orene) and any factor that causes hypothyroidismis
‘much higher.“ Radiation by either x-rays or I is
particularly effective since it simultaneously acts as
a carcinogen and, by impairing the functional capacity of the gland, also induces TSH secretion and
thyroid stimulation.
:
Several aspects of radiation damage may be considered. Cytoplasmic damage may result in enough
functional damage to result in cel] death, or repair
may occur if the damage is not too extensive. Injury
to nuclear material may result in death of the cell if
enough deoxynucleic acid (DNA) is altered so that
the process of its transformation for synthesis of
protein is seriously inhibited. Restoration of DNA
may occur after these damaging effects, as his been
so well studied after ultraviolet irradiation in bacteria.24 On the other hand, DNA that is not restored,
may not affect cell function if’ that portion of the
DNA is repressed and not undergoing transformation, but cell death may occur after division. It is
likely that that the same process of division in cells
carrying damaged DNA may be acritical factor in
such stimulation.
to
Figure 3. Marked Growth Retardation and Hypothyroidisian ina
Boy Exposed to Fallout al Eighteen’ Mouths of Lge.
The taller boy standing beside him is his brother, wha ix a year younger.
gulSde
Neg
radiation exposure has been amply demonstrated. A
series of retrospective and prospective studies have
clearly shown the causal relation of irradiation of
the thymic region in infants and the later development of thyroid cancer.?3-4 In addition, the development of adenoniatous lesions of the gland in chil-
DISCUSSION
Adenomas and cancers of the thyroid gland can
be produced in Jaboratory animals by a variety of
agents or regimens that interfere with the ability of
the gland to synthesize thyroid hormone. Iodine
deficiency,’ agents that chemically inhibit thyroxine
synthesis such as thiouracil,!’"'™ x-irradiation of the
gland,'-"7 and irradiation of the gland with ['')'8-2e
have all been shown to produce both benign and,
malignant neoplasms. Furthermore, subtotal thyroidectomy in the rat has been reported to cause adehoma formation in the remaining thyroid tissue two
years later???) All these procedures
pothyroidism of varying degree, which
effective stimulant to the secretion of
pituitary gland. Under the influence
Increased sensitivity of children's thyroid glands
the development of neoplastic changes from
produce hyserves as un
TSH by the
of TSH the
thyroid gland first undergoes diffuse hyperplasia and
hypertrophy and finally adenoma formation. In the
cases of iodine deficiency and partial thyroidectomy,
dren five to eleven years after treatment with ["! for
thyrotoxicosis has been reported.2*" Doniach*!
points out that cell division in the growing thyroid
gland of the child may be a factor in this increased
sensitivity to irradiation. On the average, 3 to 4 cell
divisions are assumed to occur as the infant land
rows from a size of about 1.5 gm. to a mature [7um. gland.? It thus seems reasonable to consider the
enhanced tumorovenic effect of irradiation in) children to be due to the necessity for division of cells
whose nuclei have previously been injured by raciation. The adult gland, on the other hand, would
not be as prone to such radiation effects, since cell
division is not believed to occur normally in the
mature gland (mitotic figures are usually not discernible).272 Any factor, therefore, that causes increased secretion of TSH and thyroid hyperplasia
will favor the development of neoplasia in a previously irradiated thyroid gland. Though the relation
it is probable that no drug or carcinogenic agent is
involved; also, the incidence of carcinomas after
between adenomatous and malignant changes is not
thyroid carcinoma after a combination of a carcinogenic agent (such as radiation or acetylaminoflu-
The occurrence of adenomatoid nodules and 1
carcinoma of the gkand in the Marshallese who re-
these treatments is relatively low. The incidence of
clear it is apparent that the same situations predispose to the development of both types of chanze.
.
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otrare
oN TeTy lS ay A eT
carcinogenesis although the mechanism is obscure.
Obviously, any factor that stimulates the thyroid
land to cause cell division will accelerate this
process.
Hypothyroidism produced by radiation
damage to some of the thyroid cells will result in