This has been one of my bugaboos too. But actually, both -tropin and -trophin are correct (and in common usage), since the releasing hormones have both tropic and trophic effects (the latter referring to releasing hormones promoting the growth of their target anterior pituitary cells). However, I agree with you that -tropic is preferred; most assays for releasing hormones measure their tropic rather than trophic effects. That goes for the -trophs or -tropes of the anterior pituitary as well; for example, gonadotropes is preferred over gonadotrophs.