Johny, this is the problem.........international units measure mass, not volume and are different according to the substance in question. Inulin iu's are far different than GH iu's or Human Chorionic Gonadotropin (HCG) iu's or b12 iu's. An international committee of scientists determines the "expected" mass per iu of many drugs.
Insulin syringes just happen to be standardized to convert insulin iu's to cc/ml's. As you know, this doesn't apply to , say, GH. You have to do a conversion.
It's the same thing with HCG......you will end up with 5000iu's in mass no matter how much dilutent you use. I would use 2 or 3 cc's sterile water with that and make up 2 injections, although it's ok to do all 5000 in one. I generally try to get in 20 to 25,000 iu's over the course of 2 weeks, divided into equal injections every 2 or 3 days.
Think of Saizen vs Serostim. Made by the same company, exact same GH. But Serostim dilutes 18iu in one cc/ml, while Saizen dilutes .75iu in one cc/ml.