Syringes are never graduated in milligrams (mg), they're graduated in fluid volume (cc/ml, which are the same thing). Insulin syringes are graduated by International Units (IU) but don't let this confuse you, it's simple. All insulin syringes I've seen are for U100 insulin, which is 100 IU per milliliter. Thus, 50 units on the syringe equals .5 milliliter (ml) or .5cc of fluid. You could also say the syringe is graduated in hundredths of a milliliter (.01ML). Make sure you verify this, however, as there may be syringes made that are graduated for different potencies of insulin (it will say on the package and sometimes on the syringe barrel itself). But most are U100.
You need to know the strength of your medication in order to determine the dose. It will state this on your vial. It's normally 200 or 250 mg per ml/cc. So if your T is dosed at 200mg/ml, and you need to inject 100mg, you need to draw half a cc or 50 units on your U100 insulin syringe. If you need to inject 50mg, you draw a quarter-cc or 25 units. Make sense?
If you're talking about HCG, the potency is going to depend on how you reconstitute it. I can break this down too if you need.