Which of the following is classified as a water-soluble hormone?

Epinephrine is classified as a water-soluble hormone due to its chemical structure and how it functions within the body. Water-soluble hormones are typically derived from amino acids and, in the case of epinephrine, are catecholamines. They easily dissolve in the blood and do not require carrier proteins to travel through the bloodstream.

Once they reach their target cells, water-soluble hormones bind to specific receptors on the cell membrane, triggering a series of intracellular responses through secondary messenger systems. Epinephrine, for instance, rapidly affects physiological responses such as the "fight or flight" response, by increasing heart rate and blood flow to muscles.

In contrast, the other options, such as thyroid hormone T3, testosterone, and cortisol, are lipid-soluble hormones. These hormones are able to cross cell membranes directly due to their hydrophobic nature. They rely on transport proteins in the bloodstream and exert their effects by binding to intracellular receptors, thus leading to changes in gene expression. This fundamental difference in solubility affects their mechanism of action and the nature of their physiological effects.