Are releasing-hormones and release-inhibiting hormones from the hypothalamus ever present in the blood?
I think your teacher means that these hormones only go from the hypothlamus to the anterior pituitary gland, where they stimulate hormone secretion. Like, for example, of the FSH or LH, who are stimulated by GnRH. GnRH is not present in the systemic sirculation, but it's definately present in the one between the hypothalamus and the APG. FSH and LH are though. I don't remember the name of the small blood-system between the hypothalamus and the APG, but I don't think it's connected to the systemic circulatory system.