Fodder for the mill...

By Scott Baltic

NEW YORK (Reuters Health) Sept 17 - A prospective population-based
study of two sub-cohorts in Denmark with mild or moderate iodine
deficiency found that Iodine fortification of salt was accompanied by
a moderate increase in the incidence of overt hypothyroidism.

This increase occurred primarily in young and middle-aged adults,
according to the 7-year study, reported in the August issue of the
Journal of Clinical Endocrinology and Metabolism. The authors say this
is the first prospective study to measure the effect of increased
iodine intake on the incidence of overt hypothyroidism.

Iodine intake in Denmark had been known to be low for many years when
the Danish government accepted the recommendations of thyroid and
nutrition experts and in 1998 introduced a program of voluntary iodine
fortification of salt. Fortification of household salt and salt used
in commercial bread production became mandatory in 2000. The
fortification level was calculated to increase Iodine intake by about
50 mcg/day.

A computer-based registry identified all new cases of overt
hypothyroidism in two sub-cohorts: an area of western Denmark typified
by moderate Iodine deficiency (n = 310,124) and an area in Copenhagen
typified by mild iodine deficiency (n = 225,707).

In the sub-cohort with previous moderate iodine deficiency, the
incidence of overt hypothyroidism rose from 29.8 to 40.3/100,000
person-years following iodine fortification. In the Copenhagen
sub-cohort (previous mild iodine deficiency), the incidence remained
similar to that at baseline.

In the combined cohort, the incidence of overt hypothyroidism was
stable in those aged 19 or less and those 60 and over, but rose
significantly among those age 20 - 59 during mandatory fortification.

"The mechanism behind such an iodine-induced increase on the incidence
of hypothyroidism is unknown," the authors note, although they suggest
several possible mechanisms.

One such is the well-established association between a high thyroidal
iodine uptake and the inhibition of many thyroidal processes.

Another is enhanced autoimmunity, since animal studies have suggested
that an increase from low to normal and high iodine intake is
associated with an increase in the risk of autoimmune thyroid disease,
though this has not been fully confirmed in humans.

Finally, the researchers speculate that "a high or excessive iodine
intake may lead to impaired thyroid function due to increased
apoptosis of thyroid follicular cells as found in in vitro systems."

They suggest that a follow-up time longer than 7 years might be needed
"to clarify the dynamics of the epidemiology of hypothyroidism after a
sudden change in iodine intake."

J Clin Endocrinol Metab 2007;92:3122-3127. Tweet