Final answer:
The molecule described is a hormone involved in endocrine signaling, released by the hypothalamus and affecting distant organs through the bloodstream, corresponding to how hormones typically function in the endocrine system.
Step-by-step explanation:
The newly discovered signaling molecule in the hypothalamus, which is produced and released by cells of the hypothalamus and travels through the blood to affect the kidneys and the liver, is best described as a hormone. When this molecule is released in response to a stimulus, travels through the body, and binds to receptors of the target cells in the kidneys and liver to elicit a response, it is engaging in endocrine signaling. The hypothalamus is an important part of the endocrine system and is known for producing hormones such as antidiuretic hormone (ADH) or vasopressin, and oxytocin, which are then transported to and released by the posterior pituitary gland into the bloodstream.
Due to the characteristics of the signaling molecule mentioned, specifically that it travels through the blood, is produced by an endocrine gland (the hypothalamus), and targets distant organs (kidneys and liver), it fits the description of a hormone involved in endocrine signaling. This endocrine signaling usually produces a slower response but tends to have a longer-lasting effect compared to other signaling mechanisms.