Revival of neurophysin-I as a complementary biomarker for circulating oxytocin.

There is renewed interest in oxytocin (OXT) associated neurophysin-I (NP-I) as a surrogate biomarker of endogenous OXT secretion into circulation. OXT measurement in humans remains challenging because current assays vary in their specificity for free and protein-bound OXT, and OXT-related peptide species, including extended forms and peptide fragments, and results are influenced by sample handling and analytical methods. NP-I, a carrier protein that is co-synthesized and co-secreted with OXT, can be measured using a validated sandwich immunoassay without extraction. Studies support NP-I as a promising marker of OXT release to circulation, although it is not a replacement for direct OXT measurement. This Editorial reviews the biological relationship between OXT and NP-I, discusses the advantages and limitations of NP-I as a biomarker, and highlights future opportunities for integrating NP-I measurement with advances in OXT assay methodology to improve understanding of the human OXT system in health and disease.