Iodine toxicity

Because high amounts of iodine inhibit synthesis of thyroid hormones and stimulate growth of the thyroid gland, iodine toxicity also can cause goiter. In other words, both too much and too little iodine cause goiter.

Overzealous supplementation is the most common cause of iodine toxicity.

High doses of free iodine, such as in form of iodine tincture, are highly toxic, if brought into body cavities, and cause swelling and bleeding of mucous membranes.
Iodine toxicity 

Iodine and goiter

The incidence of goiter, diffuse and nodular is to a great extent dependent on the sufficiency of iodine intake of the population. In areas of iodine deficiency, goiter prevalence can be very high and multinodular goiters are also very common.

The hypothalamus regulates thyroid hormone production by controlling the release of the pituitary’s thyroid stimulating hormone (TSH).

With iodine deficiency, thyroid hormone production declines, and the body responds by secreting more TSH in a futile attempt to accelerate iodine uptake by the thyroid gland. The excessive TSH in turn stimulates the thyroid gland to grow, eventually causing a goiter.

In India, endemic goiter has been reported from all over the country, which has been attributed to iodine deficiency and which has even resulted in overt hypothyroidism.
Iodine and goiter