There's never been a canon detailed listing afaik. Hell, I don't think there's even been a partial listing of components beyond "vita rays" activating it.
That being said, I don't think steroids would have done the job. If something like that was part of the formula, than it would be there in the form of a corticosteroid rather than something anabolic. Something to ease the difficulty of rapid mass gain and bone growth.
But, other than the obvious heightening of his physical self, it had a neurological effect too!
While not explicitly stated afaik, the comic origin has him immediately able to use his full capabilities. He didn't have to learn how to control that new body. He had increased proprioception to go along with the changes. Improved reflexes, improved balance. That's not stuff steroids touch.
I suspect, that if the serum was quantified, it would be some kind of cocktail, or be something that directly edited DNA, with the Vita-rays serving to both activate and accelerate those changes via the same kind of activity radiation causes.