Among the phenomena I've observed
1) A free T4 thyroid hormone at the low end of normal, or even in the below normal range, along with a highish TSH (usually >1.5 mIU/L) are the most frequent patterns that signal iodine deficiency. Occasionally, a low free T3 value will also increase, though this is the least frequent development.
2) At a dose of 500 to 1000 mcg iodine per day, it requires anywhere from 3 to 6 months to obtain normalization of thyroid measures.
3) Reversal of small goiters also occurs over about 6 months.
4) Iodine intolerance is uncommon. If it occurs, using a low starting dose, e.g., 100-200 mcg per day, usually works. The dose can be increased gradually over the ensuing months.
5) Perceptible benefits of iodine occur only occasionally. The most common perceptible effects are increased energy and increased warmth, especially of the hands and feet.
6) Some people who have taken thyroid hormones for years will develop reduced need for their medication with iodine supplementation. In other words, their physician was inadvertently treating iodine deficiency with thyroid hormone replacement. Anyone already on any thyroid preparation(s), e.g., Synthroid, levothyroxine, Armour thyroid, Naturethroid, etc., should watch for signs of hyperthyroidism when iodine is added. But having your own thyroid gland make its own thyroid hormones is better and healthier than relying on the prescription agents. Just be sure to monitor your thyroid measures.
7) Iodine toxicity can occur--Two people in my clinic population developed iodine toxicity by taking 6000 mcg iodine per day for 6 or more months. (Both patients did it on their own based on something they read). Iodine toxicity is evidenced by shutting down your thyroid, i.e., marked increase in TSH, e.g., 15 mIU/L.
Most of the people in my clinic obtain their iodine from kelp tablets. Some use potassium iodine (KI) drops. A handful have used the high-potency Iodoral (12.5 mg or 12,500 mcg iodine per tablet); this was also the form that generated the toxic effects in the two females.
All in all, iodine deficiency is actually far more common than I ever suspected. Not everybody is iodine deficient. But a substantial minority of the Midwest population I see certainly are.