Really, best course of action is not to guess and just get blood work done regularly.

It builds up slowly enough that your doc may suggest mega-dosing for a few weeks to bring things back in line.

Pure anectdata on the auto-immmune thing: since getting my vitamin D levels up, my alopecia barbae has completely disappeared. It's pretty crazy cause it's something I've had my entire life (and very severe at times). My doc mentioned a link between auto-immune and vitamin D, and after bringing my levels back in line, all the patches filled back in. Pretty wild, tbh.

i was once accidentally taking 25.000 IU daily for a few months instead of 5.000 (400-600 IU being official recommended dosages, although many sources claim that that's way to little, hence the 5.000). went to an endocrinologist for really weird bouts of whole body weakness and exhaustion after even light activity. nothing like i had ever experienced. lab results came back with a vitamin d level of the maximum measurement level (so probably more, maybe a lot more). that's when i realized i was taking 5 times more than i had planned. i looked up vitamin d toxicity and found fatigue and muscle weakness as symptoms and had the answer to my condition. i don't want to know what would have happened if i hadn't looked it up myself, because the doctor didn't even want to see me again, claiming by letter that my blood work was just fine. so yeah, pay attention to your supplement regime.

There was a good discussion a ~16 months ago (and the author even showed-up to comment) on an MD's article re: vitamin D that discusses toxicity: https://news.ycombinator.com/item?id=24138590

It's not just vitamin D toxicity, it's also Calcium toxicity.

For example, vitamin D3 (Cholecalciferol) increases Calcium absorption by 5-10x fold.

Besides, Calcium (surprisingly) are toxic for human bodies as well, in excessive amounts, and accumulates with age in body tissues.

Btw, vitamin D3 (Cholecalciferol) are commonly used as rat poison.

Wow, are there more medicine/rat poison, apart from warfarin?

Yeah, but you have to be basically gobbling these pills to get anywhere near the dangerous levels. People dumb enough to do that are probably making all sorts of other poor decisions about their health and life.