After studying this chapter, you should be able to:
Describe the structure of the thyroid gland and how it relates to function.
Define the chemical nature of the thyroid hormones and how they are synthesized.
Understand the role of iodine in the thyroid gland and how its transport is controlled.
Describe the role of protein binding in the transport of thyroid hormones and peripheral metabolism.
Identify the role of the hypothalamus and pituitary in regulating thyroid function.
Define the effects of the thyroid hormones in homeostasis and development.
Understand the basis of conditions where thyroid function is abnormal and how they can be treated.
The thyroid gland has two primary functions. The first is to secrete the thyroid hormones, which maintain optimal levels of tissue metabolism. Thyroid hormones stimulate O2 consumption by most cells, help regulate lipid and carbohydrate metabolism, and influence body mass and mentation. Consequences of thyroid gland dysfunction depend on life stage. The thyroid is not essential for life, but 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) of the anterior 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.
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.
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. The gland is well vascularized, with one of the highest rates of blood flow per gram of tissue of any organ.
The portion of the thyroid concerned with the production of thyroid hormone consists of multiple acini (follicles) surrounded by a single layer of polarized epithelial cells and filled with material called colloid. Colloid consists predominantly of the glycoprotein, thyroglobulin. Microvilli project into the colloid from the apexes of the thyroid cells and canaliculi extend into them. The endoplasmic reticulum is prominent, a feature common to most glandular cells, and secretory granules containing thyroglobulin are seen. The individual thyroid cells rest on a basal lamina that separates them from the adjacent fenestrated capillaries.
FORMATION & SECRETION OF THYROID HORMONES
The primary hormone secreted by the thyroid is thyroxine (T4), along with much lesser amounts of triiodothyronine (T3). T3 has much ...