You should not exceed 400mcg (that's MICROGRAMS) per day, including what you get in your diet naturally.
The minimum daily requirement is 55mcg.
The maximum suggested for safety concerns is 400mcg per day, total.
More than 400mcg a day leaves you at risk for developping Selenosis. There is a thread on Dr. Lupo's thyroid board about this, and I am assuming that is where you read about the upper limit being 1,000mcg.
Studies that have been conducted have always used 200mcg, and that is what doctors suggest. You could probably take 300mcg a day and still be safe, but you have to keep in mind you are getting at least 55mcg in your diet already.
Very few people in the U.S. have selenium deficiency, because they get enough in their diet. So figure you are already getting 100mcg to be on the safe side?
Selenium MAY lower your TPOab count, but it only works on TPOab, as far as the studies have shown. You need chromium to help the Selenium do it's thing, as well as zinc and copper. So it's a little complicated.
Here is what has worked for me. I am not recommending you do this, but it is what I am doing.
My doctor has me taking two multivitamins a day, because I am not absorbing nutrients well. He doesn't know what's in them, he just told me to buy some and take two a day.
The ones I am taking have 110mcg of Selenium in each tablet, along with the chromium, zinc, and copper to go with it. So I'm taking 220mcg of Selenium a day as well as what I get in my diet. I'm not sure if that will be good for you, or if you will load up on heavy metals you don't need.
If nothing else, it is a reminder to check your vitamins before you start taking additional Selenium. You may be getting some in your vitamins.
Bottom line, don't take more than 400mcg per day TOTAL.