Depending on the availability of iodine, the thyroid gland is able to enhance or limit
the use of iodine for thyroid hormone production. When compensation fails, as in severely
iodine-deficient populations, hypothyroidism and developmental brain damage will be
the dominating disorders. This is, out of all comparison, the most serious association
between disease and the level of iodine intake in a population. In less severe iodine
deficiency, the normal thyroid gland is able to adapt and keep thyroid hormone production
within the normal range. However, the prolonged thyroid hyperactivity associated with
such adaptation leads to thyroid growth, and during follicular cell proliferation
there is a tendency to mutations leading to multifocal autonomous growth and function.
In populations with mild and moderate iodine deficiency, such multifocal autonomous
thyroid function is a common cause of hyperthyroidism in elderly people, and the prevalence
of thyroid enlargement and nodularity is high. The average serum TSH tends to decrease
with age in such populations caused by the high frequency of autonomous thyroid hormone
production. On the other hand, epidemiological studies have shown that hypothyroidism
is more prevalent in populations with a high iodine intake. Probably, this is also
a complication to thyroid adaptation to iodine intake. Many thyroid processes are
inhibited when iodine intake becomes high, and the frequency of apoptosis of follicular
cells becomes higher. Abnormal inhibition of thyroid function by high levels of iodine
is especially common in people affected by thyroid autoimmunity (Hashimoto's thyroiditis).
In populations with high iodine intake, the average serum thyroid-stimulating hormone
(TSH) tends to increase with age. This phenomenon is especially pronounced in Caucasian
populations with a genetically determined high tendency to thyroid autoimmunity. A
small tendency to higher serum TSH may be observed already when iodine intake is brought
from mildly deficient to adequate, but there is at present no evidence that slightly
elevated serum TSH in elderly people leads to an increase in morbidity and mortality.
Even minor differences in iodine intake between populations are associated with differences
in the occurrence of thyroid disorders. Both iodine intake levels below and above
the recommended interval are associated with an increase in the risk of disease in
the population. Optimally, iodine intake of a population should be kept within a relatively
narrow interval where iodine deficiency disorders are prevented, but not higher. Monitoring
and adjusting of iodine intake in a population is an important part of preventive
medicine.
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