12 Thyroid disease Garry McDowell Learning objectives After studying this chapter you should be able to: ■ Describe the structure and function of the thyroid gland ■ Explain the function of thyroid hormones ■ Outline the action of thyroid hormones and control of their secretion from the thyroid gland ■ Describe the conditions which lead to abnormal thyroid hormone production ■ Discuss the investigation of suspected thyroid dysfunction Introduction The thyroid gland secretes thyroid hormones that are required for normal metabolism of body cells. Disorders of thyroid function can result in either inadequate or excess production of thyroid hormones causing altered cellular metabolism and development of associated clinical features. This chapter will describe the nature and role of thyroid hormones, their regulation in the blood and the consequences of changes in their secretion. The value of laboratory investigations in diagnosis and monitoring of treatment will be discussed. 12.1 Structure of the thyroid gland The thyroid gland is found below the larynx and is a butterfly shaped gland composed of a right and left lobe on either side of the trachea. Both lobes are joined by an isthmus in front of the trachea. The normal thyroid gland weighs approximately 30 g and is highly vascularized, receiving 80–120 mL of blood per minute, as shown in Figure 12.1. 12.1 STRUCTURE OF THE THYROID GL AND 321 Hyoid bone Common carotid artery Thyroid cartilage of larynx Internal jugular vein Right lateral lobe of thyroid gland Isthmus of thyroid gland Left lateral lobe of thyroid gland Trachea Clavicle Sternum FIGURE 12.1 Anatomical location of the thyroid gland in the neck. Microscopic examination of thyroid tissues shows small spherical sacs called thyroid follicles that make up most of the thyroid gland. The wall of each follicle is composed mainly of follicular cells, most of which extend to the lumen of the follicle. Figure 12.2 shows the structure of thyroid follicles. Follicular cell FIGURE 12.2 Follicle containing thyroglobulin Histological structure of the thyroid gland showing the follicles in which thyroid hormones are made. Courtesy of Dr A L Bell, University of New England College of Osteopathic Medicine, USA.
