Human Physiology, 14th edition (2016)

Endocrine Glands 343

Because of its stimulation of protein synthesis, children

need thyroxine for body growth and, most importantly, for the

proper development of the central nervous system. The need

for thyroxine is particularly great when the brain is undergoing

its greatest rate of development—from the end of the first tri-

mester of prenatal life to six months after birth. Hypothyroid-

ism during this time may result in cretinism. Unlike people

with dwarfism, who have inadequate secretion of growth hor-

mone from the anterior pituitary, people with cretinism suffer

severe mental retardation. Treatment with thyroxine soon after

birth, particularly before one month of age, has been found to

completely or almost completely restore development of intel-

ligence as measured by IQ tests five years later.

also exerts a trophic (growth-stimulating) effect on the thyroid.
This trophic effect is evident in people who develop an iodine-
deficiency (endemic) goiter, or abnormal growth of the thyroid
gland ( fig.  11.24 ). In the absence of sufficient dietary iodine,
the thyroid cannot produce adequate amounts of T 4 and T 3. The
resulting lack of negative feedback inhibition causes abnor-
mally high levels of TSH secretion, which in turn stimulates the
abnormal growth of the thyroid. These events are summarized in
figure 11.25.
People who have inadequate secretion of thyroid hor-
mones are said to be hypothyroid. As might be predicted from
the effects of thyroxine, people who are hypothyroid have an
abnormally low basal metabolic rate and experience weight
gain and lethargy. A thyroxine deficiency also decreases the
ability to adapt to cold stress. Severe hypothyroidism in adults
can result in myxedema, in which mucoproteins (glycosami-
noglycans) and fluid accumulate in the subcutaneous connec-
tive tissues and viscera, producing edema that causes swelling
of the hands, feet, face, and tissue around the eyes. Severe
hypothyroidism produces a slowing of physical and mental
activity, and can result in profound lethargy and even a myx-
edema coma.
Hypothyroidism can result from a thyroid gland defect or
secondarily from insufficient thyrotropin-releasing hormone
(TRH) secretion from the hypothalamus, insufficient TSH
secretion from the anterior pituitary, or insufficient iodine in
the diet. In the latter case, excessive TSH secretion stimulates
abnormal thyroid growth and the development of an endemic
goiter, as described previously. The hypothyroidism and goi-
ter caused by iodine deficiency can be reversed by iodine
supplements.

Figure 11.24 Endemic goiter is caused by
insufficient iodine in the diet. A lack of iodine causes
hypothyroidism, and the resulting elevation in TSH secretion
stimulates the excessive growth of the thyroid.

Figure 11.25 How iodine deficiency causes a

goiter. Lack of adequate iodine in the diet interferes with the

negative feedback control of TSH secretion, resulting in the

formation of an endemic goiter.

Sensor

Integrating center

Effector

Hypothalamus

TRH

TSH

Thyroid

Normal

thyroid

If iodine

inadequate

Low negative

feedback

Anterior

pituitary

Excess TSH

Thyroid Growth

(goiter)

Hypertrophy—

produces goiter

If iodine

adequate

Low

T 3 and T 4 T^3 and T^4

Anterior

pituitary

Negative

feedback