8 - Thyroid Function

Abstract

This chapter reviews the anatomy, physiology, and diseases of the thyroid as a corollary to the understanding of thyroid function. The thyroid gland of animals is a bilobed structure, which overlays the trachea and is located just below the larynx. The thyroid gland is a highly vascularized tissue with a rapid blood flow. The glandular tissue of the thyroid is composed of follicles whose lumens contain a thick clear fluid, the colloid. The size of the follicles and their cells vary according to the functional state of the gland. The cells can vary from the less active squamous type to the highly active tall columnar cells. The thyroid gland is unique among the endocrine glands in that an integral part of its hormone, L-thyroxine, is a micronutrient, iodine, which is available to the animal in only limited amounts. A second major site of action of thyroid stimulating hormone is to stimulate the release of thyroid hormones from the gland. Goiter may be defined as any enlargement of the thyroid gland, which is not because of inflammation or malignancy. There are two general types of goiters: (1) nontoxic goiters, which produce normal amounts of hormone or less than normal amounts of hormone, and (2) toxic goiters, which are characterized by excessive production of hormone. Hypothyroidism may be the result of a variety of causative factors. Iodine deficiency is a well-known cause of goiter.