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The thyroid follicles secrete thyroxine, also called tetraiodothy-ronine (T4), in response to stimulation by thyroid-stimulating hormone (TSH) from the anterior pituitary (chapter 11; see fig. 11.23). The thyroid also secretes smaller amounts of tri-iodothyronine (T3) in response to stimulation by TSH. Almost all organs in the body are targets of thyroxine action. Thyroxine itself, however, is not the active form of the hormone within the target cells; thyroxine is a prehormone that must first be converted to triiodothyronine (T3) within the target cells to be active (chapter 11). Acting via its conversion to T3, thyroxine (1) regulates the rate of cell respiration and (2) contributes to proper growth and development, particularly during early childhood.

Thyroxine (via its conversion to T3) stimulates the rate of cell respiration in almost all cells in the body—an effect believed to be due to a lowering of cellular ATP concentrations. This effect is produced by (1) the production of uncoupling proteins (as in brown fat, discussed previously); and (2) stimulation of active transport Na+/K+pumps, which serve as an energy sink to further lower ATP concentrations. As discussed in chapter 4, ATP exerts

Adrenal cortex

Glucocorticoids (e.g., cortisol)

Where Are Glucocorticoids Produced

fatty CoA acid acids acids J J

Ketone Glucose bodies

Blood f Free t Ketone t Glucose f Amino fatty bodies acids acids

■ Figure 19.15 The metabolic effects of glucocorticoids. The catabolic actions of glucocorticoids help to raise the blood concentration of glucose and other energy-carrier molecules.

an end-product inhibition of cell respiration, so that when ATP concentrations decrease, the rate of cell respiration increases.

Much of the energy liberated during cell respiration escapes as heat, and uncoupling proteins increase the proportion of food energy that escapes as heat. Since thyroxine stimulates the production of uncoupling proteins and the rate of cell respiration, the actions of thyroxine increase the production of metabolic heat. The heat-producing, or calorigenic (calor = heat) effects of thyroxine are required for cold adaptation.

The metabolic rate under carefully defined resting conditions is known as the basal metabolic rate (BMR). This rate of basal metabolism has two components—one that is independent of thyroxine action and one that is regulated by thyroxine. In this way, thyroxine acts to "set" the BMR. The BMR can thus be used as an index of thyroid function. Indeed, such measurements were used clinically to evaluate the condition of the thyroid prior to the development of direct chemical determinations of T4 and T3 in the blood. A person who is hypothyroid may have a basal O2 consumption about 30% lower than normal, while a person who is hyperthyroid may have a basal O2 consumption up to 50% higher than normal.

A normal level of thyroxine secretion is required for growth and proper development of the central nervous system in children. This is why hypothyroidism in children can cause cretinism (see fig. 11.27). The symptoms of hypothyroidism and hyperthyroidism in adults are compared in table 11.8, p. 311.

Regulation of Metabolism

A normal level of thyroxine secretion is required in order to maintain a balance of anabolism and catabolism. For reasons that are incompletely understood, both hypothyroidism and hyperthyroidism cause protein breakdown and muscle wasting.

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Your Metabolism - What You Need To Know

Your Metabolism - What You Need To Know

If you have heard about metabolism, chances are it is in relation to weight loss. Metabolism is bigger than weight loss, though, as you will learn later on. It is about a healthier, better you. If you want to fire up your metabolism and do not have any idea how to do it, you have come to the right place. If you have tried to speed up your metabolism before but do not see visible results, you have also come to the right place.

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  • cameron
    Does thyroxin regulate cell respiration?
    8 years ago
  • lisa
    Why is thyroxine needed for growth and development?
    8 years ago
  • Sirja
    Does thyroxin regulate the rate of cellular respiration?
    8 years ago
  • Proserpina
    Is thyroxine a stimulant of cellular metabolisma nd targets all cells?
    8 years ago
  • girma
    Does thyroxine stimulate respiration?
    4 years ago
  • niina
    Does thyroxine increase respiration?
    2 years ago
  • rudolph
    How does thyroxine effect cellular respiration?
    12 months ago

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