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 (approximately 167 vs 700 micrograms l- per kg) chow and distilled water or HA water (1mg/ml) for 3 or 4 months. Serum T4, T3, reverse T3, and/or TSH were measured by radioimmunoassay. Thyroidal 125I uptake was measured in mice at 2 h after injection of 1 microCi125I ip. Treatment of the rat with HA was associated with a significantly (p < 0.05) reduced serum T4 without a change in other parameters of study. Treatment with low iodine diet was associated with a clear increase in serum T3 and a decrease in serum rT3. Rats treated with both HA and low iodine diet showed a significantly reduced serum T4, increased serum T3 and decreased serum rT3. In mice, treatment with low iodine diet significantly increased thyroidal 125I 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.