= Estradiol
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<Estradiol> is an <estrogen> <steroid hormone> involved in reproductive development and regulation. It is one of the estrogens produced by the <ovary (anatomy)> and participates in the interacting hormonal signals of the <menstrual cycle>.<Estradiol in the NCI Dictionary of Cancer Terms>[definition, estradiol as an estrogen]{ref}<Anatomy and Physiology 2e by J. Gordon Betts and others>[§17.8, ovarian estrogens and reproductive functions]{ref}
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.<Hormonal dynamics at midcycle: a reevaluation by J. D. Hoff and others>[Abstract, sampling schedule and timing relative to the estradiol peak]{ref}
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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