Title: Iodine Excess
Key words: thyroiditis, goitre, hypothyroidism, hyperthyroidism, hypersensitivity, iatrogenic iodine toxicity, povidone-iodine, euthyroid, TSH, Graves Disease, exopthalmus, myxoedema
Date: July 2000
Author: Dr M Draper
What are the possible clinical consequences of iodine excess?
Iodine excess and clinical conditions due to toxicity.
Exposure to excessive iodine through foods, dietary supplements, topical medications, and /or iodinated contrast media has resulted in thyroiditis, goitre, hypothyroidism, hypersensitivity reactions and acute responses for some individuals1. Fatal iatrogenic iodine toxicity has occurred when used in skin or bowel povidone-iodine treatments2.
Wolff3 has defined four degrees of iodide excess in humans:
1. A relatively modest excess promoting temporary increase in the absolute uptake of iodine by the thyroid and the formation of organic iodine without inhibiting the capacity to release iodine in response to physiological demand.
2. A large excess, which can inhibit iodine release from the thyrotoxic human thyroid or from thyroids in which iodine release has been accelerated by TSH.
3. A slightly greater intake which inhibits organic iodine formation and which probably causes iodide goitre (Wolff- Chaikoff effect) .
4. Very high levels which saturate the active transport mechanisms. The acute pharmacological effects of iodide can usually be demonstrated before saturation becomes significant.
Wolffe has suggested that 2000 microgms/day should be considered as excesssive or potentially harmful. Normal diets are less than 1000 microgms/day but high marine fish or seaweed consumption (eg in Japan) can cause intake as high as 50,000-80,000 m g/day4.In Japan, the incidence of nontoxic diffuse goitre and toxic nodular goitre are markedly decreased by high dietary iodine.The high iodine can induce hypothryroidism in autoimmune thyroiditis.In areas which previously had low levels, supplementation programs with iodised salts or oils can cause a mild increase in the incidence of hyperthyroidism which spontaneously resolves with or without antithyroid drugs and in general should be avoided over age 40.
Iodine Excess and Hyperthyroidism
The thyroid can remain euthyroid in excess with a healthy thyroid gland5. There is blunting of TSH effects, and an escape from the Wolff-Chaikoff effect6 by inhibition of organification and down-regulation of iodide transporter (autoregulation). This escape fails in a diseased thyroid (Grave’s or Hashimoto’s thyroiditis), resulting in hypothyroidism. Iodine excess7 may contribute to raised T4 levels, which in pathological thyroid conditions (toxic adenoma or multinodular goitre [euthyroid before]) turn the thyroid gland autonomous, resulting in hyperthyroidism (Jod-Basedow effect). Hyperthyroidism presents with symptoms such as weight loss, increased appetite, irritability, restlessness, tremor, choreoathetosis, muscle weakness, palpitations (potentiation of catecholamines), arterial fibrillation, heart failure8, breathlessness, heat intolerance, oligomenorrhoea, gynaecomastia, infertility, vomiting and diarrhoea. Examination reveals exophthalmus, goitre, muscle wasting, tachycardia, hypertension, warm vasodilated peripheries, palmar erythema and peritibial myxoedema.