Essentials of Anatomy and Physiology

tioning of the brain, muscles, heart, and virtually all
other organs. Although thyroxine and T 3 are not vital
hormones, in that they are not crucial to survival, their
absence greatly diminishes physical and mental
growth and abilities (see Box 10–2: Disorders of
Thyroxine).
Secretion of thyroxine and T 3 is stimulated by
thyroid-stimulating hormone (TSH) from the ante-
rior pituitary gland (see also Fig. 1–3). When the

metabolic rate (energy production) decreases, this

change is detected by the hypothalamus, which

secretes thyrotropin releasing hormone (TRH). TRH

stimulates the anterior pituitary to secrete TSH,

which stimulates the thyroid to release thyroxine and

T 3 , which raise the metabolic rate by increasing

energy production. This negative feedback mecha-

nism then shuts off TRH from the hypothalamus until

the metabolic rate decreases again.

232 The Endocrine System

Thyroid gland

T and T 4 3

Increase

cell respiration of

all foods

Increase

protein synthesis

Glucose

Fats

Excess

amino acids

ATP

Bone and muscle

Liver and viscera

Brain

Reproductive

organs

Figure 10–5. Functions of thyroxine and T 3.

QUESTION:Which functions of thyroxine help bones and muscles to grow and maintain

their own functions?