Thyroid hormones act on testis in multiple ways and exert their effect on different cell types, including Leydig and Sertoli cells, and germ cells. An excess or deficit of thyroid hormones results in alterations of testis function, including semen abnormalities. More frequently, hyperthyroidism has been associated with reduced semen volume and reduced sperm density, motility, and morphology, whereas hypothyroidism is associated with reduced sperm morphology. Therefore, thyroid function tests should be part of the diagnostic workup of the infertile man. This article is aimed at (1) elucidating how hyperthyroidism and hypothyroidism lead to a reduction in semen quality, briefly reviewing the current literature on murine models and humans, and (2) pinpointing the limitations of the studies carried out so far and identifying new perspectives for future research.