What is the term for a protein released by an endocrine gland into the blood that acts as a chemical messenger?

The correct term for a protein released by an endocrine gland into the bloodstream that serves as a chemical messenger is hormone. Hormones are vital signaling molecules in the body that regulate various physiological processes, such as growth, metabolism, and mood. They travel through the blood to target organs and tissues, where they bind to specific receptors, initiating a response that affects the function of those areas.

Enzymes, on the other hand, are proteins that catalyze biochemical reactions but do not serve as chemical messengers. Neurotransmitters are chemicals that transmit signals across a synapse in the nervous system and are distinct from hormones, which operate through the bloodstream rather than nerve synapses. Vitamins are organic compounds that are essential for normal growth and nutrition and must be obtained from the diet, but they do not function as hormones. Thus, "hormone" is the specific and correct term to describe proteins that act as chemical messengers in the bloodstream.