Which organ is NOT part of the endocrine system?

The liver is not classified as part of the endocrine system, even though it plays critical roles in various bodily functions and produces hormones. The endocrine system is primarily comprised of glands that secrete hormones directly into the bloodstream to regulate various physiological processes, including growth, metabolism, and homeostasis.

While the liver does produce several important proteins and factors, including insulin-like growth factor and angiotensinogen, it is not primarily known as an endocrine organ. Instead, it functions more as an exocrine organ with secretory roles related to digestion, alongside other metabolic functions.

In contrast, the pancreas, thyroid, and adrenal glands are specialized endocrine organs with distinct roles in hormone production and secretion: the pancreas produces insulin and glucagon to regulate blood sugar, the thyroid produces hormones that regulate metabolism, and the adrenal glands release hormones like cortisol and adrenaline that help manage stress responses.