Which of the following is NOT an example of a protein hormone?

• A. Placenta
• B. PTH
• C. Prolactin
• D. Cortisol

Answer: (D)

Protein hormones are polypeptides that work by binding to receptors on the cell surface and triggering signaling cascades. Parathyroid hormone and prolactin are classic examples of this type. Cortisol, however, is a steroid hormone derived from cholesterol. Being lipid-soluble, it crosses the cell membrane and binds to intracellular glucocorticoid receptors. The receptor–hormone complex then acts in the nucleus to regulate gene transcription, producing effects that are slower to start but longer lasting. This fundamental difference in solubility, receptor location, and mechanism is why cortisol is not a protein hormone, even though many other hormones share a protein nature. The placenta can secrete protein hormones as well, but the key distinction here is the chemical class and action of cortisol.