Googling "PubMed" along with "lithium" and "thyroid" found me LOTS of hits. Probably the most interesting i've read so far was this recent study, which looked at naturally-occurring lithium in drinking water. Sure enough, they found correlation with markers of impaired thyroid function. Not proof of my hypothesis of course, but support.
(Funnily enough, they were puzzled that higher urinary selenium was associated with better thyroid markers; they seemed to think that it should be used up, not passed out! Did "repleteness" not occur to them?)
I did find something else that seems significant here (besides the fact that lithium can damage the heart, skin, kidneys and pancreas as well as the thyroid) -- turns out that older people can't handle the doses that younger people can. "Elderly individuals require lower doses of lithium to achieve similar serum concentrations as those in younger adults. ... There is also a difference in lithium tolerability with age, and the prevalence of hand tremor with lithium increases with age. In the elderly, neurotoxicity clearly occurs at serum lithium levels which are considered 'therapeutic' in general adult populations."
Add to this, "there is evidence that lithium has effect on glucose metabolism and has the ability to increase the release of glucagon. Studies have suggested that lithium treatment may impair glucose tolerance or produce frank diabetes in certain patients, and the risk is higher in patients above the age of 40 years." And some "scientists" were thinking it's a great idea to add lithium to water supplies, in order to improve mental health....
Mercifully, the folks who did these studies were a bit brighter. Far from thinking that increased lithium is good, the first set concluded that paying more attention to levels in drinking-water is "highly warranted." And considering what the second bunch said, i wonder how much worse the results of the drinking-water study would have been if the median age of the participants had been greater, rather than the mid-30s it was.