Hormones and other signaling molecules generate physiological responses by binding to specific receptor proteins in or on target cells. Only cells that have receptors for the secreted molecule (hormone, neurotransmitter, neurohormone, etc.) are target cells; other cells are nonresponsive.

There are five different classifications for molecular signaling. These are as follow:

Endocrine signaling: Secreted molecules diffuse into the bloodstream and trigger responses in target cells throughout the body.

Paracrine signaling: Signaling molecules diffuse locally and trigger a response in neighboring cells.

Autocrine signaling: Secretion of molecules that diffuse locally and trigger a response on the same cells that secreted them.

Synaptic signaling: Secretion of neurotransmitters that diffuse across synapses and trigger a response in cells or target tissues (neurons, glands, and muscles).

Neuroendocrine signaling: Secretion of hormones from neuronal cells that diffuse into the bloodstream and trigger a response on cells throughout the body.

Melatonin is a modified amino acid that is secreted by the pineal gland. The pineal gland contains light-sensitive cells and has nervous connections to the eyes that affect its secretory activity. As a result, melatonin regulates functions related to light, circadian rhythm, and seasonal alterations based on the amount of daylight.

Melatonin is secreted at night, and the amount released depends on the length of the night. In winter, for example, more melatonin is released. Melatonin is believed to target a group of neurons in the hypothalamus called the suprachiasmatic nucleus (SCN), which functions as a biological clock.

Melatonin also affects skin pigmentation in many vertebrates.

Since melatonin is a modified amino acid, its structure is very different from that of corticosteroids, which are synthesized from the lipid cholesterol. Melatonin has two rings (similar to tryptophan), a modified ether group (from the carboxylic acid of the amino acid), and an amide group (from the amine group of the amino acid). Corticosteroids have four rings and multiple hydroxyl and ketone groups.

The two hormones known for their effect on plasma calcium levels are calcitonin and parathyroid hormone (PTH). When calcium levels are high, calcitonin is released by the thyroid gland to stimulate the uptake of serum calcium into bone. This effectively decreases calcium levels in the blood. PTH has the opposite effect and is released by the parathyroid gland.

The three remaining answer choices are not known for their effect on calcium levels in the blood.

The hormone responsible for the reduction of calcium in the blood is calcitonin. This hormone promotes the assimilation of calcium ions into the bone matrix from the blood. This reduces the concentration of calcium ions in the blood and increases the concentration found in the bone. Meanwhile, PTH has the opposite effect by taking calcium ions from the bone matrix and releasing them into the blood.

Prolactin is responsible for the production of milk, while oxytocin is responsible for the ejection of milk. The question asks for the hormone that causes production, so the answer is prolactin. Prolactin is released from the anterior pituitary.

Oxytocin is released from the posterior pituitary and the mammary glands are responsible for releasing milk.

There are two types of thyroid hormone: T3 and T4. Both hormones are derived from the amino acid tyrosine, and are created by the addition of iodine atoms to the amino acid structure. The full name of T3 is triiodothyronine (three iodine atoms) and the full name of T4 is tetraiodothyronine (four iodine atoms). Because iodine is specific to the production of thyroid hormone, radioactive iodine is used to gather images of the thyroid gland.

If iodine levels are low, the thyroid cannot produce sufficient thyroid hormone. The result is hypothyroidism. Symptoms of hypothyroidism include weight gain, lethargy, and intolerance to cold. In extreme cases, the thyroid will enlarge in an attempt to produce more hormone, resulting in a goiter.

Endocrine interactions involve a molecule (hormone) being secreted into the bloodstream to trigger a response in target cells in a different location. Reduction of blood-glucose levels by insulin and stimulation of cortisol release by adrenocorticotropic hormone are both examples of endocrine function.

Though epinephrine can act as a hormone when secreted by the adrenal medulla, the answer option indicates that it is being secreted by a neuron into a synapse. In this case, epinephrine would be acting as a neurotransmitter, causing an effect on a neighboring post-synaptic neuron. This answer does not describe an endocrine interaction.

Following a large uptake of glucose, the body will begin to release insulin to facilitate storage of the glucose molecules within the cells of the liver, as well as skeletal muscles.

Glucagon is released when blood glucose levels are low and promotes an increase in free blood glucose. The breakdown of glycogen into free blood glucose is a process known as glycogenolysis; this process is stimulated by glucagon and inhibited by insulin. Gluconeogenesis is the generation of glucose from non-carbohydrate carbon substrates; it is also stimulated by glucagon and inhibited by insulin.

Water-soluble hormones can be either polypeptides (proteins) or tyrosine-derivatives. The binding of a water-soluble hormone to a receptor occurs on the cell membrane, and receptor binding triggers a physiological response through a signal transduction cascade. Peptide hormones include insulin, growth hormone, oxytocin, adrenocorticotropic hormone, and many more. Tyrosine-derivative hormones include epinephrine and the thyroid hormones (T3 and T4).

Water-soluble hormones, like most hormones, are secreted into the bloodstream and carried throughout the body. As such, they are capable of eliciting responses in non-local regions of the body, and would not be classified as local regulators.

The question tells us that slowed output of thyroid hormones can lead to obesity. One can safely assume that receptor insensitivity will have a similar effect. In either scenario, the T3 and T4 hormones are unable to elicit the proper response form the body. Similar to individuals with hypothyroidism, individuals with insensitive receptors would show slowed metabolism and increased weight gain.