Background Socio-economic inequalities in functional ageing are substantial in the UK and many other developed nations. The neuroendocrine system is influenced by environmental factors operating across life, is hypothesised to be a key regulator of functional ageing, and may partly explain observed socio-economic inequalities. We examined associations between childhood and adult socio-economic position (SEP) with markers of the growth hormone and reproductive axes (insulin-like growth factor-I [IGF-I] and testosterone) which are thought to have important roles in regulating both early life growth and adult physical functioning.

Methods Data were from 791 men and 799 women participating in the MRC National Survey of Health and Development (also known as the 1946 British Birth Cohort Study), who provided fasting morning blood samples during clinic or home visits at 60–64 years. Plasma IGF-I concentration was measured by immunoassay and plasma testosterone concentration by liquid chromatography-tandem mass spectrometry. Associations between prospectively ascertained childhood SEP (paternal occupational class at 4 years) or adult SEP (household income at 60–64 years) with IGF-I and testosterone at 60–64 years were examined using the relative index of inequality to enable comparison across different size groups.

Results Lower childhood or adulthood SEP was associated with lower IGF-I among women but not men – mean percentage difference in IGF-I comparing the lowest with the highest childhood SEP (women: -9.16, 95% CI: -17.86 to -0.46; men: 5.57, -3.05 to 14.19), adult SEP (women: -12.16, -20.63 to -3.68; men: -5.16, -13.67 to 3.35). Childhood SEP was not associated with testosterone (men: -3.65, -14.53 to 7.24; women: 15.13, -3.52 to 33.78), but lower adult SEP was associated with lower testosterone among men (-18.77, -29.43 to -8.10) but not women (-2.58, -20.82 to 15.66).

Conclusion Socio-economic inequalities were found in circulating IGF-I (among women) and testosterone concentrations (among men) in early old age. Socio-economic differences in neuroendocrine functioning may contribute to inequalities in functional ageing as both low testosterone and IGF-I have been associated with adverse health outcomes and poorer physical functioning. Further research will address this question, and investigate whether SEP is associated with the typical mid-life declines in these profiles.