Thyroid hormones have an important function in numerous physiological processes in humans and animals, including the regulation of metabolism, growth and development of the individual and thus also the reproductive function. The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), have a direct effect on the reproductive organs and an indirect effect in interaction with other hormones. Thyroid hormones influence the reproductive system by regulating the metabolism and tissue development of the ovaries, uterus and placenta. Changes in the serum concentrations of T3 and T4 lead to disturbances in the function of the entire organism. Thyroid dysfunction, including hypothyroidism and hyperthyroidism, can lead to significant reproductive problems. The aim of this review is therefore to describe the functions of thyroid hormones and the effects of hypothyroidism and hyperthyroidism on the reproductive system of animals with special reference to dogs and cats. Hypothyroidism, which is often caused by autoimmune diseases such as lymphocytic thyroiditis, is associated with later puberty, reduced fertility and abnormalities in the development of the reproductive organs. Hypothyroidism can lead to a prolonged interval, absent cycles, silent estrus cycles, prolonged estrus bleeding and a lack of libido with the occurrence of infertility, miscarriages, stillbirths and mummified fetuses. In males, hypothyroidism leads to a decrease in libido, semen quality, ejaculate volume, testicular atrophy, hypospermia and azoospermia. Hyperthyroidism, on the other hand, is usually the result of thyroid cancer, which produces excessive amounts of thyroid hormones. Increased concentrations of thyroid hormones lead to disturbances in the physiological balance of reproductive hormones, irregular oestrus cycles, anovulation and reduced fertility as well as a decrease in the weight of the ovaries and the number of healthy follicles with a simultaneous increase in the number of atretting follicles. The effects of hyperthyroidism on male fertility include disorders of spermatogenesis, changes in sex hormone levels and changes in sperm quality, such as hypospermia, oligozoospermia, asthenozoospermia and teratozoospermia. It is important to emphasize the importance of timely diagnosis and treatment of thyroid disorders to prevent negative effects on fertility and reproduction in animals.