More ‘subclinical’ deficiencies (not bad enough to be diagnosed with something or have your health in danger, but still not optimal) include a reduction in cognitive capacities and maybe a lowering of the metabolic rate. The magnitude for both of them is quite small in adults actually, but they are both things that should be avoided.
Most risk comes with the unborn children of pregnant mothers who themselves are iodine deficient, since thyroid hormones are most critical in making the brain of the child before birth and an iodine deficiency at this time (or in early life when the brain is most rapidly growing) can make the ‘cognitive deficits’ much more pronounced. When comparing cohorts of children who were sufficient in iodine their entire life compared to those who were deficient noted that the children with insufficient iodine were 70 times more likely to have an IQ below the 25th percentile (source) and salt iodization in the US is commonly credited with basically increasing the average IQ of an entire generation by 10 points (source) while other countries with high iodine deficiency rates (China) are at risk for a large reduction in overall IQ (source)
How can I make sure I am not Iodine Deficient?
The first line of defense is to modify your diet to accommodate more iodine. This is most readily done by eating some sushi or sashimi (the nori wrapping the rice contains iodine at around 12 micrograms per gram) or you can simply add more fresh fish or iodized salt into your diet.
If the above are not practical for various reasons such as being vegan or avoiding added salts, then a dietary supplement containing low amounts of iodine (ie. barely enough to avoid deficiency) would be best since there really aren’t any benefits with excessively high doses of iodine, potentially harm actually.