Caroline, yes 1 mg is 1000 micrograms. However, that is the UL - people whose daily iodide intake is significantly greater than that are at risk of hypothroidism.

The RDA for iodide is indeed 150 micrograms/day. I don't know whether or not that is too low. If somebody chooses to increase their daily iodide intake to (say) 500 micrograms/day that probably will do no harm and might help, as long as they stay below the UL of about 1 mg/day.

More generally, thyroid hormone production is controlled by a complicated feedback loop involving the thyroid gland, the pituitary gland and the hypothalamus.

Thyroid hormone production is usually self-regulating though hypothyroidism or hyperthyroidism can arise from pathology involving any or all of those three tissues. Hypothyroidism is usually not a direct consequence of iodide deficiency, as along as the daily intake is comfortablely above the RDA but below the UL.