Which of the following is NOT a water-soluble hormone?

Thyroxine (T4), a hormone produced by the thyroid gland, is classified as a lipid-soluble hormone rather than a water-soluble one. This classification is primarily based on its chemical structure and the mechanism through which it exerts its effects on target cells. Lipid-soluble hormones, such as thyroid hormones, are typically small, non-polar molecules that can easily pass through cell membranes. They generally travel in the blood bound to carrier proteins and exert their effects by binding to intracellular receptors, influencing gene expression.

In contrast, dopamine, epinephrine, and norepinephrine are all derived from the amino acid tyrosine and exhibit water-solubility. Water-soluble hormones are polar molecules that cannot readily cross the lipid bilayer of cell membranes, so they bind to receptors on the cell surface. This interaction often triggers signal transduction pathways to elicit a rapid physiological response.

Understanding these fundamental differences in solubility and the subsequent effects on hormone action is crucial in pathophysiology, as it provides insight into how various hormones function and the implications for health and disease.