Perhaps thoughts is the wrong word, since they as a concept often are associated with words, but the things that go through my mind are concepts, and I often have a word label for them, sometimes it's right there, sometimes it's not. Sometimes I have concepts in my head that have no label, and that I can express, but the concept which I can imagine in a few seconds often requires a couple paragraphs to describe directly.

Another argument for OPs view (with the caveat above) is that these concepts must predate speech. We have an imagination before we have speech, and that obviously doesn't require words.