The thyroid gland is present in all vertebrates and it is unique among endocrine glands in that it stores its secretory products (the thyroid hormones) extracellularly. These hormones play an indispensable role in a variety of biochemical reactions at the level of peripheral tissues such as the skeletal and heart muscle, the liver and the kidney which collectively control the basal metabolic activity of the organism. Although many physiologic processes in ruminant require a normal activity of the thyroid gland, their role has generally been emphasized in the reproductive physiology. However, in most of the current literature, the synthesis and mechanism of action of the thyroid hormones in the ruminant physiology have been extrapolated of the extensive knowledge that we have in the canine and feline species. This work seeks to give up-to-date information on the endocrine physiology of the ruminant thyroid gland, emphasizing its role in the bovine and ovine species and the more common causes of clinic hypothyroidism in domestic ruminants.