The effects of fluoride on thyroid metabolism were studied in rats.
Wistar rats were maintained for 54-58 days on a high (34.5 ppm F) or low (0.31 ppm F) fluoride diet, and were provided distilled water as drinking water in which sodium fluoride was contained at concentrations of 0, 1, 5, 10, 50, 100, and 200ppm F.
The results were as follows:
1. No dose-response relationships were found between fluoride concentration of drinking water and thyroid hormone (T₃ and T₄) values in serum. In the groups exposed to high concentrations of fluoride, moreover, few distinct effects were found. Therefore, it is probable that the fluoride has no serious effect on thyroid metabolism. In groups maintained with 1 ppm F water and low fluoride diet, however, a somewhat high value of T₃ and T₄ was observed. From this, it is possible that the thyroid metabolism is activated by trace amounts of fluoride.
In the groups maintained with low fluoride water (1, 5 and 10ppm F) and high fluoride diet, a little low value of T₃ was observed, but no change of T₄ value was seen. It is possible that some effects appear with ordinary diet (high fluoride diet) itself.
2. No histopathologic changes were observed in any of the experimental groups.
3. In the groups exposed to 100 or 200 ppm F water, TSH and GH values decreased. The weight of the pituitary gland also decreased in the 200 ppm F group. It is possible that the pituitary gland is affected directly by high fluoride.
4. Fluoride concentration in serum was a little higher in the low fluoride diet group than in the high fluoride group, and a similar finding was observed in teeth and bones, although the total intake of fluoride from drinking water was lower in the low fluoride diet group than in the high fluoride diet group. It is possible that the absorption of fluoride from water through the digestive tract is inhibited by the high fluoride diet.
5. A relatively firm homeostasis in serum fluoride level was found in the zero to 10ppm F water groups, and fluoride concentrations in soft tissues were reflected in the serum fluoride level. Therefore, fluoride levels in soft tissues have no worth as an index of fluorosis except at high fluoride doses. Fluoride concentration in hard tissues reflected clearly the fluoride content in drinking water, especially in the low fluoride diet group. This may be a best index of fluoride intake.