Which of the following is NOT a hormone?

• A. Estrogen
• B. Progesterone
• C. Testosterone
• D. Sperm

Answer: (D)

In endocrine signaling, hormones are chemical messengers released into the bloodstream by glands to act on distant target organs. Sperm does not fit this role because it is a cell, a reproductive gamete, not a signaling molecule released to regulate physiology.

Estrogen, progesterone, and testosterone are steroid hormones produced mainly by the gonads (and to some extent the adrenal glands). They travel through blood to specific tissues, bind to receptors, and influence processes like sexual development, the menstrual cycle, and reproduction. Sperm, on the other hand, is produced by the testes to fertilize an egg; it’s a cell, not a hormone, and it doesn’t function as a signaling messenger.