43 yo male, low fsh, low lh, low testosterone, all other labs normal. what are possible causes?

2o hypogonadism: I assume you're asking on behalf of a male since you've described (presumably) yourself as 37yo female. In any case, also need to check prolactin as a prolactinoma can cause this situation. If any change in appearance to suspect acromegaly, also need to check growth hormone level. If either are high, then consider a pituitary mri. Also check iron. If testes injured, then lh & fsh are usually high.