The correct answer is glucagon. Glucagon is secreted by the pancreas, not the pituitary gland. All the other answer choices are major hormones secreted by the pituitary that are essential to bodily functions.

Oxytocin is produced by the hypothalamus, but stored and released by the posterior pituitary, making it the correct answer. Antidiuretic hormone (ADH) is also released by the posterior pituitary, so its pairing with the hypothalamus was incorrect. Calcitonin is released by the thyroid, and aldosterone (like other corticosteroids) is released by the adrenal cortex. Thyroid-stimulating hormone (TSH) is released from the anterior pituitary.

The posterior pituitary secretes oxytocin, which is important for milk ejection from breasts and uterine contractions during labor.

Calcitonin, which serves to decrease serum calcium, is secreted by the thyroid gland. The adrenal cortex secretes aldosterone, which increases blood pressure by increasing sodium reabsorption and potassium secretion. The adrenal cortex also secretes cortisol, a steroid hormone that acts to increase serum glucose, aid in fat, protein, and carbohydrate metabolism, and suppress the body's immune system. 

The posterior pituitary gland does in fact secrete oxytocin; however, all of the hormones secreted by the posterior pituitary gland are created in the hypothalamus. The hormones are then sent to the posterior pituitary gland in order to be secreted.

The anterior pituitary creates and secretes follicle-stimulating hormone. The pancreas creates and secretes glucagon and insulin. The adrenal cortex creates and secretes aldosterone.

The pituitary gland is comprised for the anterior and posterior pituitary. The anterior pituitary arises from the oral ectoderm, while the posterior pituitary arises from the neuroectoderm. Hormones are secreted from the anterior pituitary in response to stress or to promote milk production, growth, or reproduction. Follicle-stimulating hormone (FSH) is required for growth of the reproductive system.

The posterior pituitary is responsible for production and secretion of neurohypophysical hormones, including oxytocin and vasopressin. Somatostatin is produced from within the digestive system, and signals to the anterior pituitary to inhibit growth hormone secretion. 

The hormones of the anterior pituitary can be remembered by using the pneumonic "FLAT PEG." The anterior pituitary secretes follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone, thyroid-stimulating hormone, prolactin, endorphins, and growth hormone.

The parvocellular neurons of the hypothalamus are responsible for secreting numerous hormones into the portal system, which are then delivered to the anterior pituitary. Thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH) are released from the parvocellular neurons. TRH stimulates the release of thyroid-stimulating hormone from the anterior pituitary, while CRH stimulates the release of adrenocorticotropic hormone from the anterior pituitary.

Vasopressin and oxytocin are secreted from the magnocellular neurons of the hypothalamus, and are released from the posterior pituitary.

The posterior pituitary is under neuronal control and is responsible for releasing vasopressin (antidiuretic hormone) and oxytocin.

In contrast, the anterior pituitary is under hormonal control from the hypothalamus, and is responsible for releasing growth hormone, adrenocorticotropic hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin. 

The anterior pituitary is responsible for secreting hormones into the blood stream, once their respective releasing factors from the hypothalamus arrive through the hypothalamus-pituitary portal system. The anterior pituitary releases growth hormone, adrenocorticotropic hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing-hormone, and prolactin.

The posterior pituitary is under neuronal control and releases vasopressin (antidiuretic hormone) and oxytocin.

The posterior pituitary releases antidiuretic hormone (vasopressin), which causes water retention in the kidneys, making this the correct answer.

Follicle-stimulating hormone causes follicle development, and is released from the anterior (not the posterior) pituitary.

Oxytocin causes uterine contractions, and is released from the posterior (not the anterior) pituitary.

Calcitonin helps to reduce blood calcium, and is released from the thyroid. Parathyroid hormone acts counter to calcitonin, and increases blood calcium.

Glucagon helps to increase blood glucose levels by stimulating glycogen break-down, and is released from the pancreas (not the adrenal gland).

The anterior pituitary releases a number of key hormones. These hormones can be memorized using the anagram "FLAT PEG."

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH)

Adrenocorticotropic hormone (ACTH)

Thyroid-stimulating hormone (TSH)

Prolactin (PRL)

Endorphins

Growth hormone (GH)

Thyrotropin-releasing hormone is actually released from the hypothalamus to trigger the anterior pituitary to secrete TSH.