EFFECT OF HUMIC ACIDS ON THYROIDAL FUNCTION

Humic substances (HS) have been implicated as environmental goitrogens. Increased prevalence of goiter has been recently noticed in the blackfoot disease endemic area on the southwest coast of Taiwan, where well 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 measured by radioimmunoassay. Thyroidal I-125 uptake was measured in mice at 2 h after injection of 1 mu Ci(125)I ip. Treatment of the rat with HA was associated with a significantly (p<0.05) reduced serum T-4 without 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 serum rT(3). In mice, treatment with low iodine diet significantly increased thyroidal I-125 uptake and additional treatment with HA significantly enhanced the effect of low iodine diet. Treatment with HA did not influence thyroid weight of rats or mice given normal or iodine deficient diets. We conclude that HA per se do not induce goiter, but they may enhance the goitrogenic effect of low iodine.