Final answer:
Luteinizing hormone (LH) dramatically spikes in response to estrogen when it reaches a threshold level, triggering ovulation around day 14 of the menstrual cycle. This is part of a positive feedback loop in the menstrual cycle.
Step-by-step explanation:
The hormone that spikes dramatically in response to estrogen reaching a "threshold" level is luteinizing hormone (LH). During the menstrual cycle, a positive feedback loop occurs when the level of estrogen produced by the dominant follicle rises to such an extent that it causes a regulatory switch in the anterior pituitary gland. This prompts the pituitary to release large amounts of LH (and FSH) into the bloodstream. This LH surge is crucial as it triggers ovulation, which is the release of an egg from the most mature follicle in the ovary.
Typically, this spike in LH occurs at about day 14 of a typical 28-day menstrual cycle, marking the transition from the follicular phase to the luteal phase. Following ovulation, if pregnancy does not occur, estrogen levels decrease and the corpus luteum, which produces progesterone, forms in the ovary.