Selenium biochemistry is also an interesting topic. Trace amounts of selenium are essential to the diet of, at least to my knowledge, vertebrate animals. I'm not aware whether or not other kinds of animals use it. In several of the enzymes that metabolize iodine-containing thyroid hormones, the amino acid cysteine which contains the R-SH (thiol group) is replaced with R-SeH. Thus creating the unique amino acid "selenocysteine" at the active site of the enzymes. The -SeH seems to be important in handling iodine in thyroid hormones.

Thus, selenium deficiency can mimic symptoms of iodine deficiency. Although the former is rare in humans. Grazing mammals can get selenium deficiency in areas with certain kinds of clay soils.