thyroid hormone means you start to feel kind of sluggish, your metabolic rate goes down, you might gain weight, you might be cold all the time because your body isn't going to have these metabolic reactions as quickly. Thyroid hormone, you can survive without it, but it is a seriously, like, heavy hitter. When it comes into the mix, it's not messing around. So let's look at how, let's remind ourselves of how thyroid hormone is regulated in the body.