Which hormone initiates its response by binding to cytoplasmic receptors?

• A. Estradiol
• B. Epinephrine
• C. Growth hormone
• D. Follicle-stimulating hormone

Answer: (A)

Steroid hormones like estradiol cross the cell membrane easily because they are lipophilic, so they bind to receptors inside the cell, typically in the cytoplasm. When estradiol binds its cytoplasmic estrogen receptor, the hormone–receptor complex moves into the nucleus and acts as a transcription factor, altering gene expression and leading to changes in protein synthesis. This intracellular, transcription-driven mechanism gives a slower but longer-lasting response compared with other hormones.

In contrast, epinephrine, growth hormone, and follicle-stimulating hormone are not lipophilic and bind to receptors on the cell surface, triggering second messenger pathways (such as cAMP or JAK-STAT signaling) that rapidly produce cellular responses but do not primarily involve direct genomic changes via cytoplasmic receptors.