Diseases of Thyroid in Animals and Their Management

Disorders of the thyroid gland are well known in companion animals but less so in livestock. In livestock, nutritional iodine deficiencies have been of greater importance than thyroid-gland diseases, particularly in the iodine-deficient areas. Thyroid hormones have many functions in the body and, in general, regulate growth, differentiation, and the metabolism of lipids, proteins and carbohydrates. The thyroid gland of animals is a bilobed structure that overlays the trachea at a point just below the larynx. Anatomical varia‐ tions of the gland are quite marked between species and, to some extent, within a given species. The isthmus connecting the two lobes of the thyroid is the region that varies most markedly between species. Humans and the pig have a large discrete isthmus that forms a pyramidal lobe connecting the two lateral lobes. The cow has a fairly wide band of glandular tissue that forms the connecting isthmus. In the horse, sheep, goat, cat, and dog, the isthmus is a narrow remnant of tissue and may be nonexistent. The size of the gland relative to body weight is extremely small in all animals, approximating 0.20% of body weight. Accessory or extrathyroidal tissue is quite commonly seen in the dog, particular‐ ly near the thoracic inlet, though it may be found anywhere along the esophagus. This tissue is fully functional physiologically, synthesizes hormone, and can be located by its uptake of radionuclides. The thyroid gland is a highly vascularized tissue with a large blood flow. The functional unit of the thyroid gland is the thyroid follicle, a spherical structure composed of an outer monolayer of follicular cells surrounding an inner core of colloid, the thyroglobulin-hormone complex, which is the storage reservoir of thyroid hormone. The colloid stored in the lumen is a clear, viscous fluid. The individual follicu‐ lar cells vary from 5 to 10/zm in height and the entire follicle may vary from 25 to 250/zm in diameter. The size of the follicles and the height of their cells vary according to the functional state of the gland. The cells may vary from an inactive squamous cell to the highly active, tall columnar cell. Interspersed between the follicles are the thyroid C cells,