Effect of excess iodine on the synthesis of thyroid hormones
During normal functioning of the thyroid gland, a permanent excess of iodine (several hundred milligrams with an actual daily requirement of 200 micrograms) inhibits iodine absorption and thyroid hormone production. This effect is known as the Wolff-Chaikoff effect. In the past, this effect was used before surgical treatment of hyperthyroidism with excessive release of thyroid hormones.
This therapy is called “plummer” because it goes back to the US-American internist and endocrinologist Henry Stanley Plummer. After a period of two to four weeks, the thyroid hormone synthesis is no longer inhibited by the excess iodine, so that the thyroid gland produces the hormones again despite the yodexcess. This effect is known as an escape phenomenon and is no longer guaranteed in the case of thyroid dysfunction. In patients who have, for example, chronic autoimmune thyroiditis (Hashimoto’s thyroiditis), patients who have had their thyroid partially removed during an operation or patients who have been treated with radioiodine therapy, excess iodine can cause an underfunctioning of the thyroid (hypothyroidism). On the other hand, if the thyroid gland is enlarged due to an iodine deficiency or due to thyroid nodes and hormone-releasing thyroid nodes (autonomous adenoma), administration of iodine can trigger hyperthyroidism within weeks to months.
Effect of iodine deficiency on the synthesis of thyroid hormones
In the case of iodine deficiency, the production of thyroid hormones, for which iodine is required, is limited. Due to feedback mechanisms, when there is a deficiency of thyroid hormones, thyroid-stimulating hormone (TSH) is released from the front lobe of the pituitary gland with the aim of stimulating thyroid growth (hyperplasia) so that more thyroid hormones can be produced. It is now known, however, that TSH is not the decisive factor in thyroid enlargement, but that thyroid growth is also triggered by local growth factors released by low-iodine thyroid tissue. Thus, iodine deficiency leads to an enlargement of the thyroid gland (iodine-deficient goiter) with initially still normal thyroid function (euthyroid goiter). However, if the iodine deficiency persists over a longer period of time, even a healthy thyroid gland can no longer compensate for it and iodine deficiency diseases occur.
