What is oxytocin’s biological function?

Oxytocin is a hormone synthesized in brain regions such as the supraoptic nucleus and paraventricular nucleus of the hypothalamus, which are critical for behavioral and physiological homeostasis. In the popular imagination, this hormone is probably best known for its involvement in the control of social, sexual, and romantic attachment and behavior. By studying animal models, neurobiologists have shown the possible involvement of oxytocin in pair-bonding, along with other hormones like arginine-vasopressin and dopamine. Several studies involving the administration of oxytocin in humans have also implicated a role for this hormone in forming and maintaining human romantic relationships.

Oxytocin also functions as a reproductive health regulator, stimulating uterine smooth muscle and serving as one of the most potent uterotonic agents. It also impacts reproduction-related functions for both women and men such as influencing estrous cycle length, promoting follicle luteinization in the ovary and ovarian steroidogenesis, and stimulating erections and ejaculation.

Most recently, emerging evidence shows that oxytocin helps regulate the body’s response to stress, and that it is particularly involved in regulating the hypothalamo-pituitary-adrenal axis and cardiovascular function.

In what clinical scenarios could oxytocin testing be used?

Although oxytocin testing has not yet become a routine clinical assay, in the future it could aid healthcare providers in assessing behavioral, psychiatric, and stress-related conditions. Specifically, this could include the work-up of mood disorders, anxiety disorders, obsessive-compulsive disorder, and the evaluation of burnout.

How is oxytocin measured?

Immunoassays are the most common method for determining oxytocin concentrations. The first oxytocin assays were based on the radio-immunoassay format, but now most are enzyme-linked immunosorbent assays (ELISAs). These ELISA-based tests have acceptable limits of quantification of approximately a few pg/mL and imprecisions usually below 10%.

However, they aren’t standardized, and there is clear heterogeneity/lack of commutability between results obtained using different oxytocin immunoassays. This could be related to the specificity of the antibodies used in these assays and also to cross-reacting components, which reinforces the need for a sample’s extraction before testing. Sample extraction has a significant impact on oxytocin results: The concentrations of oxytocin measured by enzyme immunoassay without plasma extraction are more than 100-fold higher than in extracted plasma.

To improve the specificity of oxytocin measurements, assays based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) have now been validated for this hormone. Some of these two-dimensional LC-MS/MS assays are ultra-sensitive, with lower limits of quantification close to 1.00 pg/mL and with imprecisions also lower than 10%.

What sample types are used for oxytocin testing?

Oxytocin can be detected in various physiological fluids such as plasma, serum, saliva, urine, and cerebrospinal fluid. However, as oxytocin assays evolve, saliva could ultimately become the sample of choice because it is less invasive and would also facilitate data comparison due to the fact that a large number of studies are already using this matrix to measure oxytocin.

Damien Gruson, PhD, is head of the department of clinical chemistry at the Cliniques Universitaires St-Luc and Université Catholique de Louvain in Brussels, Belgium. He also serves in the research unit of endocrinology, diabetes, and nutrition at the university’s Institut de Recherche Expérimentale. +E-mail: damien.gruson@uclouvain.be