Final answer:
Prolactin (PRL) is the anterior pituitary hormone that lacks a specific hypothalamic releasing factor and is predominantly regulated through inhibition by dopamine, also known as prolactin inhibiting hormone (PIH).
Step-by-step explanation:
The anterior pituitary hormone that has no known hypothalamic releasing factor is prolactin (PRL). The anterior pituitary gland produces several hormones under the regulation of the hypothalamus, which releases specific hormones that either stimulate or inhibit the production of anterior pituitary hormones. While most anterior pituitary hormones have corresponding hypothalamic releasing or inhibiting hormones, prolactin is unique because it is primarily regulated by prolactin inhibiting hormone (PIH), which is actually dopamine. There is no known separate prolactin releasing hormone under normal physiological conditions. Instead, the production of prolactin is principally controlled by the inhibition of dopamine.