What this result can and cannot tell you

Estradiol, FSH, and LH move together through a feedback loop, so a value read without its gonadotropin and clinical context is read out of context. Reference intervals for estradiol differ by cycle phase, life stage, sex, pregnancy, and method; no quoted interval is universal. Direct immunoassay has a real, documented low-end limitation from limit of quantitation and cross-reactivity, and LC-MS/MS offers better low-end specificity while still having its own limits below about 5 pg/mL.

Do not use these findings to make fertility-treatment protocol decisions, hormone dosing or titration decisions, diagnose a specific endocrine disease, or calculate free or bioavailable estradiol. The gender-affirming therapy target range cited is a guideline framework for laboratory context, not a laboratory-directed dosing rule.

The exact limit of quantitation, limit of detection, and cross-reactivity figures above come from a cited manufacturer's method sheet for one reagent version. A laboratory's analyzer, reagent lot, and software may differ, and CLIA (42 CFR 493.1253) and CAP accreditation require performance specifications and reference intervals to be verified for that laboratory's own method and population before use. The escalation pathway to LC-MS/MS and any gender-affirming-therapy monitoring interval are local policy decisions.

An estradiol result is only as informative as its context and its method allow. Correlate it with FSH/LH and the clinical situation, check it against the assay's own limit of quantitation before trusting a low value, and route to a more specific method when the clinical question needs precision the immunoassay cannot deliver.