a hormone found in the brain, platelets, digestive tract, and pineal gland. it acts both as a neurotransmitter (a substance that nerves use to send messages to one another) and a vasoconstrictor (a substance that causes blood vessels to narrow). a lack of serotonin in the brain is thought to be a cause of depression. also called 5-hydroxytryptamine.
the thyroid gland {hormone}, called thyroxine
a hormone that stimulates the sympathetic nervous system, called
a hormone secreted from a generative gland, called sex hormone