|
| |
It is well known that in several countries with a low iodine intake
the incidence and/or prevalence of thyroiditis is low. Actually, it cannot
be overlooked that some authors reported the differences between the prevalence
of AT in the countries with high iodine and low iodine intake. Thus, Robuschi
et al. (1987) found a 14 % prevalence of subclinical hypothyroidism in
Worcester (Massachussetts) and 0.6 % of that in Reggio Emilia (Italy).
Similarly, Herrmann (1981) reported a multicenter study from Germany (at
that time a low iodine-intake country) in which an incidence of subclinical
hypothyroidism only 4 % was found in 2579 senior citizens apparently using
RIA assay of TSH and TRH tests. However, even these findings do not contradict
the above subconclusions I/3 and II/1.
However, let us have a look at the incidence of AT in Japan, a country
where people have been eating megadoses of iodine for many years. Actually,
in respect to the situation in Japan, the WHO Statement says:
"In Japan, where dietary iodine intakes are high, it has been shown
that: * normal people who are not iodine-deficient can maintain normal
thyroid function states even at intakes of several milligrams of dietary
iodine/day; * the incidence of non-toxic diffuse goiter and toxic nodular
goiter is markedly decreased by high iodine intake; * the incidence of
Graves' disease and Hashimoto's disease does not appear to be affected
by high intakes of dietary iodine."
Based on some very recent data, the average urinary iodine was in the
range of 700-3200 µg/day (Nagataki, 1993) or up to 5600 µg/liter
out of 4110 subjects examined (Konno et al., 1993), but in several fishing
villages at the sea coast (1061 subjects examined) it was reported to be
even as high as 30,000 µg/liter, with about 85 % of values between
1200 and 12,000 µg/liter (Konno et al., 1994).
The next question is the prevalence and/or incidence of AT in Japan.
Actually, Inoue et al. (1975) examined the thyroid by palpation in 10,220
children and adolescents aged from 6 to 18 years from Kanazawa and Wajima
cities in the Ishikawa region. Palpable goiter was found in 441 of them.
Out of these, positive antithyroglobulin and antimicrosomal antibodies
were found in 43 cases and needle biopsy showed 30 cases of AT, out of
these 26 girls (0.8 % of all girls or 5.2/1000) and 4 boys (0.3 % or 3/1000,
resp.). However, when limited to adolescent girls only (i.e. from 16 to
18 years), it was 8.2/1000 out of 1965 cases. Okamura et al. (1989) found
88 cases (3.6 %) of elevated TSH (over 4.1 mU/l) among 2421 residents of
both sexes over 40 years of age. Later, Okayasu et al. (1991) analyzed
histologically the autopsy material from 1826 Japanese cases collected
from four institutions (Sapporo in Hokkaido, Tokyo and Nagano in Honshu
and Kurume in Kyushu island), and from 810 British cases (London), all
from 20 to more than 80 years old. The overall prevalence of lymphocytic
infiltration in British subjects was 42.5 % in females and 19.4 % in males,
while the respective values for Japanese subjects were significantly less,
being 22.2 % for females and 13.9 % for males. Recently, Konno et al. (1993)
reported the prevalence Hashimoto's thyroiditis of 13.11 % out of 1179
females and 6.15 % out of 2931 males, the mean age of all subjects being
45 + 10 years.
It may be concluded that, in general, the prevalence of AT in individual
age groups in Japan does not seem to be higher than that in USA and Europe.
Since a great majority of the cases of subclinical hypothyroidism (usually
defined as increased level of ultrasensitive TSH above 4-6 mU/l without
any apparent clinical signs of hypothyroidism) are very likely caused by
autoimmune thyroiditis, even the data on the prevalence of increased TSH
level will be presented below provided that they are closely related to
the prevalence of AT. In addition, predominantly more recent reports will
be selected in which ultrasensitive assay of TSH has been used.
Thus, concerning the US, in the Framingham study of 2139 unselected
elderly subjects, Sawin et al. (1985) found 10.3 % of elevated TSH levels.
In addition, Rosenthal et al. (1987) found 34 cases (13.2 %) of TSH over
4.1 mU/l among 258 subjects of both sexes over 60 years. Bagchi et al.
(1990) found 70 cases (7.3 %) of TSH over 6.0 mU/l among 968 subjects of
both sexes aged over 55.
In Europe, Parle et al. (1991 ) found 94 cases (7.6 %) of TSH over 5.0
mU/l among 1210 patients over 60 years registered with a single general
practice in Birmingham (England). In Sweden 50 cases (6.1 %) of increased
TSH over 4.0 mU/l were found among 823 women aged 40/65 (Wikland and Allgen,
1990) and, similarly, in Norway 15 cases of TSH over 4.5 mU/l (7.6 %) were
found out of 197 subjects of both sexes (Brochmann et al., 1988). In a
small Slovakian village, we found 20 cases (7.2 %) of TSH over 4.5 mU/l
out of 279 women aged over 35 (Hanzen et al., to be published). Recently,
Vanderpump et al. (1995) extended the original Whickham (England) study
by Tunbridge et al. (1977). Actually, after about 20 years they were able
to re-examine 97 % of the original sample of 2779 subjects. They found
9.3 % prevalence of hypothyroidism in both sexes (7.7 % in women).
Subconclusions III
- From the evidence published so far, it seems that the prevalence of
AT in Japan (with the range of iodine intake between about 1000-5000 µg
daily) is about the same as in USA (the range of iodine intake of about
500-1000 µg daily) or in several European countries with marginally
sufficient iodine intake, i.e. about 100-200 µg daily (England, Norway,
Sweden, Slovakia).
- So far, no reports have been published on the increasing incidence
of AT in Japan in spite of strikingly high iodine intake.
- From the above data it may be suggested that: (A) If AT occurs only
in genetically predisposed individuals and if high iodine intake plays
some role in its etiology, the actual prevalence of AT in middle-aged populations
of iodine sufficient countries (i.e. approximately 5-15 percent) might
show more or less the range or even the upper limit of global prevalence
of such predisposed individuals. (B) The reports on increasing incidence
of AT in the US within last 60-70 years might simply reflect the conversion
of that country from a low iodine-intake to high iodine-intake one.
|
|