Although selenium and iodine are two minerals where we do need to be mindful of toxicity, fears of supplementing them are largely inflated. This is particularly true considering that most horses are likely to be deficient in one or both.
Unless your horse is eating hay/feeds grown in the alkaline soils of the Midwest, there is a good chance selenium intake is marginal to deficient. Similarly, hays/feeds from Canada and the northern half of the United States are going to be deficient in iodine and levels in other areas may only meet bare minimums.
Both iodine and selenium are critical for thyroid function. Iodine is an integral part of thyroid hormones and selenium is needed to convert the relative inactive T4 form into the active T3 hormone.
Iodine’s other roles in the body are less well studied but deficiency results in fetal death or mental retardation, goiter and mammary gland disease. Selenium’s other functions include maintenance of the body’s glutathione antioxidant system and other antioxidant functions as well as male fertility.
The currently recommended bare minimum intakes for these minerals for a 1000 lb horse are about 1 mg/day for selenium and 4 mg/day for iodine.
Although either of these minerals could be toxic, it would take several months of ten times these minimal intakes to put the horse at any risk. To put this in perspective, a horse getting 5 lbs/day of a highly mineral fortified grain mix would take in at most 1.25 mg of selenium and no more than 1.5 mg if being feed a mineral balancer product. This is 10% or less of the level suspected to potentially be toxic after months of exposure. Iodine contents of feeds and balancers are often not declared but are even lower than selenium when compared on a minimum requirement basis.
Bottom line here is that unless you know for sure your horse is getting adequate selenium and iodine you should at least supplement with the minimum recommended amount. For a 1000 to 1100 lb horse, this would be 1 mg/day of selenium (2 mg if in work) and 4 mg of iodine.