The pituitary gland (located at the base of the brain, controlled by the hypothalamus) is called the 'master gland' because it secretes hormones that regulate the activity of other endocrine glands — including the thyroid, adrenal glands, and gonads. It has two lobes: the anterior pituitary (produces GH, TSH, ACTH, FSH, LH, prolactin) and posterior pituitary (releases ADH and oxytocin).