Selenium

The available research on selenium spans several health domains including thyroid function, immune response, male fertility, glycemic control, and athletic performance. Across 15 retrieved studies — including multiple meta-analyses and systematic reviews of randomized controlled trials rated as strong quality — the evidence suggests that selenium plays meaningful roles in specific clinical contexts, particularly thyroid-related conditions and male reproductive health. However, for most broader health claims made about selenium supplementation, the evidence is incomplete or insufficiently powered, and the majority of expert claims assessed in this review were rated as having insufficient evidence to confirm. The one area receiving even partial support was the association between selenium deficiency and multiple adverse health outcomes, reflecting how selenium's importance is clearest when deficiency is present rather than when supplementing replete individuals. The strongest and most consistent evidence involves thyroid conditions and male fertility. A strong-quality meta-analysis found benefits of selenium supplementation versus placebo specifically in Graves' orbitopathy, and a strong-quality systematic review and network meta-analysis found selenium among effective interventions for Hashimoto's thyroiditis. For male fertility, two strong-quality meta-analyses examined effects on sperm parameters and reproductive outcomes, suggesting modest improvements with supplementation. A strong-quality systematic review addressed selenium status and immune function, supporting a foundational role in immunity, while a dose-response meta-analysis assessed glycemic control with generally limited or inconclusive findings. An RCT in overweight individuals tested zinc plus selenium co-supplementation, complicating attribution of any effects to selenium alone. Several important limitations temper these findings. Most studies do not clearly characterize participants' baseline selenium status, which is critical — benefits are most likely in populations who are deficient, and supplementation in selenium-replete individuals may offer little benefit or even carry risk. Many studies used combined supplement protocols, making it difficult to isolate selenium's specific contribution. The evidence on cancer risk, athletic performance, photoprotection, chronic fatigue, and HIV was either preliminary or insufficiently specific to selenium to draw firm conclusions. Long-term safety data across diverse populations remain limited, and optimal dosing and formulation (e.g., selenomethionine vs. sodium selenite vs. selenium nanoparticles) are not well established in human trials.