Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 20The Thyroid Gland 313

EFFECTS ON

CARBOHYDRATE METABOLISM

Thyroid hormones increase the rate of absorption of carbohy-
drates from the gastrointestinal tract, an action that is probably
independent of their calorigenic action. In hyperthyroidism,
therefore, the plasma glucose level rises rapidly after a carbohy-
drate meal, sometimes exceeding the renal threshold. However,
it falls again at a rapid rate.

EFFECTS ON

CHOLESTEROL METABOLISM

Thyroid hormones lower circulating cholesterol levels. The
plasma cholesterol level drops before the metabolic rate rises,
which indicates that this action is independent of the stimu-
lation of O 2 consumption. The decrease in plasma cholesterol
concentration is due to increased formation of low-density
lipoprotein (LDL) receptors in the liver, resulting in in-
creased hepatic removal of cholesterol from the circulation.
Despite considerable effort, however, it has not been possible
to produce a clinically useful thyroid hormone analog that
lowers plasma cholesterol without increasing metabolism.

EFFECTS ON GROWTH

Thyroid hormones are essential for normal growth and skele-
tal maturation (see Chapter 23). In hypothyroid children,
bone growth is slowed and epiphysial closure delayed. In the
absence of thyroid hormones, growth hormone secretion is
also depressed. This further impairs growth and development,
since thyroid hormones normally potentiate the effect of
growth hormone on tissues.

CHAPTER SUMMARY

■ The thyroid gland transports and fixes iodide to amino acids

present in thyroglobulin to generate the thyroid hormones

thyroxine (T 4 ) and triiodothyronine (T 3 ).

■ Synthesis and secretion of thyroid hormones is stimulated by

thyroid-stimulating hormone (TSH) from the pituitary, which

in turn is released in response to thyrotropin-releasing hormone

(TRH) from the hypothalamus. These releasing factors are con-

trolled by changes in whole body status (eg, exposure to cold or

stress).

■ Thyroid hormones circulate in the plasma predominantly in

protein-bound forms. Only the free hormones are biologically

active, and both feed back to reduce secretion of TSH.

■ Thyroid hormones exert their effects by entering cells and bind-

ing to thyroid receptors. The liganded forms of thyroid recep-

tors are nuclear transcription factors that alter gene expression.

■ Thyroid hormones stimulate metabolic rate, calorigenesis, cardi-

ac function, and normal mentation, and interact synergistically

with catecholamines. Thyroid hormones also play critical roles in

development, particularly of the nervous system, and growth.

■ Disease results with both under- and overactivity of the thyroid

gland. Hypothyroidism is accompanied by mental and physical

slowing in adults, and by mental retardation and dwarfism if it

occurs in neonatal life. Overactivity of the thyroid gland, which

most commonly is caused by autoantibodies that trigger secre-

tion (Graves disease) results in body wasting, nervousness, and

tachycardia.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. In which of the following conditions is it most likely that the
   TSH response to TRH will be reduced?
   A) hypothyroidism due to tissue resistance to thyroid hormone
   B) hypothyroidism due to disease destroying the thyroid gland
   C) hyperthyroidism due to circulating antithyroid antibodies
   with TSH activity
   D) hyperthyroidism due to diffuse hyperplasia of thyrotropes of
   the anterior pituitary
   E) iodine deficiency


2. A young woman has puffy skin and a hoarse voice. Her plasma
   TSH concentration is low but increases markedly when she is
   given TRH. She probably has
   A) hyperthyroidism due to a thyroid tumor.
   B) hypothyroidism due to a primary abnormality in the thyroid
   gland.
   C) hypothyroidism due to a primary abnormality in the pitu-
   itary gland.
   D) hypothyroidism due to a primary abnormality in the hypo-
   thalamus.
   E) hyperthyroidism due to a primary abnormality in the hypo-
   thalamus.


3. The enzyme primarily responsible for the conversion of T 4 to T 3
   in the periphery is
   A) D 1 thyroid deiodinase.
   B) D 2 thyroid deiodinase.
   C) D 3 thyroid deiodinase.
   D) thyroid peroxidase.
   E) none of the above


4. The metabolic rate is least affected by an increase in the plasma
   level of
   A) TSH.
   B) TRH.
   C) TBG.
   D) free T4.
   E) free T3.


5. Which of the following is not essential for normal biosynthesis of
   thyroid hormones?
   A) iodine
   B) ferritin
   C) thyroglobulin
   D) protein synthesis
   E) TSH