The oxytocin pathway is an example of a positive feedback mechanism. Unlike negative feedback, which counteracts a stimulation, positive feedback reinforces a stimulus, leading to an even greater response.

One key function of oxytocin in mammals is to regulate the release of milk during nursing. In this case, the initial stimulus in the oxytocin pathway is an infant suckling, which stimulates sensory nerve cells in the nipples. The signal is received in the hypothalamus, which signals the posterior pituitary to secrete more oxytocin. Additional oxytocin is released into the bloodstream, signaling the mammary glands to secrete milk and perpetuating the stimulation of the hypothalamus. Oxytocin participates in a similar positive feedback loop when it stimulates uterine contractions during birth.

Vasopressin (antidiuretic hormone) is moderated by a negative feedback loop based on blood pressure. Based on pressure sensors in the body, vasopressin will be released or inhibited. Insulin and glucagon work in a negative feedback loop via sensors in the pancreas to moderate blood glucose levels. Calcitonin and parathyroid hormone work in a negative feedback loop to moderate blood calcium.

Follicle-stimulating hormone (FSH) is a tropic hormone produced in the anterior pituitary. Its primary functions are to regulate development, growth, pubertal maturation, and reproductive processes of the human body.  

In males, follicle-stimulating hormone acts on the testes to promote spermatogenesis. In females, follicle-stimulating hormone is an important modulator of the menstrual cycle.

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.

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.

Hormones are generally slow acting, spread throughout the body by the blood, and can affect a variety of tissues at once. Neurotransmitters on the other hand, are quick acting, localized, and affect only a specific cell. We would not expect hormones to affect only one cell or tissue at a time, as their wide distribution allows them to have multiple target areas.

A critical concept to understand about hormones in the body is negative feedback. Remember that hormones are not causing the body's condition, but rather, are responding to the body's condition. As a result, parathyroid hormone would be higher than normal in order to increase the concentration of calcium currently in the blood, correcting the existing deficiency.

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.

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.

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.

Neuroendocrine signaling describes the process when specialized neural sensors, usually in the brain, detect changes in the body. These sensors stimulate the release of a hormone to help correct deviations. The hormone can travel via the bloodstream to affect distant tissues.

In this example, sensor neurons in the hypothalamus trigger the release of the hormone vasopressin to affect the kidney.