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Neuroendocrinology and Pituitary

SAT-114 - Novel LC/MS/MS method for oxytocin aligns with neurophysin-I levels during pregnancy and postpartum

Saturday, June 13, 2026
9:30 AM - 4:30 PM CT


Background: Understanding of oxytocin (OXT), a neurohormone implicated in many physiologic processes in animal models, in human physiology and disease is limited by challenges in measuring endogenous OXT. OXT analysis techniques yield vastly different absolute values, which may in part reflect method-specific detection of protein-bound vs. free OXT and OXT-related peptides (e.g., degradation products). Liquid chromatography/tandem mass spectrometry (LC/MS/MS) is a highly specific technique enabling measurements of free, biologically active hormones. An LC/MS/MS assay for free OXT has been recently developed but not yet validated. Neurophysin-I (NP-I) is the reference marker of hypothalamic-pituitary OXT release into the circulation and a robust surrogate biomarker for OXT, released to circulation in equimolar concentrations; however, its longer half-life may not capture rapidly changing OXT dynamics. NP-I is known to increase during pregnancy and decrease starting a few days post-partum. Here, we leveraged samples from a randomized clinical trial testing a probiotic delivered early postpartum to mothers at risk for postpartum depression followed from the 3rd trimester to 6 weeks after childbirth to compare LC/MS/MS OXT to NP-I.

Methods: Forty-six women were evaluated during the 3rd trimester, 0-4 days following childbirth (start of treatment; 6 weeks L. reuteri vs. placebo randomly assigned), and 6 weeks post-partum (wk6). At all visits, plasma was collected and placed immediately on ice before processing. OXT levels were analyzed using extracted LC/MS/MS. NP-I was measured using ELISA (Abcam).

Results: Compared to pregnancy levels (adjusted mean [adjusted standard error]: 5931 [148] pg/mL), NP-I levels did not change at birth (5548 [220] pg/mL, -6%; ß=-383, p=0.111) and decreased at wk6 (1940 [150] pg/mL, -67%; ß=-3991, p< 0.0001). Compared to pregnancy levels (52.8 [2.3] pg/mL), LC/MS/MS OXT was lower following birth (46.0 [2.6] pg/mL, -13%; ß=-2.4, p=0.017) and at wk6 (14.3 [1.0] pg/mL, -73%; ß=-14.9, p< 0.0001). There was a strong positive correlation between NP-I and LC/MS/MS OXT levels across all timepoints (Rf=0.69, p< 0.0001).

Conclusions: While 100-fold lower in absolute values (reflecting half-life differences), LC/MS/MS OXT closely aligned with NP-I levels and followed the expected trajectory after birth, supporting LC/MS/MS for measurement of free OXT. Our findings support the use of LC/MS/MS OXT and NP-I in research, the establishment of normative data, and ultimately translation to the clinic.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*