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I don't believe there is a specific cause because there will always be several, measurable, causal factors. But if are going to zoom way out and pick one thing, it's body weight. Increased physical activity is going to increase free Test and reduce the likelihood of being overweight.

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Causality is complex here. Obesity reduces testosterone levels, but low-T also causes adipose tissue growth in a positive feedback loop. That's one of the reasons why some obese people have so much trouble losing weight.

Exposure to endocrine disruptor chemicals such as pthalates is also proven to reduce testosterone levels.

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