The established consequences of iodine deficiency in population-based studies and the implications of iodine deficiency in the etiology of cancer certainly justify repletion of iodine in all populations through diet and/or supplementation. According to the National Institutes of Health (NIH), adults should consume a minimum of 150 mcg/day of iodine, pregnant women 220 mcg/day, and breastfeeding women 290 mcg/day.71 The safe upper limit of consumption according to the NIH is 1,100 mcg/day for adults, with lesser amounts for teens and children. However, the NIH concedes that higher doses may be necessary for some, explaining "These [upper safe limit] levels do not apply to people who are taking iodine for medical reasons under the care of a doctor."
When assessing the physiological effects of ingested iodo-compounds, the form that is consumed is highly relevant. Most pertinent to the nutritional supplementation of iodine compounds is the finding that iodide salts (ie, KI, NaI) affect the thyroid while molecular iodine (I2) has less influence. This is the Wolff-Checkoff effect, which is essentially the shutdown of thyroid hormone synthesis in the presence of large amounts of ioidide (I-). This effect is due to impaired transport of the iodide (I-) molecule, which is the only means for the thyroid to obtain the necessary iodine for thyroid hormone synthesis. While this effect is generally thought to be transient until the thyroid gland reequilibrates to the available iodide, there are also published studies showing a permanent impairment of thyroid function.72–75 Those most likely to have adverse effects were more likely to be elderly, have severe iodine deficiency, and/or have a larger increase in iodine consumption.
While the NIH recommends 1,100 mcg/day as the safe upper limit, doses up to 4.0 mg/100 lb body weight of I2 appears to be both safe and therapeutic for benign breast diseases. The most effective anticarcinogenic form of iodine appears to be molecular iodine and intracellular 6-iodolactone. Doses exceeding 4.0 mg/100 lb of body weight have not been clinically documented as safe. In fact, there is documentation that doses 9 mg and higher may induce transient hypothyroidism as well as minor side effects such as respiratory tract infection, headache, sinusitis, nausea, acne, diarrhea, rash, or abdominal pain.76 These side effects abated with the discontinuance of iodine, but it is important to realize that high-dose iodine is not without risk of side effects.
Many of the iodine supplements on the market provide iodide (I-), usually potassium iodide, either alone or in combination with molecular iodine (I2). While this is safe for physiological repletion of iodine (doses less than 1,100 mcg/day), the salts carry greater risk of interfering with thyroid function at higher doses. The ideal supplement would contain molecular iodine with very little iodide. The difficulty is that iodine (I2) is rendered much more soluble with iodide (I-) and water (ie, Lugol's solution). Given that whole food sources are generally safe, perhaps the best means of iodine repletion for prevention of thyroid disorders as well as reduction in cancer risk is though moderate consumption of whole foods such as seaweed and fish.77–79 This brings with it one difficulty―dosing. Seafoods vary dramatically in their iodo-compound type and concentration. Seaweed alone varies from 16 mcg to 2,984 mcg iodine/g depending on the type of seaweed and where it is sourced.80
One last cautionary note, even with molecular iodine (I2), is that patients with antithyroid antibodies can have an exacerbation of symptoms with use.81,82 All patients should undergo testing for autoantibodies before beginning any supplementation of iodine. In the benign breast disease studies referred to above, there were no such adverse reactions because women with autoimmune disease were excluded from the study designs.