Iodine: Safety Assessment

The European food safety authority (EFSA) last evaluated vitamins and minerals for safety in 2006 and set a so-called Tolerable Upper Intake Level (UL) for each micronutrient, provided sufficient data were available. This UL reflects the maximum safe level of a micronutrient that will not cause adverse effects when taken daily from all sources for a lifetime.

The above safe maximum daily amount applies to adults 18 years of age and older and to pregnant and lactating women. The safe maximum daily intake does not apply to populations with symptoms of iodine deficiency or to persons receiving therapeutic treatment with iodine. In countries with prevalent iodine deficiency, the daily maximum amount should not exceed 500 µg to avoid the occurrence of hyperthyroidism (overactive thyroid gland). The BfR (Federal Institute for Risk Assessment) also considers only a daily maximum amount of 500 µg of iodine safe for Germany due to the widespread iodine deficiency situation and the resulting increased sensitivity to iodine within the population. According to the criteria of the WHO, Germany is one of the iodine deficiency areas. The data of the NVS II (National Nutrition Survey II, 2008) on the daily intake of iodine from all sources (conventional diet and food supplements) indicate that an unintentional exceeding of the safe daily maximum amount is unlikely. Intake of such an amount is even conceivable only with a combination of an extraordinarily high dietary intake and a deliberate additional high intake of iodine via dietary supplements. The LOAEL (Lowest Observed Adverse Effect Level) – the lowest dose of a substance at which adverse effects were just observed – is 1,700 µg of iodine per day for adults. The LOAEL was established by studies in healthy individuals with normal thyroid function. For individuals with autoimmune diseases or impaired thyroid function, as well as those with iodine deficiency, the LOAEL may be much lower because this population is more sensitive to high iodine intake. Adverse effects of excessive iodine intake may be manifested by a variety of circumstances:

• Acute iodine poisoning due to ingestion of very large amounts of iodine.
• Ingestion of extremely high amounts of iodine (up to 15,000 mg) resulted in side effects such as vomiting, convulsions, anuria (decreased urine output of less than 100 ml in 24 hours), fever, and coma, some of which were fatal. Such high levels of iodine have been supplied, for example, intentionally or unintentionally via tinctures of iodine for disinfection. An intake of iodine in these orders of magnitude is not possible via the conventional diet and properly dosed dietary supplements.

Disruption of thyroid function by permanently too low or too high amounts of iodine from the diet

Both long-term too-low intake below 50 µg of iodine per day and too-high intake above 500 µg of iodine per day are associated with an increasing risk of thyroid dysfunction. 1,000 µg of iodine or more per day can usually be tolerated by healthy adults without adverse side effects. However, this upper limit is much lower in populations with iodine deficiency because sensitivity to high iodine intake depends on past iodine supply. Thus, an excessive iodine intake of more than 1,000 µg per day can lead to various clinical pictures, depending on the amount and the sensitivity of the person. Possible conditions include hyperthyroidism (hyperthyroidism), immunothyreopathy (abbreviated: IHT; synonyms: Graves’ disease, Graves’ disease; it is an autoimmune disease of the thyroid gland that leads to hyperthyroidism), Hashimoto’s thyroiditis (synonyms: Struma lymphomatosa Hashimoto, lymphocytic thyroiditis, and Ord thyroiditis; autoimmune disease leading to chronic thyroiditis), acute blockage of iodine uptake in the thyroid gland, and, in rare cases, hypersensitivity reactions. In healthy individuals, acute iodine doses of 2,000 to 10,000 µg per day can cause hyperthyroidism (hyperthyroidism).In the presence of functional autonomy of the thyroid tissue (autonomous adenoma/hot nodule, which produces hormones independently/uncontrolled), Graves’ disease or iodine deficiency, even excessive iodine doses of 500 µg per day can trigger hyperthyroidism. Rare hypersensitivity reactions after ingestion of extremely high doses: iodine hypersensitivity or iodine allergy has been observed in rare cases after use of iodine-containing X-ray contrast media, iodine-containing disinfectants and iodine-containing cosmetics. Such side effects have not been observed with oral intake of physiologic amounts of iodine through the diet.