All of you who read my emails who are also my clients know that I urge everyone to get a blood test of your Vit D level before starting Vit D or at least after you've taken it for about 3 mo.  It's rare to accumulate too much by taking 4000 - 5000 i.u.  per day, but there are always exceptions.
 
Vit D is actually a pro-hormone, not a vitamin.  It interacts with just about every system in the body, which is why it's so valuable for everything, including and especially the immune system.  We all know about its value in promoting healthy bone structure and remember that ricketts was a common ailment for children in northern climates where there was little sunshine most of the year.  At one point, governments around the world advised the food processing companies to add 400 iu to milk and other processed, boxed foods to avoid this bone deforming disease. (you can't get it plants or animal protein)  We now know that 400iu daily is a pathetically low level of supplementation and that avoiding ricketts isn't all Vit D is good for.  Low Vit D is common to all immune disorders, everything from getting frequent colds and flu to major illnesses like MS.
 
When people have their blood levels tested, more than 80%of us have suboptimal levels. We like to strive for levels of 50-80 and we rarely see someone reach a level of 100, but at levels of over 100 we can begin to see problems, though not all of the time.  One syndrome that can begin to occur if levels are too high is atrial fibrulation.  If you already have a diagnosis of A-fib, it doesn't mean that you should try to achieve low levels of Vit D!  This can get confusing, I realize.  But if you achieve levels of over 100 and then develop A-fib, you need to drop your levels back down to 80 or so.  Another possible issue with levels of Vit D over 100 is that calcium levels can get too high and then calcium can deposit in soft tissue - not a good thing.  I think people should know there is a potential downside of oversupplemeting any single nutrient or vitamin.
 
These conditions are very rare and never seen in levels under 100, (according to the study with a large sample size that I read)  but it is still a pro-hormone vitamin acting like a pharmaceutical so it's wise to know your level before taking Vit D, and then check it every six months afterwards until it has stabilized. Keeping your levels in the high-normal range is the idea.  That's when you have the best immune function.  Once you achieve that level, you can probably take your Vit D just 3-4 times a week, then check it annually.  You might find you need less in the summer months if you're prone to sporting a bikini for a few minutes during mid-day everyday....  most of us would rather take a tiny pill.....
 
Typically people can't get their Vit D level to an optimal range by taking just 1000 iu daily.  It generally takes 4000 - 5000 iu daily to cause movement toward upper normal levels.
 
I know, staying healthy and staying abreast of all of the research takes some effort on each of our parts.  But I do believe it's worth it.  The more we know about the "easy things to check" like Vit D levels, cholesterol/HDL ratio, red blood cell magnesium, blood glucose and Hemo A1C, C-Reactive protein inflammation marker, the easier it is to make modest modifications in our diet/lifestyle to achieve optimal health.
 
Cheers and enjoy the festivities of this low-sunshine time of year!
mary