Consequences of excess Iodine

Karthik Balachandran

February 13,2015

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Background

• RDA of iodine is 150 µg/day

• Tolerable upper level of 1100 µg/day

• Excess :• Children-Urinary iodine > 300 µg/day• Pregnant women- Urinary iodine > 500 µg/day

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Measures of iodine excess

• Dried blood spot thyroglobulin concentration

• 4-40 µg/dl indicates iodine sufficiency in children 514 years old

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Thyroidal adaptation to excess iodine

• Wolff-Chaikoff effect

Mechanism

Inhibitory effect of intrathyroidal iodolactones, iodoaldehydes and/oriodolipids on thyroid peroxidase activity

Reduced intrathyroidal deiodinase activity as a result of the increasediodine load might also contribute to decreased synthesis of thyroidhormones

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Thyroidal adaptation to excess iodine

• Wolff-Chaikoff effect

Mechanism

Inhibitory effect of intrathyroidal iodolactones, iodoaldehydes and/oriodolipids on thyroid peroxidase activity

Reduced intrathyroidal deiodinase activity as a result of the increasediodine load might also contribute to decreased synthesis of thyroidhormones

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Wolff-Chaikoff effect

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Wolff-Chaikoff effect : Aftermath

• Reduction of NIS levels in 24 hours

• Reduced intake - less inhibitory substances

• Impaired follicular cell - thyroid dysfunction

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Susceptible patients

• Autoimmune thyroid disease

• History of surgery I 131 or antithyroid drug therapy for Graves disease

• Subacute thyroiditis

• Postpartum thyroiditis

• Type 2 amiodarone-induced thyrotoxicosis (AIT) hemithyroidectomy

• IFNα therapy

• Concomitant use of potential goitrogens, such as lithium

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Iodine induced hyperthyroidism

Jod-Basedow phenomenon

• Nontoxic or diffuse nodular goiter

• Latent Graves disease

• Long standing iodine deficiency

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Sources of Iodine excess

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Iodine supplementation

• Iodized oil

• Water supply

• Crop irrigation

• Animal fodder

• Salt

• Bread iodophors

• Micronutrient biscuits

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Drawbacks

• Increased risk of hypothyroidism

• Increased risk of papillary carcinoma of thyroid

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Dietary sources

• Dairy products

• Seaweed - mainly in Japan

• Iodized salt

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Vitamins and supplements

• Congenital hypothyroidism caused by ingestion of excess maternaliodine tablets during pregnancy

• Discrepancy b/w measured and labelled values

• American Thyroid Association recommends against ingestion of aniodine or kelp daily supplement containing > 500µg iodine for allindividuals, except for certain medical indications

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