From Edward Priestley.

I agree that different forms of fluoride can be more or less dangerous to consume, but even if as free from contaminants as possible it can still have adverse effects on health.   Fluoride displaces iodine & thereby causes endocrine disorders including hypothyroidism.  Any form of fluoride can act synergistically with other chemicals increasing their potency. This is especially bad when it is combined with organic chemicals in drugs & pesticides etc.   Millions of people worldwide are affected by naturally occurring fluoride in water supplies.  India has a problem with 60,000 of its people suffering health problems from natural fluoride. 

Edward