Which gland is NOT part of the endocrine system?

The brain is the correct answer because it is not classified as a gland within the endocrine system. While the brain plays a crucial role in regulating various bodily functions and has some endocrine functions, it is primarily composed of neural tissue and is considered an organ rather than a gland.

The thyroid, adrenal, and pituitary glands, on the other hand, are all integral components of the endocrine system. The thyroid regulates metabolism and energy levels through hormone production. The adrenal glands produce hormones that help manage stress and regulate various physiological functions such as blood pressure and metabolism. The pituitary gland, often referred to as the "master gland," controls the release of hormones from other glands in the endocrine system, influencing growth, metabolism, and reproductive processes. Thus, while the brain is vital for overall homeostasis, it is not itself an endocrine gland.