Relationship between circulating levels of sex hormones and insulin-like growth factor-1 and fluid intelligence in older men

The relationship was investigated between baseline serum levels of total testosterone (T), free testosterone (FT), dehydroepiandrosterone sulfate (DHEAS), ESTRADIOL (E2), sex hormone-binding globulin (SHBG), insulin-like growth factor-1 (IGF-1) and cognitive functioning in 25 healthy older men (mean age 69.1 years). Cognitive tests concerned measures not sensitive to ageing (crystallized intelligence), and measures sensitive to ageing (fluid intelligence and verbal long-term memory). Partial correlation coefficients (controlled for level of education) revealed significant associations of total T (r = -.52, p = -.009), SHBG (r - .59, p = .002) and IGF-1 (r = .54, p = .007) with the composite measure of fluid intelligence test performance, but not with crystallized intelligence, nor verbal long-term memory. Stepwise hierarchical regression analysis with the composite measure of fluid intelligence as the dependent variable showed that the contributions of SHBG, total T, and IGF-1 were not additive.