Iodine supplementation can be an effective treatment of iodine deficiency hypothyroidism and can halt the growth of goiter if the cause is not complicated by malnutrition or environmental and dietary goitrogens.^6,7 Iodine supplements will help to shrink goiters during early stages, but they have no effect in later stages.⁸ Ingestion of 2,000 to 6,000 mcg of iodine daily over long periods of time can be toxic to the thyroid and can be a cause of goiter.^9,10

When iodine deficiency is present, other nutrient levels become important in the development of goiter. Deficiencies of zinc¹¹ and manganese¹² can both contribute to iodine-deficiency goiter; however, an animal study found that manganese excess can also be goitrogenic.¹² It has been suggested that selenium deficiency may contribute to goiter.¹⁴ However, when selenium supplements were given to people deficient in both iodine and selenium, thyroid dysfunction was aggravated, and it has been suggested that selenium deficiency may provide some protection when there is iodine deficiency.^15,16 A study of the effects of selenium supplementation at 100 mcg daily in women without selenium deficiency but with slightly low iodine intake found no effect on thyroid function.¹⁷ The authors concluded that selenium supplementation seems to be safe in people with only iodine deficiency but not in people with combined selenium and iodine deficiencies. In those cases, iodine supplementation has been shown to be most useful.¹⁸ No studies have been done to evaluate the usefulness of supplementation with zinc or manganese to prevent or treat goiter.

Blood levels of vitamin A are lower in people with goiter than in similar people without goiter.^18,19 The same relationship has been found for vitamin E and goiter.¹⁹ Animal research has found that, in iodine-deficient conditions, a supplement combination of vitamin C, vitamin E, and beta-carotene prevented goiter formation (though hypothyroidism was not improved), and vitamin E alone had a similar effect.²¹ No studies have been done to investigate this benefit in humans.

Blood levels of vitamin A are lower in people with goiter than in similar people without goiter.^21,22 The same relationship has been found for vitamin E and goiter.²² Animal research has found that, in iodine-deficient conditions, a supplement combination of vitamin C, vitamin E, and beta-carotene prevented goiter formation (though hypothyroidism was not improved), and vitamin E alone had a similar effect.²⁴ No studies have been done to investigate this benefit in humans.

When iodine deficiency is present, other nutrient levels become important in the development of goiter. Deficiencies of zinc²⁴ and manganese²⁵ can both contribute to iodine-deficiency goiter; however, an animal study found that manganese excess can also be goitrogenic.²⁵ It has been suggested that selenium deficiency may contribute to goiter.²⁷ However, when selenium supplements were given to people deficient in both iodine and selenium, thyroid dysfunction was aggravated, and it has been suggested that selenium deficiency may provide some protection when there is iodine deficiency.^28,29 A study of the effects of selenium supplementation at 100 mcg daily in women without selenium deficiency but with slightly low iodine intake found no effect on thyroid function.³⁰ The authors concluded that selenium supplementation seems to be safe in people with only iodine deficiency but not in people with combined selenium and iodine deficiencies. In those cases, iodine supplementation has been shown to be most useful.³¹ No studies have been done to evaluate the usefulness of supplementation with zinc or manganese to prevent or treat goiter.