Testosterone is the main sex hormone in men and plays a big role in sexual arousal
(Estrogen is the primary female hormone)

Testosterone and ED go hand in hand because testosterone is not only a necessary ingredient for libido, it also has a role in maintaining nitric oxide levels in the penis which raises blood supply (vasodilation), lowers blood pressure and relaxes vascular smooth muscle

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If you have low testosterone levels you may experience erectile dysfunction along with low libido and other side effects associated with low levels of this hormone. 
 

However it’s far from being the slam dunk ED cause.

You can have low T and yet have no problems with erectile function,

or your testosterone levels may be quite high and you could be impotent. 

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ED is usually caused by low blood flow to the penis, is commonly associated with other issues such as

cardiovascular disease

diabetes

high blood pressure

high cholesterol

hardening of the arteries

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Low testosterone can put a significant damper on your sex drive, but it rarely causes ED.

https://prostate.net/health-centers/erectile-dysfunction/causes-erectile-dysfunction/low-t-blame/