Impotence is loss of sex drive (libido). Most cases are caused by overuse, and by physiological factors already discussed in Shy Breeders and Dogs Who Won’t Mate.
The male sex drive is under the influence of testosterone, produced by the testicles. A dog with a normal libido is easily aroused when presented with a bitch in standing heat. A dog with a poor libido exhibits little interest in sex, despite the overtures of a willing female. Lack of libido and a low sperm count occur together in dogs with testicular diseases, hypothyroidism, and pituitary gland disorders.
In rare cases, a Sertoli cell tumor of the testicle may be responsible for impotence. These tumors manufacture estrogen, which neutralizes the effects of testosterone. Feminization of the male occurs, with enlargement of the mammary glands, a pendulous foreskin, and bilateral symmetrical hair loss.