FST and polycystic ovary syndrome: For follistatin, it was found that the relationship between follistatin and PCOS is modified by the difference of mean age between the PCOS patients and healthy controls (Coefficient: - 0.06, SE: 0.01, p = 0.007) so that by an increase in the difference of mean age between the PCOS patients and healthy controls, there was a significant reduction in the mean difference of follistatin between the groups (Fig 7).