When thinking of hormones, it helps to remember that they are generally slow acting, indirect, and long lasting. All endocrine hormones travel through the bloodstream in order to reach their target cells. It can help to compare the activity of hormones to the activity of neurotransmitters, which are fast acting and have immediate effects on their target cell. Consider the effects of acetylcholine release at a neuromuscular junction in comparison to growth hormone release into the blood. The muscle twitch is much more transient and quick to react, whereas the effects of growth hormone can take much longer to appear.

All steriod hormones are lipid soluble and will thus bind either nuclear or cytoplasmic receptors. Most peptide hormones are water soluble and cannot diffuse through the plasma membrane of the target cell; they must bind to receptors embedded in the cell membrane. The one exception to this rule is thyroid hormone. It is a peptide hormone, but is a lipophilic tyrosine derivative and binds to nuclear receptors.

A positive feedback loop features further deviation from normal. In the case of childbirth, oxytocin produces uterine contractions, which then causes more oxytocin to be released and more uterine contractions to occur until the baby is delivered. A negative feedback loop returns the body to normal conditions.

Neural or synaptic communication occurs when a neurotransmitter (or chemical messenger) diffuses across the synaptic cleft of the presynaptic neuron and postsynaptic neuron or target cell. Examples of these neurotransmitters include acetylcholine and norepinephrine. Endocrine communication occurs when chemical messengers are released into the blood stream. These chemical messengers are called hormones. 

The secretion of hormones can often be controlled by concentrations of nutrients, ions, and other molecules. The secretion of parathyroid hormone is regulated and controlled by the concentration of calcium found in the plasma. The function of parathyroid hormone is to increase the concentration of calcium in the blood. This is done by breaking down bone and increasing calcium reabsorption in the kidneys and intestine. 

Calcitonin, thyroxine, triiodothyronine are the hormones that are secreted by the thyroid. The c-cells of the thyroid secrete calcitonin. The function of calcitonin is to decrease the concentration of calcium in the blood by activating osteoblasts and/or deactivating osteoclasts, ultimately storing the calcium from the blood into bone.

The correct answer is parathyroid hormone (PTH). PTH is secreted by the parathyroid gland when calcium levels become too low and acts to increase levels to normal range.

Calcitonin is also involved in calcium regulation, but is secreted by the thyroid gland when calcium levels become too high. Calcitonin reduces the action of the parathyroid gland and PTH until calcium levels are reduced to normal range. Thyroid stimulating hormone (TSH) causes the thyroid gland to grow in size in an attempt to make more hormones if iodine levels become too low. Melatonin is synthesized in the pineal gland and is important for the regulation of the sleep cycle. Cortisol is secreted by the adrenal cortex, and increases blood glucose levels, and has other effects.