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The two hormones produced by the thyroid, thyroxine and triiodothyronine, contain substantial amounts of iodine (65% and 58% by mass, respectively). One might guess that if you have insufficient iodine in your diet the thyroid attempts to scavenge the iodine it needs to synthesize these hormones from emergency sources that have undesirable side-effects, eventually leading to permanent damage to the gland. One of the difficulties in appropriate care and maintenance of the body is that it has substantial abilities to compensate for stresses in the short-term: it can tolerate starvation, lack of essential nutrients, dehydration, over-exericse, under-exericse, over-eating, inhaling poisonous fumes, et cetera and so forth for a while -- but not indefinitely. Eventually it will suffer permanent damage. But that means it is NOT always clear from the body's IMMEDIATE reaction whether what you are doing is bad for it, long-term. The body may cope quite well, and the owner (you) may be fooled into thinking what you are doing does no harm. Science suffers the same problem. It can be challenging to do the multi-year, if not multi-decade, studies needed to know whether a given perturbation -- drinking coffee, salt intake, fat intake, sugar intake, hydrogenated oil intake, exercise -- is healthy or unhealthy over the long-term. An interesting illustration is ionizing radiation, the kind you get from radioactive materials. Originally it was though radiation was *good* for you, in part because it definitely seems, in the short-term, to perk a body up -- for example, make it successfully fight off infection. It was known later that very large doses were fatal, but it was only much later after that that it became known that sublethal doses were also not good for you -- could cause cancer, for example. There is still a strong argument going on about whether low doses of radiation are good or bad or neither over the long run. It's very hard to do the kinds of experiments that would find out for sure.