I've been reading my textbook and two terms have appeared that, after a bit of looking, I still can't seem to be able to distinguish. Here's what the textbook says (McGraw Hill Bio 12, 2011):

After receiving signals from various sensors in the body, the hypothalamus secretes what are referred to as releasing hormones, which often travel to the pituitary gland. The releasing hormones stimulate the pituitary gland to secrete hormones that act on other endocrine glands. Hormones that stimulate endocrine glands to release other hormones are called tropic hormones. (pg. 394)

From the work I've done, it all seems to indicate that releasing hormones regulate the secretion of other hormones, and that tropic hormones target and stimulate other endocrine glands (to release hormones). And yet my textbook appears to use the terms separately later on as if they're related rather than identical (e.g.: "the pituitary gland is controlled by the hypothalamus via releasing hormones and it secretes tropic hormones" (pg. 396)). So are they identical? And if not, what distinguishes between them?


The releasing hormones could also be considered tropic hormones, and indeed they fit the definition as you noticed, but aren't usually named as such.

The special thing about the ones your textbook is specifically highlighting as releasing hormones is that they communicate from the hypothalamus to the pituitary through the hypophyseal portal system, vessels that travel directly from the hypothalamus to the anterior pituitary.

Therefore, they aren't really released into the broader circulation (they can get there, but they'd be in too low a concentration). They are easily identified in a list of hormone because they all share a naming convention where they are called "____-releasing hormone" and their primary function is to cause the pituitary to release whatever is in the "_____".

In terms of overall neuroendocrine function, the "releasing" hormones all cause release of tropic hormones, which in turn cause release of hormones that affect other tissues.

Therefore, the sequence is releasing -> tropic -> non-tropic. They could have been named something else, like "primary tropic" and "secondary tropic" but this terminology keeps them separate as "tropic hormones that cause release of other tropic hormones."


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