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Relaxin – Hormone, Function & Significance

Relaxin is a naturally occurring hormone that plays a key role during pregnancy. It relaxes connective tissue and ligaments, helping the body prepare for childbirth.

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Things worth knowing about "Relaxin"

Relaxin is a naturally occurring hormone that plays a key role during pregnancy. It relaxes connective tissue and ligaments, helping the body prepare for childbirth.

What is Relaxin?

Relaxin is a peptide hormone belonging to the insulin superfamily. It is primarily produced by the corpus luteum (a temporary endocrine structure in the ovary) and, during pregnancy, also by the placenta and uterus. In men, relaxin is produced in smaller amounts by the prostate gland. The hormone is best known for its effects on connective tissue, ligaments, and smooth muscle.

Biological Functions

Relaxin fulfills several important roles in the human body:

Loosening of pelvic ligaments: During pregnancy, relaxin softens and increases the flexibility of pelvic ligaments, facilitating the birthing process.
Cervical ripening: Relaxin contributes to the softening of the cervix (cervical ripening), preparing it for labor and delivery.
Inhibition of uterine contractions: In early pregnancy, relaxin can suppress premature uterine contractions, helping to maintain the pregnancy.
Connective tissue remodeling: The hormone promotes collagen breakdown and supports tissue adaptation to changing mechanical demands.
Cardiovascular effects: Relaxin increases cardiac output and causes vasodilation (widening of blood vessels), supporting the increased circulatory demands of pregnancy.
Kidney function: Relaxin enhances renal blood flow and increases the glomerular filtration rate.

Relaxin During Pregnancy

Blood levels of relaxin rise sharply in early pregnancy, peak near the end of the first trimester, and then gradually decline — though the hormone remains detectable throughout pregnancy. The high relaxin concentrations during the first trimester help explain why many pregnant women experience joint and ligament laxity, often presenting as pelvic girdle pain or lower back pain.

Relaxin in Men

Relaxin is also present in the male body. Produced in the prostate and secreted into seminal fluid, research suggests that relaxin plays a role in improving sperm motility, contributing to male fertility.

Relaxin as a Medication

Synthetic relaxin — specifically serelaxin, a recombinant form of human relaxin-2 — has been investigated clinically for the treatment of acute heart failure. Early studies showed promising benefits, including relief of breathlessness and improved renal perfusion. However, large-scale clinical trials did not fully confirm the hoped-for long-term outcomes, and serelaxin has not received widespread regulatory approval for routine use. Research into further therapeutic applications of relaxin, including fibrosis, systemic sclerosis, and wound healing, remains ongoing.

Mechanism of Action

Relaxin binds to specific G protein-coupled receptors known as RXFP1 and RXFP2 (Relaxin Family Peptide Receptors). This binding activates adenylate cyclase, raises intracellular cyclic AMP (cAMP) levels, and triggers a cascade of cellular responses that include increased collagenase production, smooth muscle relaxation, and vasodilation.

Measurement and Diagnostics

Relaxin levels can be measured using blood tests (immunoassays). Elevated levels are physiologically normal during pregnancy. Abnormally low levels may be associated with fertility issues or pregnancy complications. Routine clinical measurement of relaxin is not yet widely standardized in everyday medical practice.

References

Samuel, C. S. & Royce, S. G. - Relaxin in cardiovascular and renal disease. In: Kidney International, 2018.
Sherwood, O. D. - Relaxin's physiological roles and other diverse actions. In: Endocrine Reviews, 2004; 25(2): 205–234.
Bathgate, R. A. D. et al. - Relaxin family peptides and their receptors. In: Physiological Reviews, 2013; 93(1): 405–480.

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