Insulin released from the pancreas is carried in the bloodstream to distant target cells. This type of communication is considered to be a(n):

Insulin being released from the pancreas and carried through the bloodstream to reach distant target cells exemplifies the concept of a circulating hormone. Circulating hormones are secreted by endocrine glands, travel through the bloodstream, and exert their effects on target cells located at significant distances from the site of secretion. In the case of insulin, it regulates glucose uptake in various tissues throughout the body, illustrating the fundamental role of circulating hormones in systemic physiological regulation.

Autocrine communication refers to a mechanism where a cell releases a signal that binds to receptors on its own surface, thereby affecting its own function. This is not applicable to insulin, as it acts on distant cells rather than on itself.

Local hormones act in a paracrine manner, affecting neighboring cells within a localized area. Insulin, by contrast, travels through the bloodstream and acts at a distance, which distinguishes it from local hormones.

Neurotransmitters serve as chemical messengers that transmit signals across synapses in the nervous system, primarily between neurons and other cells. They operate on a much shorter scale compared to hormones like insulin, which circulate throughout the body.

Thus, the characteristic of insulin being carried in the bloodstream to reach various target cells is what defines it as a circulating hormone.