Reading an American website in the UK? Or reading a European study in the US? It's a PITA! If you're looking to put values into the system you're used to, here's about the best list I've ever come across.
Clinical Data Unit Converter
I found this looking to convert insulin from µU/mL to pmol/L. The conversion factor is 6.945.
So: 10 µU/mL = 10 * 6.945 = 69.45 pmol/L
Going the other way just divide.
So 100 pmol/L = 100 / 6.945 = 14.4 µU/mL