I think is it time for USA to learn the metric system, i.e. what the rest of the world is using. Why so? Because it is easy.
1 milli = 1000 micro (1mX = 1000uX)
Replace X by meter, grams, Ohms, Newton, or whatever unit you need to measure.
can you please exlain to me what is the mIU symbols mean are miligramms?
It depends on what substance. Some international body agrees on a unit of biological activity, and then determines the mass to reach that unit. Sometimes different compounds act in similar manner, but the amount of substance to produce i unit of activity differs for the different forms (vitamin A is an example)
But there is no universal conversion; you have to look it up in a reference.