The normal level of testosterone at the age of forty should be 9.45~37.5nmol/L. Testosterone is very important for men, it is mainly a steroid hormone hormone secreted by male testes, the normal value range is usually 9.45~37.5nmol/L, if it is lower than 9.45nmol/L it is an abnormal performance. Testosterone maintains male secondary sex characteristics as well as promotes spermatogenesis. If the testosterone level is low in men, it will lead to male secondary sex characteristics, lack of development or low spermatogenesis. However, if it exceeds 37.5nmol/L it is also abnormal and may be caused by testicular mesenchymal stromal cell tumor, idiopathic male precocious puberty, and so on. If the testosterone level is not normal at the age of forty, you should go to the hospital in time to identify the cause and then targeted treatment.