Most people can tolerate up to 1000 | g iodine daily without adverse effects, but daily intakes of up to 2000 | g are regarded as excessive and potentially harmful.5 Such intakes are unlikely to be obtained from normal diets of natural foods except where they are exceptionally high in marine fish or seaweed, or where foods are contaminated with iodine from iodine-containing medications or other adventitious sources. The effects of high iodine intake on thyroid function are variable and depend on the health of the thyroid gland. Dietary intakes of up 1000 |g/day have few long-term effects when the thyroid is healthy. Adverse effects include hypothyroidism and elevated TSH, goiter and increased incidence of autoimmune thyroid disease.66 People who have underlying autoimmune disease such as Grave's Disease or Hashimoto's thyroiditis, or who have previously been iodine deficient, may be more sensitive to iodine.8 Iodine-induced thyrotoxicosis (Jod-Basedow) has been described following the iodization programs, particularly in women over 40 years of age who had always been living in a low-iodine environment. Some individuals have thyroid nodules that can start making too much thyroid hormone when dietary iodine increases, which produces a condition called iodine-induced hyperthyroidism. Excess iodine can also cause hypothy-roidism because large amounts of iodine block the thyroid's ability to produce hormones.

Iodine overload may occur in vegans when seaweed and iodine-containing dietary supplements are consumed.67 The use of kelp supplements is not recommended as these can contain very high but variable amounts of iodine.

There might also be a danger from consumption of thyroid preparations. Scally et al.68 reported case studies of two physically fit adults who supplemented with tiratricol, an over-the-counter thyroid preparation marketed as a metabolic accelerator and fat loss aid. They presented with lethargy, loss of appetite and muscle weakness, symptoms that were accompanied by low serum TSH and profoundly elevated T3 concentrations. These cases illustrate the danger of consumption of substances marketed as "nutritional" supplements, but which may have pharmacological effects capable of inducing thyroid abnormalities when consumed inappropriately.

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