Estradiol is an estrogen steroid hormone involved in reproductive development and regulation. It is one of the estrogens produced by the ovary and participates in the interacting hormonal signals of the menstrual cycle.[ref][ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. §17.8, ovarian estrogens and reproductive functions
A concentration measured once cannot show the shape or ordering of a changing hormonal signal. In 1983, J. D. Hoff and collaborators sampled five women at two-hour intervals for five days around midcycle. The onset of the luteinizing hormone and follicle-stimulating hormone surges was temporally associated with peak estradiol levels.[ref]
Hormonal dynamics at midcycle: a reevaluation by J. D. Hoff and others. Abstract, sampling schedule and timing relative to the estradiol peak
Plotting these measurements as time series distinguishes an increase, a peak and a subsequent decline. The observations support a temporal relationship in the measured cohort; their timing alone does not isolate every causal link in the feedback system. This makes sampling interval and the chosen time reference part of an interpretable hormone measurement.
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