and this

Patients who used water from wells and municipal plants may be exposed to potassium perchlorate, a very powerful goitrogen that behaves like fluoride, binding to the halide-binding site of the symporter without itself being symported. A recent Internet publication by Kirk et al (16) reported the presence of high concentrations of perchlorate in dairy milk sold in grocery stores and in human milk. The mean levels of perchlorate were 5 times higher in breast milk than dairy milk. Perchlorate has a selectivity factor of at least 30 over iodide. To compete effectively against this goitrogen, the peripheral concentration of inorganic Iodide must be at least 100 times higher than the concentration of perchlorate. Kirk et al (16) observed that breast and dairy Iodide levels were inversely correlated with the levels of perchlorate. Perchlorate and fluoride, due to their high redox potential, may cause oxidative damage to the halide binding site, decreasing its efficiency for Iodide transport.

from:
http://www.optimox.com/pics/Iodine/IOD-08/IOD_08.htm
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