Except Steiner's origins aren't in theosophy (if you mean H.P. Blavatsky brand of theosophy). He found that theosophists were the most likely to be interested in what he had to say, so they were kind of a built-in audience, and he often spoke to the German Theosophical Society, but was eventually expelled from it (largely due to his insistence on focusing on the central role of Christ & Christianity).
Probably the best way to describe the difference between Rudolf Steiner (Anthroposophy) and Blavatsky (Theosophy) is:
- Blavatsky was teaching Buddhism without the trappings.
- Steiner was teaching Christianity without the trappings.