It is definitely possible to take too much vitamin D, just as it is possible to take too much of anything (even drinking too much water can be toxic!). That being said, it is very difficult to reach toxic levels of vitamin D. The Institute of Medicine has set its no-observed-adverse-effect-level (NOAEL) for vitamin D to 10,000 IU per day. Their Upper Intake Level for adults and pregnant women is 4000 IU per day.
1000 IU of vitamin D is well below that recognized level, however, it actually may not be enough for you. The recommended serum level range, per our Scientists’ Call to Action, is 40-60 ng/ml (100-150 nmol/L) for best overall health. The only way to know if you are within that level is to have your 25-hydroxy vitamin D serum level measured. Once you know your level, you can determine if supplementation is needed and how much. This is where our supplementation charts come in handy: http://www.grassrootshealth.net/media/download/poc-supplementation-chart-range-150lbs.pdf
If you still have concerns about toxicity, consider the following information:
– Vitamin D related toxicity has been observed at levels of 200 ng/ml (500 nmol/L) and higher per J. Hathcock, Am J. Clin Nutr. 2007;85:6-18. To get to that serum level is very difficult, and would take massive, daily doses of vitamin D (such as 100,000 IU per day).