 So you might not have even ever heard of the parathyroid gland. Parathyroid gland is actually four little distinctive knobs, knobbies, nodules, twangers embedded in the posterior side of the thyroid gland. Thyroid gland can be removed and you can survive, but if the parathyroid glands are removed, done. Game over because calcium homeostasis is so important and calcium homeostasis is monitored by the parathyroid glands. So imagine this. The parathyroid glands have receptors that monitor calcium levels. So there are parathyroid cells that have calcium receptors and they monitor calcium levels. And if calcium levels get low, then the message is, whoa, we need to make something happen. And they release parathyroid hormone, parathyroid hormone, PTH. Parathyroid hormone goes into the system, goes into the blood. Guess that makes sense since we're talking about a hormone here. And guess where they go? Well, I would draw it, but I might as well just tell you it acts on the bones. It's responding to low calcium concentrations, right? So what's it going to do? Who's it going to activate in the bones? Who are you going to call? Osteoclasts. And the osteoclasts are going to increase their activity and calcium concentrations in the blood are going to increase. That's one thing it does. There's something else it does. In fact, I'd like to say there's another thing that it does. It also, okay, it increased osteoclasts. It also increased a hormone called calcitriol. And I hope you're like, dude, what is this calcitriol that you speak of? Would you like me to tell you? Okay, I'll be right back.