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[Discussion]
Effects of PCBs on thyroid functions are studied chiefly for
highly exposed populations such as yusho patients and
occupationally exposed persons. While many reports affirm some
or other effects of PCBs on thyroid functions, well-designed
studies (cohort studies or nested case-control studies) are not
many: it is difficult to evaluate statistical significance of
the results reported. Apart from endocrine disruption via
binding of PCBs to hormone receptors, (a) manifestation of
latent thyroid autoimmune abnormality and (b) action on the
thyroid gland (as histologic enlargement observed in rats),
particularly enlargement of thyroid epithelial cells, which
prompts antigen presentation resulting in new autoimmune
abnormality, should not be excluded as potential mechanism of
action. While infants and lactating mothers have often been
studied, effects of PCBs on the general public are still
unknown. This gap must be filled since the persistence of PCBs
cannot be neglected in healthy people.
In addition to papers on PCB, studies on HCB exist, but most of
them are chronic studies and do not permit evaluation of
significant effects on the thyroid functions. In short, findings
on the association of chemical substance with PCBs are still
scant; evaluation of causality is impossible. Suspected effects
of HCB on the thyroid functions indicate the need of study on
other organochlorine compounds.
[Conclusions]
Survey of epidemiologic studies on exposure to endocrine
disruptors and thyroid functions found some papers that report
effects of PCB on thyroid functions. Studies on HCB also exist
which, however, do not permit adequate evaluation of effects on
thyroid functions. Other substances, including organochlorines,
have not been studied in relation to thyroid functions and,
therefore, evaluation of the causality is impossible.
Experimental designs with high reliability are needed for future
studies on this problem.
[Literature]
Table 2.7 1: Cohort
studies on the relationship of endocrine disruptors with thyroid
functions
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