Humic substances (HS) have been implicated as environmental goitrogens
. Increased prevalence of goiter has been recently noticed in the blac
kfoot disease endemic area on the southwest coast of Taiwan, where wel
l water is rich in HS. This study investigated the in vivo effects of
humic acids (HA) on the-thyroid gland of rats and mice. Groups of mice
and rats were fed regular or moderately iodine deficient (similar to
167 vs 700 mu g I- per kg) chow and distilled water or HA water (1 mg/
ml) for 3 or 4 months. Serum T-4, T-3, reverse T-3, and/or TSH were me
asured by radioimmunoassay. Thyroidal I-125 uptake was measured in mic
e at 2 h after injection of 1 mu Ci(125)I ip. Treatment of the rat wit
h HA was associated with a significantly (p<0.05) reduced serum T-4 wi
thout a change in other parameters of study. Treatment with low iodine
diet was associated with a clear increase in serum T-3 and a decrease
in serum rT(3). Rats treated with both HA and low iodine diet showed
a significantly reduced serum T-4, increased serum T-3 and decreased s
erum rT(3). In mice, treatment with low iodine diet significantly incr
eased thyroidal I-125 uptake and additional treatment with HA signific
antly enhanced the effect of low iodine diet. Treatment with HA did no
t influence thyroid weight of rats or mice given normal or iodine defi
cient diets. We conclude that HA per se do not induce goiter, but they
may enhance the goitrogenic effect of low iodine.