Ganong's Review of Medical Physiology, 23rd Edition

EFFECTS ON
CARBOHYDRATE METABOLISM
Thyroid hormones increase the rate of absorption of carbohydrates
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 carbohydrate
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 stimulation
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 increased
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 skeletal
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 controlled
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 binding
to thyroid receptors. The liganded forms of thyroid receptors
are nuclear transcription factors that alter gene expression.
■ Thyroid hormones stimulate metabolic rate, calorigenesis, cardiac
function, and normal mentation, and interact synergistically
with catecholamines. Thyroid hormones also play critical roles in
development, particularly of the nervous system, and growth.
CHAPTER 20 The Thyroid Gland 313
■ 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 secretion
(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 pituitary
gland.
D) hypothyroidism due to a primary abnormality in the hypothalamus.
E) hyperthyroidism due to a primary abnormality in the hypothalamus.
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 T 4.
E) free T 3.
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