Achemical produced in one part of a multicellular organism that will have effects elsewhere within that creature is a(n):

enzyme

hormone

nutrient

antinutrient

The best answer is Hormone.

Hormones are chemicals secreted by one tissue and affect the functioning of other tissues and cells.

Hormones are secreted by glands of the endocrine system. They are secreted into the blood and also extracellular fluid. They then circulate in the blood and come into contact with essential all cells, but a specific hormone will only affect a limited number of cells, namely the cells that bear a receptor for that hormone.

Red blood cells for example have receptors for the insulin hormone that is produced by the pancreas. They will therefore be directly  affected by insulin while all other cells in the blood remain oblivious of this hormone..

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.