301

CHAPTER

SECTION IV ENDOCRINE &

REPRODUCTIVE PHYSIOLOGY

20

The Thyroid Gland

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the structure of the thyroid gland and how it relates to its function.

■

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.

■

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.

INTRODUCTION

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 hor-

mones stimulate O
2
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 hypofunc-

tion during fetal and neonatal life results in severe mental retarda-

tion and dwarfism. In adults, hypothyroidism is accompanied by

mental and physical slowing and poor resistance to cold. Con-

versely, excess thyroid secretion leads to body wasting, nervous-

ness, 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 hor-

mone 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 calcito-

nin, a hormone that regulates circulating levels of calcium. This

function of the thyroid gland is discussed in Chapter 23 in the

broader context of whole body calcium homeostasis.