Goitrogens

Some foods contain substances that interfere with iodine utilization or thyroid hormone production;
these substances are called goitrogens. Some species of millet and cruciferous vegetables (for example, cabbage, broccoli, cauliflower, and Brussels sprouts) also contain goitrogens. Further, the soybean isoflavones, genistein and daidzein, have been found to inhibit thyroid hormone synthesis. Most of these goitrogens are not of clinical importance unless they are consumed in large amounts or there is coexisting iodine deficiency. Recent findings also indicate that tobacco smoking may be associated with an increased risk of goiter...

Radioactive iodine, especially, may be released into the environment as a result of nuclear reactor accidents. Thyroid accumulation of radioactive iodine increases the risk of developing thyroid cancer, especially in children.

The increased iodine trapping activity of the thyroid gland in iodine deficiency results in increased thyroid accumulation of radioactive iodine. Thus, iodine-deficient individuals are at increased risk of developing radiation-induced thyroid cancer because they will accumulate greater amounts of radioactive iodine. ... administered in pharmacologic doses (50-100 mg for adults) within 48 hours before or eight hours after radiation exposure from a nuclear reactor accident can significantly reduce thyroid uptake of ¹³¹I and decrease the risk of radiation-induced thyroid cancer . The prompt and widespread use of in Poland after the 1986 Chernobyl nuclear reactor accident may explain the lack of a significant increase in childhood thyroid cancer in Poland compared to fallout areas where  iodine was not widely used. In the U.S., the Nuclear Regulatory Commission (NRC) requires that consideration be given to potassium iodide as a protective measure for the general public in the case of a major release of radioactivity from a nuclear power plant.

Fibrocystic breast condition is a benign (non-cancerous) condition of the breasts, characterized by lumpiness and discomfort in one or both breasts. In estrogen-treated rats, iodine deficiency leads to changes similar to those seen in fibrocystic breast condition, while iodine repletion reverses those changes.

 An uncontrolled study of 233 women with fibrocystic breast condition found that treatment with aqueous molecular iodine (I₂) at a dose of 0.08 mg of I₂/kg of body weight daily over six to 18 months was associated with improvement in pain and other symptoms in over 70% of those treated. About 10% of the study participants reported side effects that were described by the investigators as minor. A double-blind, placebo-controlled trial of aqueous molecular iodine (0.07-0.09 mg of I₂/kg of body weight daily for six months) in 56 women with fibrocystic breast condition found that 65% of the women taking molecular iodine reported improvement compared to 33% of those taking the placebo. More recently, a double-blind, placebo-controlled, clinical trial in 111 women with documented breast pain reported that molecular iodine (3 mg/day or 6 mg/day) for five months improved overall pain. In this study, more than half of the women receiving the highest dosage of molecular iodine reported a ≥ 50% reduction in self-assessed breast pain compared to 8.3% in those receiving placebo. Large-scale, controlled clinical trials are needed to determine the therapeutic value of molecular iodine in fibrocystic breast condition. The doses of iodine used in these studies (3 to 7 mg/day for a 60 kg person) are several times higher than the tolerable upper level of intake (see chart above) recommended by the Food and Nutrition Board (FNB) of the Institute of Medicine and should only be used under medical supervision.