Genetic influences on hippocampal volume differ as a function of testosterone level in middle-aged men

► The brain is highly responsive to androgens such as testosterone. ► In men, the hippocampus is vulnerable to age-related changes in testosterone. ► We found no significant correlation between testosterone and hippocampal volume. ► The heritability of hippocampal volume increases as a function of testosterone. ► Testosterone level is likely a valuable biomarker for brain aging in men.