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The
thyroid gland is the most vulnerable organ in respect to radiation and
has suffered most from the Chernobyl disaster. The gland has manifested
pathological effects specifically clearly after a significant period. In
particular, the diseases of the thyroid gland in 1990 in the Gomel region
amounted to 126.1 cases per 100,000 with the rise in 1995 to 1,154.5 cases,
meanwhile among children these cases numbered 3106.1 per 100,000. These
figures are still higher in the areas proportionally denser radioactive
levels amounting to 4,056.9 generally in the Braguin district and to 1,9072.6
among the children. The leading factor of internal irradiation has been
'~'I with the collective dose for the thyroid gland amounting to 22,000
(R.I. Halitov, et al., 1993). Exactly this radioisotope has led to substantial
disorders in the areas adjacent to Chernobyl. In particular, examinations
of the individuals after their involvement in the cleaning-up operations
have demonstrated the compensated condition of the thyroid system just
among 35.6%, hyperthyreosis among 39.7%, hypothyreosis among 24.7% (V.N.
Petrov, N.M. Petrov, 1993). Dr. O.V. Kopylova and her co-researchers have
obtained similar results showing the condition of hyperthyrexinemia among 40.0% of
the children in the areas of radiation risk, other medical researchers
(N.B. Pashinskaja et al., 1991) also report a suppressed function of the
thyroid gland among 37.904 of the children.
G.S.
Bandazhevskaya (1966) registered a higher concentration of thyroxin in
the blood among children aged 3-7 years old in 1995 in Gomel (1-5 Ci
per km2 of 137Cs) compared with the control having
a directly proportional relation with the concentration of incorporated
radioisotopes (Fig. 11, 12). Meanwhile the concentration of triiodine thyronin
is much below the control level (Fig. 13). E.L.
Strukov et al. (1994)
believe that this condition accompanies a complicated evolution of diseases
of the cardiovascular and digestive systems.
It
should be remarked that the above hyperthyrexinemia commonly evolves without
any pronounced clinical manifestations (E.V.
Epshtein et al., 1993). A somewhat higher level of triiodine thyronin in
the blood with a simultaneous reduction of the secretion of thyreotropin
was observed among the children and adolescents during the first three
years after Chernobyl indicating a functional activation of the thyroid
system (A.F. Tsyb et al., 1991). The intricate metabolism of thyroid hormones
should be taken into consideration. The basic hormone of the thyroid gland
of a normal
man is T4. Less than 2004 of the total
zero of T3 is produced directly by the thyroid gland, 80-9004
result from the monodeiodinizing of T4 in peripheral tissues.
The liver and kidneys are the main transformers of T4 into T3
(I.I. Dedov et al., 1992). The basic effects of the thyroid hormones
are determined by T3. Small doses of radiation lead to suppressed
secretion of the thyroxin binding globulin, a higher total concentration
of thyroxin in the blood would occur versus the reduced concentration of
progesterone and prolactin among control pregnant
women (weeks 12-32 of pregnancy) (N.S. Akulich et al., 1990). Individuals
with a dose burden 400 Gy upon the, thyroid gland become predisposed to
hypothyreosis (L.N.. Astakhova et al., 1993).
In
addition to the altered hormonal secretion by the thyroid gland, structural
modifications have been detected proving the evolution of the gland's pronounced
pathology. The ultrasound screening of girls aged 6-18 years who stayed
within the 30 km zone during the explosion revealed changes in their thyroid
glands typical for hypoplasia and moderate thyreofibrosis (I.V. Vovk et
al., 1992).
Pathoanatomical investigations of the thyroid glands of fetuses and children in the Gomel region who died of various diseases, have revealed morphological manifestations of a stronger functional activity of the gland, such as noticeable stimulation of the processes of desquamation of thyrocytes, reduction of the relative volume of the thyroid epithelium in the gland due to cytolysis, valid reduction of the sizes of follicles followed by their collapse and substitution with the connective tissue stroma (E.D. Cherstvoj et al., 1993).
