Iodine is a halogen.
On the periodic table of elements (remember the big chart of the elements in science class?), the ingenious table that lays out all known atomic elements, elements with similar characteristics are listed in the same column. The elegant genius of the periodic table has even allowed prediction of new, undiscovered elements that conform to the "laws" of atomic behavior.
Column 17 (also called "group VIIa") contains all the halogens, of which iodine is one member. Other halogens include fluorine, chlorine, and bromine.
Odd phenomenon in biologic systems: One halogen can often not be distinguished from another. Thus, a chlorinated compound can cleverly disguise itself as an iodinated compound, a brominated compound can mimic an iodinated compound, etc.
What this means in thyroid health is that, should sufficient iodine be lacking in the body, i.e., iodine deficiency, other halogens can gain entry into the thyroid gland.
While a polychlorinated biphenyl (PCB) molecule may be recognized as an iodinated compound, it certainly doesn't act like an iodinated compound once it's in the thyroid's cells and can disrupt thyroid function (Porterfield 1998). Another group of chlorine-containing compounds, perchlorates , that contaminate groundwater and are found as pesticide residues in produce, are extremely potent thyroid-blockers (Greer 2002). Likewise, bromine-containing compounds, such as polybrominated diphenyl ethers (PBDEs), widely used as flame retardants, also disrupt thyroid function (Zhou 2001). Perfluoro octanoic acid (PFOA), found in Teflon non-stick cookware and stain-resistant products, has been associated with thyroid dysfunction (Melzer 2010). PFOA, incidentally, can disrupt thyroid dysfunction that will not show up in the TSH test used by primary care physicians and endocrinologists to screen for thyroid dysfunction. (In fact, the presumed champions of thyroid health, the endocrinology community, have proven a miserable failure in translating and implementing the findings from toxicological science findings to that of preserving or restoring thyroid health. They have largely chosen to ignore it.)
We therefore navigate through a world teeming with halogenated thyroid blocking compounds. We should all therefore avoid such exposures as perchlorates in produce by rinsing thoroughly or purchasing organic, avoid non-stick cookware, avoid use or exposure to pesticides and herbicides.
Another crucial means to block the entry of various halogenated compounds into your vulnerable thyroid: Be sure you are getting sufficient iodine. While it doesn't make your thyroid impervious to injury, iodine circulating in the blood in sufficient quantities and residing in sufficient stores in the thyroid gland provides at least partial protection from the halogenated impostors in your life.
I make this point in the context of heart disease prevention, since even the most subtle degrees of thyroid dysfunction can easily double, triple, or quadruple heart disease risk. See related posts, Is normal TSH too high? and Thyroid perspective update.