Which gland is not part of the endocrine system?

The liver is not classified as part of the endocrine system. While the liver performs various vital functions in the body—such as metabolism, detoxification, and production of important proteins—it primarily functions as an organ in the digestive system. The liver does secrete some hormones and plays a role in the regulation of certain substances in the blood, but it does not primarily release hormones directly into the bloodstream in the same way that glands in the endocrine system do.

In contrast, glands like the pituitary, thyroid, and pineal glands are integral components of the endocrine system, which is responsible for regulating many bodily functions through the release of hormones. These glands are directly involved in producing and secreting hormones that influence everything from growth and metabolism to mood and reproductive processes.