Experimental
studies corroborate the results of clinical observations. In particular,
intra-abdominal administration of 500 kBq of 131I would alter
the calcium-phosphorus metabolism and its hormonal regulation among rats
and cause a shift in the production of thyroid hormones (I.M. Bagel et
al., 1990). In addition to 131I, significant changes
in the thyroid gland can be induced by repeated external gamma irradiation
(1 Gy) leading to stable structural and functional disorders among laboratory
animals. In the long term these disorders result in the appearance of hypothyreosis
demonstrated by the reduced concentration of thyroxin in the blood, stimulation
of the thyreotropic function of the hypophysis, inhibited activity of -glycerophosphate
dehydrogenase as a thyroxin dependent enzyme in the liver (J.H. Turakulov
et al., 1992).
The
incidence of the thyroid nodular pathology among the children in the Gomel
Region exceeds 15%, including 2.54% thyroid cancers ((L.N.. Astakhova et
al., 1993).
The
Kaluga Region manifests similar results where nodular goiters have been
increasing since 1994, together with thyroid cancers (follicular and papillary
forms). The dose of radioactive iodine absorbed by the thyroid gland varied
from 25.6 to 169 cGy (E.G. Matvienko et al., 1996).
Nevertheless,
the children in the Khojniki district (aged 0-17 years) show the highest
doses of irradiation of the thyroid gland (242-527 cGy) versus 15
to 60 cGy in Gomel, 2.4 to 9.2 cGy in Minsk. The children which were aged
under five years at the moment of the disaster received the maximum doses
(I.I. Dedov et al., 1992).
This
effect of radioactive iodine resulted in more frequent thyroid disorders,
specifically thyroid cancers, nodular goiters, autoimmune thyreoditis.
A sharp rise in the number of thyroid cancers occurred in the Republic
of Belarus on the fifth year after Chernobyl. During the last decade 422
cases have been registered among children and 3,492 among adults and adolescents
(I.M. Drobyshevskaja et al., 1996). Thyroid cancers are most frequent among
adolescents in the Khojniki district.
The
clinical and morphological analysis of thyroid cancers among the children
in Belarus has revealed several features of the diseases:
1.
A relatively short latent period between the probable cause factor and
the clinical manifestation of tumors.
2.
Domination of capillary carcinomas with manifestations of reduced histological
differentiation having a pronounced localization and a high frequency of
metastasizing (A.V. Furmanchuk et al., 1992; L.S. Baleva, E.E. Karneeva,
1996).
The
pathogenesis of the
above pathological conditions is primarily due to the initial effect of
131I. It has been noted that the disaster led to the release
of 20 to 30% into the atmosphere from 60-70 million Curies of 131I
contained in the reactor (G.A. Gerasimov, 1991). The "iodine impact" severely
injured the structural components of the thyroid tissue. Taking into consideration
the short life of 131I a period of repair -restoration followed,
when the immunity system would play a paramount role of controlling the
processes of proliferation and differentiation of the follicular epithelium
and the neighboring cells.
Apart
from 131I, the radioactive
releases created conditions when other long living radioisotopes would
affect continuously the human organism, specifically 137Cs and
90Sr. They would modify the functions of many vital systems
and tissues of the organism, including the immunity system and the thyroid
cells which would accumulate them intensively (Yu.I. Bandazhevsky, A.M.
Perepletchikov, 1996). It would lead to the damage of the most essential
ultrastructures of the cells, modification of the antigen determinants
on the surfaces of the cytoplasm membrane.
The
result is that the structural components of thyroid cells become antigens
for the immunity system. An immunological response appears when the cellular
structures of the thyroid gland are damaged by the autoantibodies and immunocompetent
cells, finally leading to the appearance of autoimmune thyreoditis and,
after that, in a number of cases, to thyroid cancers. A relationship has
been established between the radiation burden upon the thyroid gland, the
populations of antigens and the microsomal fractions of the thyroid gland
(A.M. Poverennyi et al., 1996).
The
reactions between immunoglobulins and thyroid hormones, such as thyroxin,
should be taken into consideration in the pathogenesis of thyroid diseases.
It has been established that immunoglobulins of different classes (J, A,
M) are capable to bind thyroxin, triiodine thyronin (O.V. Sviridov et al.,
1992). The children in Gomel have manifested correlation between the concentration
of Ig and the level of T4, while the children in the control areas have
manifested none.
When
the hormones are bound by immunoglobulins, they are eliminated from the
metabolic chain and naturally the functions of the hypophysis - thyroid
system become upset.
The
processes lead to the liberation of significant quantities of thyreotropic
hormone enhancing the pressure upon the thyroid gland and boosting proliferation
of the follicular epithelium creating conditions for neoplastic transformations.
