What type of hormone is insulin when it is released from the pancreas and travels in the bloodstream?

Insulin is classified as a circulating hormone because it is produced in the pancreas and then released into the bloodstream, allowing it to travel throughout the body to reach various tissues and exert its effects. Circulating hormones are generally secreted into the bloodstream and have effects on distant target organs or tissues, playing a significant role in regulating various physiological processes. In the case of insulin, it primarily regulates blood glucose levels by facilitating the uptake of glucose by cells, as well as influencing fat and protein metabolism.

The other options refer to different mechanisms of hormone action. Autocrine hormones act on the same cells that produce them, local hormones (also known as paracrine hormones) exert effects on nearby cells rather than traveling through the bloodstream, and neurotransmitters are chemical messengers released from neurons to communicate with other neurons or muscles. Each of these forms plays distinct roles in the body, but they do not encompass the systemic function of insulin as a circulating hormone.