"According to a previous thread, the toxins are actually excreted through the Stool, not the urine. If that's the case, then how is it possible for people to test Mercury and other heavy metal levels in the urine?"
I've also been wondering about this but haven't bothered digging around yet to find out why. I've been pondering over it for the past few days though and have come to my own hypothesis/assumption/guesstimation...
I think what we, in our intellects, consider "toxic", aren't exactly the kind of toxins the liver removes. I think the toxins the liver removes are those that are more like poisons that would be more of an immediate danger if allowed to circulate through the blood. Other things that we consider toxic, due to bad effects that become apparent after long-term exposure, may not be the more immediately urgent "poisons" that the liver sees as toxins.
But the difference between the kidney vs the liver, is that, the "bad" stuff that's allowed through by the liver and on into the kidneys and in turn out through the urine, WE HAVE CONTROL OVER, because it's a direct result of the things we chose to put in our bodies. So if you're worried about "bad" stuff passing through the urine, don't put that stuff in your body to begin with -- drugs, alcohol, carbonated/syrupy drinks, metals, acidic foods, processed foods filled with a billion different synthetic chemicals for "taste", bad oils (lard, canola oil, etc.), processed sugars, fake sugars, etc. etc. etc. This is why proper diet seems to go hand in hand with UT practice... because, while urine may be sterile, and while the more immediately dangerous, poisonous toxins may be filtered by the liver, there are still "bad" things that will be present in the urine if those "bad" things are present in your diet.
However, even in the most healthy of diets, toxins and waste will still be produced by the body as it utilizes and converts various elements of foods and materials, and THAT, I think, is what the liver is meant to filter out -- those INEVITABLE toxins and wastes that need to be eliminated REGARDLESS of what your diet is like.
So basically the liver makes it so you don't have to worry about urine being "poisonous", but various other elements will still pass through that we might not deem "good". But those elements are only present if you ingested them in the first place. So if you are really worried about ingesting them through urine, you wouldn't have eaten them in the first place! ... what difference does it make whether they enter the body through urine, or through the form of a big mac?
Again, this is all just my hypothesis or assumption and I really haven't done any research on it yet, so take it with a grain of salt. Just stuff to think about!