Iodine Deficiency

Iodine deficiency is an inadequate intake of iodine, the essential component of thyroid hormones, which causes goitre, hypothyroidism and, in the fetus and infant, irreversible brain damage. It remains the commonest preventable cause of intellectual impairment worldwide, and mild deficiency has re-emerged in many countries, especially in pregnant women.

• Daily requirement: 90 micrograms (children 1 to 8 years), 120 micrograms (9 to 12 years), 150 micrograms (adults), 250 micrograms (pregnancy and breastfeeding). The upper safe limit is about 600 to 1100 micrograms daily in adults.
• Population status by median urinary iodine concentration in school age children and non-pregnant adults: below 20 micrograms/L severe deficiency; 20 to 49 moderate; 50 to 99 mild; 100 to 199 adequate; 200 to 299 above requirements; 300 or more excess. In pregnancy, adequate intake is 150 to 249 micrograms/L.
• Sources: iodised salt, dairy products, white fish, shellfish, eggs, seaweed (varies, sometimes excessive), bread made with iodate dough conditioners, and iodine supplements. Plant foods depend on soil iodine.
• Risk groups: pregnant and lactating women, vegans and those avoiding dairy and fish, people in mountainous or inland regions with iodine poor soils, adolescents, and those eating goitrogen rich diets (cassava, millet, soy, cruciferous vegetables, thiocyanate in smokers); high dose selenium deficiency worsens effects.
• Consequences: goitre, hypothyroidism, subclinical hypothyroidism, impaired cognition, increased miscarriage, stillbirth, congenital anomalies, neonatal hypothyroidism, and severe endemic cretinism.

• Goitre (diffuse, later nodular), neck fullness, dysphagia or cough from compression.
• Hypothyroid symptoms: fatigue, cold intolerance, weight gain, constipation, dry skin, hair loss, low mood, menstrual disorders, infertility.
• Children: poor school performance, reduced IQ (about 8 to 13 points lower in severe deficiency), growth delay.
• Pregnancy: maternal goitre, hypothyroxinaemia; infant with low birth weight, neonatal goitre, hypothyroidism.
• Severe endemic cretinism: deaf mutism, spastic diplegia, squint, severe intellectual disability (neurological type), or dwarfism with myxoedema (myxoedematous type).

• Dietary history of iodised salt, dairy, fish and supplements.
• Spot urinary iodine concentration is a population marker (day to day variation is large, so individual diagnosis needs multiple samples or 24 hour collection, with iodine to creatinine ratio).
• Thyroid function tests: thyroid stimulating hormone, free thyroxine (and free triiodothyronine); thyroid antibodies (anti-thyroid peroxidase) to exclude autoimmune disease; serum thyroglobulin (above 13 micrograms/L in children indicates deficiency); neonatal heel prick thyroid stimulating hormone for screening.
• Thyroid ultrasound for volume and nodules; radioiodine uptake (raised in deficiency) when needed.
• Differentials: autoimmune thyroiditis, goitrogens, lithium or amiodarone effect, thyroid nodules, thyroid cancer.

Population prevention

• Universal iodised salt (20 to 40 mg of iodine per kg of salt) is the main strategy, with salt intake kept under 5 g daily. Iodised oil capsules or fortified foods where salt iodisation is not possible.

Individual treatment and prevention

• Dietary advice: 2 to 3 servings of dairy daily, fish or seafood twice a week, eggs, iodised salt (about 1 g of iodised salt gives 20 to 40 micrograms).
• Pregnant women, breastfeeding women and those planning pregnancy: potassium iodide supplement 150 micrograms (200 micrograms in some regions) once daily from before conception through breastfeeding, in addition to dietary iodine, started preferably before 10 weeks; use combined prenatal vitamins containing iodine. Check labels (kelp supplements have variable and sometimes very high iodine content, so they are not advised).
• Vegans: iodine supplement 150 micrograms daily or iodised salt.
• Infants: iodised formula or maternal intake; in severe deficiency, 100 micrograms daily.
• Severe deficiency: iodised oil 400 mg orally once yearly (200 mg for children under 1 year and for women of reproductive age), or potassium iodide 150 to 300 micrograms daily.
• Goitre in iodine deficiency: iodine repletion (150 to 200 micrograms daily) for 6 to 12 months, sometimes with levothyroxine 1 to 1.5 micrograms/kg daily if thyroid stimulating hormone is raised (check at 6 to 12 weeks).
• Hypothyroidism: levothyroxine 1.6 micrograms/kg daily (25 to 50 micrograms in older adults or cardiac disease), adjusted every 6 to 8 weeks to a thyroid stimulating hormone within the laboratory range (pregnancy target 0.1 to 2.5 mIU/L and increase the dose by 25 to 30 percent when pregnancy is confirmed).

Safety

• Excess iodine causes goitre, thyroiditis, hypothyroidism and iodine induced hyperthyroidism, mainly in people with nodular goitre or autoimmune thyroid disease. Avoid doses above 500 micrograms daily without monitoring; check thyroid function 4 to 6 weeks after starting treatment in people with multinodular goitre. Avoid supplements in untreated hyperthyroidism, Graves disease and thyroid nodules that function autonomously. Iodine and amiodarone or contrast agents can disrupt thyroid function.

Special groups

• Children and adolescents: dietary iodine, iodised salt; supplement only if deficiency is proven.
• Infants of mothers with deficiency: neonatal thyroid screening.
• Smokers and people eating goitrogens: stop smoking, cook cassava thoroughly.

Follow up

• Review thyroid size and thyroid stimulating hormone at 6 to 12 months; repeat urinary iodine on programme level; refer for large or compressive goitre, nodules, or lack of response.