Vitamins A, D, E, K are fat soluble vitamins and get stored not pissed out. It builds up if you OD and can actually be toxic. It’s hard to get toxic with D, you need to take a ton of it, but for some people it can cause their calcium to go up, or it also is a concern if the person has a parathyroid problem.
Vitamin A is a real thing to worry about, especially for women. Acutane, the acne med, is like a huge megadose of A, and women need to sign forms (unless that has changed) stating they understand it causes birth defects, and doctors prescribe women BC pills while they take it (I think doctors have to by law prescribe BC pills, or put in an IUD, or some sort of reliable method). If a woman becomes pregnant while taking a urbane she is counseled to terminate. Megadoses of regular vitamin A a woman would also be told it might have caused fetal damage. A also has been linked to some bowel diseases. Basically, I’m just saying not all vitamins just pee out.
Minerals, like calcium, iron, magnesium, also don’t just pee out.
Your calcium probably has been tested many times, because it’s included in a panel that is usually done if a patient is tired, or not feeling well in general.
If you take calcium, don’t take a lot. Possibly, since you are very thin, some extra calcium might be a good idea, but if I were you I wouldn’t take large doses. If you have some at home you can take it a few days a week, and that’s basically the same as a lower amount every day.