Effect of long-term fluoride administration on thyroid hormones level blood in rats

The effect of 2 months fluoride administration (0.1 and 1.0 mg daily/rat) on thyroxine and triiodothyronine level in blood and T3-resin uptake ratio was investigated in rats. In addition, free thyroxine index was calculated from serum thyroxine level and T3-resin uptake ratio. It was found that fluoride administration caused: 1. decrease in thyroxine and triiodothyronine level in plasma; 2. decrease in free thyroxine index values; 3. Increase in T3-resin uptake ratio. From these observations as well as from previous experiments it was concluded that fluoride given continuously to the rats may influence the thyroid gland rather indirectly by changing thyroid hormone transportation in the blood.