Roles for bacteria?
Interestingly roles for bacteria in iodine insufficiency, working directly, by impacting gut function, and by binding iodine in water, have been proposed. These if indeed relevant (and why not in principle) would likely only be significant where other factors meant iodine uptake/metabolism was marginal. Cuuld such factors help account for wide variation in the same regions, as historically in Switzerland.
CONGENITAL GOITRE IN SHEEP IN SOUTHERN TASMANIA
Michael Statham, B. Agr. Sc. (Hons.), Tas.
https://eprints.utas.edu.au/21682/1/...974_thesis.pdf
"It has been suggested that bacteria may induce goitre if they are present in large enough numbers, as Macchia, Bates and Pastan (1967) have isolated a thyroid stimulating factor from Clostridium perfringens. This compound, which was thought to be a protein with a molecular weight of about 30,000, acted in a manner similar to Thyroid Stimulating Hormone when incubated with thyroid slices, and when injected into chickens it depleted the thyroid of radioiodine.
Other bacteria which commonly inhabit the human intestinal tract (particularly Paracolobacterium) have been shown to exhibit myrosinase activity which converts progoitrin into the goitrogenic agent goitrin (Oginsky, Stein and Greer 1965).
An unusual theory has been proposed by Beres (1969) who concluded that goitre in Hungary was associated with wet areas and high levels of magnesium, calcium and potassium ions. He proposed that the goitre of these areas was caused by algae, particularly Microcystis, which removed iodine from water and secreted antithyroid compounds including thiourea, thiouracil, methyl mercaptan and cyanides."
Pollution
https://www.ncbi.nlm.nih.gov/pmc/art...07356-0001.pdf
Sir Robert McCarrison, an important early researcher into goiter, who acted as Doctor to the Hunza and other tribes in the Gilgit area noted that fecal pollution of water, and wider related pollution increased the occurrence of goiter. For example once a new clean water supply was made to a school the symptoms of goiter disappeared. McCarrison noted the Hunza were free of goiter but adjacent groups with lower standards of of ensuring access to clean water suffered from it.
Quality of water supplies, freedom or presence of high levels of nitrates, fecal and bacteriological contaminants, would arguably also have effected predisposition to goiter, so may also have been factors historically, before centrally provided treated water, helping explain the historic difference in rates of goiter occurrence in the same goiter prone geographic regions.
Iodine is an important co-factor in immune function, and possible usage of it in these pathways may reduce the wider availability of it.