The endocrine system involves secretions by ductless glands into the bloodstream. The pancreas, pituitary gland, thyroid, and hypothalamus are all endocrine organs that secrete hormone signals to the body. The pancreas, however, also functions as an exocrine gland. When the pancreas secretes insulin and glucagon into the blood stream, it serves as an endocrine organ. When it secretes digestive enzymes through the pancreatic duct into the small intestine, it serves an exocrine function.

The question states that both hormone A and hormone B act on the collecting ducts of the kidneys. There are two major hormones that act on the collecting ducts: aldosterone and antidiuretic hormone (ADH). Aldosterone is a steroid hormone that functions to increase sodium and chlorine reabsorption from urine in the collecting ducts, whereas ADH is a peptide hormone that functions to increase water reabsorption from urine in the collecting ducts (by increasing the permeability to water). This means that hormone A is ADH and hormone B is aldosterone.

Recall thatsteroid hormones, such as aldosterone, are nonpolar molecules. On the other hand, peptide hormones are made up of protein subunits and consist of many polar groups; therefore, ADH is a polar molecule. Since water is a polar molecule, only ADH (hormone A) will dissolve in water.

Insulin is released into the blood from the pancreas and functions to decrease the glucose concentration in the blood. Elevated levels of blood glucose induces insulin release. Once released, insulin transports glucose back into the cells or facilitates the formation of glycogen. Glycogen consists of multiple glucose subunits and is stored in the liver for later use. Calcitonin, on the other hand, functions to decrease the calcium concentration in the blood. Osteoclasts are cells in bones that break down bones and release bone calcium into the blood. Calcitonin inhibits the activity of osteoclasts and decreases the calcium concentration in the blood. Statement I is false.

Antagonistic hormones are a pair of hormones that have the opposite effects. For example, insulin and glucagon are antagonistic hormones because insulin functions to decrease blood glucose levels, whereas glucagon functions to increase blood glucose levels. Similarly, calcitonin and parathyroid hormone (PTH) are antagonistic hormones because calcitonin functions to decrease blood calcium levels whereas PTH functions to increase blood calcium levels. Insulin and calcitonin are not antagonistic hormones because they do not have opposite effects. Statement II is false.

Recall that insulin and calcitonin are part of the endocrine system. Hormones in the endocrine system are characterized by their ability to travel through blood and act on cells that are far away from the glands that released them; therefore, insulin and calcitonin do not act on cells surrounding their respective glands alone. Statement III is false.

Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas in response to high blood glucose levels. Alpha cells in the islets of Langerhans secrete glucagon, the antagonist of insulin, in response to low blood glucose levels.

The thyroid secretes thyroxine (also known as tetraiodothyronine or T4), which controls the body's metabolism. The thyroid also secretes calcitonin, which promotes bone formation. Cortisol is secreted by the adrenal cortex and works to regulate blood glucose levels and has an anti-inflammatory effect. Cortisol is a corticosteroid that is secreted in response to adrenocorticotropic hormone.

Seeing as type I diabetes results from the destruction of the -cells in the pancreas, this is the only feasible answer. These cells are responsible for producing insulin.

The thyroid gland is responsible for helping to regulate basal metabolism and calcium levels. The thyroid gland secretes calcitonin, synthesized from C-cells, to reduce free calcium levels in the blood. Calcitonin antagonizes parathyroid hormone (PTH), which serves to increase the free calcium levels in the blood.

Parathyroid hormone is released from the parathyroid gland. Thyroid-stimulating hormone is released from the anterior pituitary. Aldosterone is released from the adrenal cortex.

Parathyroid hormone is secreted by the parathyroid glands, and acts in a few ways to directly and indirectly increase blood calcium levels. Parathyroid hormone increases calcium reabsorption in the collecting tubule of the kidney, increases calcium absorption in the gut, and increases bone resorption to free stored calcium.

Parathyroid hormone and calcitonin act in a negative feedback loop to maintain calcium levels. Calcitonin is released from the thyroid, and serves to decrease blood calcium.

Maintaining blood calcium levels is particularly important because heart attacks can become common in low calcium settings, as calcium is crucial to maintaining cardiac muscle contractions.

Iodine is an element that plays a key role in the function of thyroxine (T4), a thyroid hormone. There are two main types of thyroid hormones: T3 and T4 (or thyroxine). Both hormones contain iodine atoms (T3 contains three and T4 contains four) and both are essential in order to maintain basal metabolic rate. Since the doctor observed a decrease in iodine concentration, the patient must experience a decrease in the production of functional thyroxine. 

Glucose concentration does not depend on iodine; therefore, an increase or decrease in glucose concentration in blood is irrelevant to this question. Thyroid-stimulating hormone (TSH) is a peptide hormone released by the anterior pituitary gland. Its main function is to stimulate the thyroid gland to release T3 and T4. Iodine atoms are not necessary for the proper functioning and production of TSH. Iodine deficiency will cause TSH to stimulate the release of dysfunctional T3 and T4 hormones. 

Of all the hormones listed, antidiuretic hormone (ADH) is the only one that neither acts on, nor is released by, the adrenal cortex. ADH is released from the posterior pituitary and causes the kidneys to retain more water.

Aldosterone (a mineralcorticoid) and cortisol (a glucocorticoid) are both incorrect choices because they are released by the adrenal cortex and would be greatly affected by trauma to that area. ACTH is released by the anterior pituitary and acts to stimulate the adrenal cortex; these hormones act as part of a negative feedback chain, so damage to the target area would temporarily cause more ACTH to be produced. The same goes for CRH, which is released by the hypothalamus and stimulates secretion of ACTH.

Endocrine glands secrete hormones into the blood stream, lined with endothelium, allowing them to travel through the blood and to act at a distant site. Exocrine organs, in contrast, secrete products into lumens that are lined with epithelium.

Mammary glands are used in lactation. Because the milk is not secreted into the blood, the mammary glands are not endocrine glands.

The hypothalamus releases hormones into the blood, such as corticotropin-releasing hormone (CRH). The pituitary releases hormones such as prolactin. The adrenal gland secretes hormones such as cortisol.