Relaxin: Physiology and What Research Reports

What Is Relaxin?

Relaxin is a small peptide hormone belonging to the insulin–relaxin superfamily. Like insulin, it is built from two peptide chains held together by disulfide bonds, which is why it is described as a peptide hormone rather than a steroid. The human genome encodes several family members, including relaxin-1, relaxin-2 and relaxin-3, alongside related insulin-like peptides. In human physiology and in most clinical literature, the circulating hormone measured and discussed as “relaxin” is relaxin-2, which has been reviewed as a candidate biomarker in cardiovascular disease (relaxin-2 as a potential biomarker in cardiovascular diseases). Relaxin-3 is studied largely as a neuropeptide, and researchers have built grafted, disulfide-stabilised scaffolds to create relaxin-3 agonists and antagonists as pharmacological tools (Frontiers in Chemistry, 2020).

Relaxin family peptides signal through relaxin family peptide receptors, and a central theme in the literature is that different family members prefer different receptors, which is why relaxin-2 research and relaxin-3 research often address entirely different questions — one largely peripheral and cardiovascular, the other largely central.

Where Relaxin Is Produced

The best-characterised source of relaxin is the reproductive tract during pregnancy. In a porcine model, researchers described the expression and cellular pattern of relaxin mRNA in corpora lutea across pregnancy (Cell and Tissue Research, 2007), illustrating the corpus luteum as a major site of synthesis in gestation. Relaxin and its receptor have also been examined in the human endometrium and in endometriotic tissue, and a review in the Annals of the New York Academy of Sciences summarised the work on relaxin in endometriosis (2009).

Beyond reproduction, relaxin biology has been studied in the heart and vasculature, where reviews have catalogued cardioprotective actions of the hormone (Molecular and Cellular Endocrinology, 2019). Relaxin-2 has also been reported to promote osteoblastic differentiation through epidermal growth factor and EGF receptor signalling in a 2025 laboratory report (Biotechnology and Applied Biochemistry), extending interest to bone cell biology.

What Relaxin Does in the Body

Across the literature, relaxin is generally framed as a hormone of vasodilation and extracellular matrix remodelling. Reviews of cardioprotective actions have grouped its reported effects into vascular, anti-fibrotic and tissue-remodelling categories (Molecular and Cellular Endocrinology, 2019), and a pharmacology review examined relaxin and its recombinant form serelaxin in the context of cardiac dysfunction and heart failure in hypertension (Advances in Pharmacology, 2022).

Connective-tissue effects are not uniformly favourable in animal work. One rat study of patellar tendon healing reported, on histological and biochemical evaluation, that relaxin inhibited tendon healing (BMC Musculoskeletal Disorders, 2019) — a finding often cited as a caution against assuming that matrix softening is always desirable. Vascular effects also appear tissue-specific: in rats, relaxin-2 failed to lower intraocular pressure and failed to dilate retinal vessels (International Ophthalmology, 2019), a negative result that argues against a universal vasodilator role.

How Relaxin Is Measured and Studied

Relaxin-2 is typically quantified in blood by immunoassay, and in some species in urine. A comparative paper reported relaxin concentrations in serum and urine across a wide range of non-domestic species, where the hormone served as a pregnancy marker (Annals of the New York Academy of Sciences, 2009). That work illustrates why assay validation matters: antibody cross-reactivity differs between species and sample types.

Measured concentrations are also sensitive to hormonal context. In elite female athletes, oral contraceptive therapy was reported to reduce serum relaxin-2 (Journal of Obstetrics and Gynaecology Research, 2017), which researchers treated as a confounder when interpreting circulating relaxin levels. For cardiovascular questions, a 2022 review assessed how well relaxin-2 performed as a biomarker and where the evidence remained limited (Journal of Personalized Medicine).

Snapshot of reported findings

Serelaxin and Engineered Relaxin Analogues

Native relaxin-2 is cleared quickly, which has shaped how it is investigated. Serelaxin, a recombinant form of human relaxin-2, has been the main clinical-stage molecule discussed in cardiovascular pharmacology reviews (Advances in Pharmacology, 2022). To address the short half-life, researchers engineered a long-acting, non-biased relaxin agonist using Protein-in-Protein technology (Biochemical Pharmacology, 2024), with “non-biased” referring to signalling that resembles the native hormone rather than favouring one downstream pathway. On the neuropeptide side, grafted disulfide-stabilised scaffolds were used to generate relaxin-3 agonists and antagonists for receptor pharmacology (Frontiers in Chemistry, 2020). These are laboratory and preclinical research tools, not consumer products.

Relaxin Research Outcomes: What Studies Report

The verified literature summarised here does not describe a human tolerability or adverse-event profile for relaxin, and no dosing is discussed on this page because the cited abstracts do not support it. What the animal and negative-result literature does report is that relaxin's effects are context-dependent. The rat tendon study reported impaired patellar tendon healing with relaxin (BMC Musculoskeletal Disorders, 2019), and the rat ocular study reported an absence of the expected pressure-lowering and vasodilatory effects (International Ophthalmology, 2019). Reviews of cardioprotection likewise framed findings as mechanistic and model-based rather than settled clinical conclusions (Molecular and Cellular Endocrinology, 2019).

Why the Term Comes Up

Relaxin appears in peptide discussions for three reasons: it is a genuine endogenous hormone with measurable physiology, it has a recombinant clinical-stage counterpart in serelaxin (Advances in Pharmacology, 2022), and its connective-tissue effects invite speculation that outpaces the evidence. Readers who encounter the term in a clinical report, a veterinary pregnancy test (Annals of the New York Academy of Sciences, 2009), or a review of relaxin in endometriosis (2009) are usually meeting three different bodies of work under one name.

This page is for educational purposes only and is not medical advice; consult a licensed physician or qualified healthcare professional about any medical question, symptom or treatment decision.

References

• Relaxin in endometriosis (Annals of the New York Academy of Sciences, 2009)
• Relaxin inhibits patellar tendon healing in rats: a histological and biochemical evaluation (BMC Musculoskeletal Disorders, 2019)
• Relaxin/serelaxin for cardiac dysfunction and heart failure in hypertension (Advances in Pharmacology, 2022)
• Relaxin concentrations in serum and urine of endangered and crazy mixed-up species (Annals of the New York Academy of Sciences, 2009)
• Relaxin 2 fails to lower intraocular pressure and to dilate retinal vessels in rats (International Ophthalmology, 2019)
• Cardioprotective actions of relaxin (Molecular and Cellular Endocrinology, 2019)
• Engineering a long acting, non-biased relaxin agonist using Protein-in-Protein technology (Biochemical Pharmacology, 2024)
• Development of Relaxin-3 Agonists and Antagonists Based on Grafted Disulfide-Stabilized Scaffolds (Frontiers in Chemistry, 2020)
• Oral contraceptive therapy reduces serum relaxin-2 in elite female athletes (Journal of Obstetrics and Gynaecology Research, 2017)
• Expression and cellular pattern of relaxin mRNA in porcine corpora lutea during pregnancy (Cell and Tissue Research, 2007)
• Relaxin-2 promotes osteoblastic differentiation mediated by epidermal growth factor and epidermal growth factor receptor signaling (Biotechnology and Applied Biochemistry, 2025)
• Relaxin-2 as a Potential Biomarker in Cardiovascular Diseases (Journal of Personalized Medicine, 2022)