After studying this chapter, you should be able to:
Describe the development, anatomy, and histology of the thyroid gland, and how these relate to its function, as well as the changes in histology that accompany the activation of secretion with reabsorption of thyroglobulin from the colloid.
Define the chemical nature of the thyroid hormones and how they are synthesized. Understand the critical role of iodine in the thyroid gland and how its transport is controlled, as well as how it is incorporated into the tyrosine residues of thyroglobulin at the interface between thyrocytes and colloid via the process of organification.
Describe the relative roles of binding to albumin, transthyretin, and thyroxine-binding globulin in the transport of thyroid hormones, the availability of free hormones, and peripheral metabolism.
Identify the role of the hypothalamus and pituitary in regulating thyroid function and the feedback loops that establish the level of secretion of thyroid hormones.
Define the effects of the thyroid hormones in homeostasis, metabolism, and growth.
Understand the basis of conditions where thyroid function is abnormal and how they can be treated.
The thyroid gland is one of the larger endocrine glands of the body. The gland has two primary functions. The first is to secrete the thyroid hormones, which maintain the level of metabolism in the tissues that is optimal for their normal function. Thyroid hormones stimulate O2 consumption by most of the cells in the body, help regulate lipid and carbohydrate metabolism, and thereby influence body mass and mentation. Consequences of thyroid gland dysfunction depend on the life stage at which they occur. The thyroid is not essential for life, but its absence or hypofunction during fetal and neonatal life results in severe mental retardation and dwarfism. In adults, hypothyroidism is accompanied by mental and physical slowing and poor resistance to cold. Conversely, excess thyroid secretion leads to body wasting, nervousness, tachycardia, tremor, and excess heat production. Thyroid function is controlled by the thyroid-stimulating hormone (TSH, thyrotropin) that is secreted by the anterior lobe of the pituitary. The secretion of this hormone is in turn increased by thyrotropin-releasing hormone (TRH) from the hypothalamus and is also subject to negative feedback control by high circulating levels of thyroid hormones acting on the anterior pituitary and the hypothalamus.
The second function of the thyroid gland is to secrete calcitonin, a hormone that regulates circulating levels of calcium. This function of the thyroid gland is discussed in Chapter 21 in the broader context of whole body calcium homeostasis.
DEVELOPMENT, STRUCTURE, AND CELL TYPES OF THE THYROID GLAND
The thyroid is a butterfly-shaped gland that straddles the trachea in the front of the neck. It develops from an evagination of the floor of the pharynx, and a thyroglossal duct marking the path of the thyroid from the tongue to the neck ...