Association between systemic immune inflammation index and serum testosterone and free testosterone in middle-aged and elderly men.

Background: Chronic inflammation is a pervasive feature of aging and may be associated with testosterone in middle-aged and older men. Systemic immune-inflammation index (SII) is a novel inflammatory biomarker. We aimed to assess the association between SII and serum testosterone and free testosterone (FT) in middle-aged and older men.

Methods: Our study included males ≥ 40 years old in the 2011-2016 and 2021-2023 National Health and Nutrition Examination Survey. Multivariable regression analysis was used to explore the associations between SII and serum testosterone and FT in middle-aged and older men. Subgroup analysis was performed according to age.

Results: About 5354 participants were included, of which 2450 contained FT data. Multivariable linear regression found that SII exhibited an inverse association with serum testosterone (β -0.05, 95% CI -0.07 to -0.03, P < 0.001) and FT (β -0.03, 95% CI -0.05 to -0.01, P = 0.032) in middle-aged and elderly men. After SII was grouped as quartiles, serum testosterone was significantly lower in SII quartile 4 than in SII quartile 1 (β -0.05, 95% CI -0.08 to -0.02, P < 0.001). However, FT was not significantly lower in SII quartile 4 than in SII quartile 1 (β -0.03, 95% CI -0.07 to 0.01, P = 0.135). In subgroup analysis, the serum testosterone results were consistent with the overall results. However, only in the ≥60 years group, SII exhibited an inverse association with FT (β -0.06, 95% CI -0.1 to -0.02, P = 0.002) and was significantly lower in SII quartile 4 than in SII quartile 1 (β -0.06, 95% CI -0.12 to -0.01, P = 0.049).

Conclusions: Our study revealed an inverse association between SII and serum testosterone and FT in middle-aged and elderly men, particularly among men ≥ 60 years.