Final answer:
Oxytocin is the hormone among the listed options that is regulated by a positive feedback mechanism. It is involved in amplifying the contractions during labor and childbirth, marking a classic example of such regulation.
Step-by-step explanation:
The hormone regulated by a positive feedback mechanism among the options provided is oxytocin. Oxytocin, which plays a crucial role in labor and childbirth, exemplifies a hormone regulated by positive feedback wherein the presence of the hormone stimulates further release. During labor, as uterine contractions increase, they signal the brain to stimulate the pituitary gland to produce more oxytocin, further enhancing contractions, which in turn releases more oxytocin - a cycle that continues until childbirth is completed. In contrast, hormones like TSH (thyroid-stimulating hormone), thyroxine, and TRH (thyrotropin-releasing hormone) are primarily regulated by negative feedback mechanisms that maintain hormone levels within a tight range.