Which of the following is NOT a gland of the endocrine system?

The liver is not classified as a gland of the endocrine system. Instead, it serves primarily as an organ involved in digestion and metabolism. While the liver does produce certain hormones and has functions related to the regulation of blood sugar and lipid levels, it does not fit the classical definition of an endocrine gland, which is responsible for secreting hormones directly into the bloodstream to regulate various physiological processes.

In contrast, the pineal gland, thyroid gland, and adrenal glands are all key components of the endocrine system. The pineal gland is responsible for melatonin production, which regulates sleep-wake cycles. The thyroid gland produces hormones such as thyroxine, which play a critical role in regulating metabolism and energy levels. The adrenal glands produce a variety of hormones, including cortisol and adrenaline, which are involved in the body’s stress response and metabolism.

Understanding these distinctions is crucial for recognizing the specific functions of various organs and glands within the body, particularly in relation to their roles in hormone secretion and regulation.