Both the Mayo Clinic and WebMD are wrong. To put 2,000 IU into perspective, when you go out and expose most of your body to the sun for 30 minutes your skin produces anywhere from 10,000 IU to 50,000 IU. The body evolved this process for a reason, and anything less than 4,000 IU per day is probably not enough.
The average man uses 5,000 IU every day and this increases when fighting illness or chronic disease.[1] There are zero reported cases of vitamin D toxicity on doses less than 40,000 IU per day [2]. I'm not saying go out and take 40,000 IU every day, but 5,000 - 10,000 IU per day is right in the range (you should be on the high end of this if your at high-risk for deficiency i.e. if you are black, obese, work indoors everyday and don't see the noon-sun much, always wear sunscreen when going outside, or live far from the equator)
This is from memory, I don't have my sources right now, but most of what I've read is that the body can deal very well with too much Vitamin D. What it doesn't do well is a deficiency.
A few of my pharmacist friends told me about how some people are being prescribed doses of 50,000 UI, and I've read somewhere that below 14,000 UI a day over extended periods, no toxicity was found. So I kind of doubt that 2,000 UI is an upper bound, unless you have a very specific condition. Just spending a day on the beach probably gives you a MUCH higher dose than this.
But as I said, this is from memory. Take this with a grain of salt and do your own research. Personally, I'll keep taking my 4,000 UI/day, something that I've been doing for about 4-5 years (and while this is anecdotal, I've seen improvements in seasonal mood changes and the number of times I'm sick per year).
The 50,000 IU dose is a single dose, not a daily dose to be taken over some period of time. The half-life of vitamin D in the body is a few months, so a large single dose is effective for some time.
I'm not sure if I wrote it like that because I used to writing the acronym for "user interface" or because in French, my native language, they write it as "UI" (unités internationales). But thanks for the correction.
I assume these are both result from the fact that "The U.S. Dietary Reference Intake Tolerable Upper Intake Level (upper limit) of vitamin D for children and adults is set at 50 micrograms/day (2,000 IU)," the U.S. Dietary Reference being a product of the USDA.
There's a long history of government medical pronouncements being mistaken or outdated (e.g. encouraging grains over fat consumption), perhaps due to the incentives under which regulatory bodies operate - there's little disincentive for being suboptimal or even mistaken, as in the case of fat.
The incidences I've found of Vitamin D toxicity have been found at far higher levels, 40-50,000 IU.
For reference a toxic aspirin dose is roughly 30x a standard dose.
