What is the recommended iodine dosing for a child with hypothyroidism?

Iodine Dosing for Pediatric Hypothyroidism

Iodine supplementation is NOT the primary treatment for hypothyroidism in children—levothyroxine is the standard therapy. However, if iodine deficiency is the underlying cause of hypothyroidism, iodine supplementation at 150 mcg/day (with an upper limit of 300 mcg/day) should be provided alongside thyroid hormone replacement 1.

Critical Distinction: Iodine vs. Levothyroxine

The question asks about iodine dosing, but it's essential to understand that hypothyroidism treatment requires levothyroxine (thyroid hormone replacement), not iodine supplementation in most cases 2, 3. Iodine is only indicated when deficiency is documented as the etiology 4.

When Iodine Supplementation Is Appropriate

Iodine supplementation should only be considered when:

• Documented iodine deficiency exists (24-hour urinary iodine <100 mcg/24hr or undetectable) 1, 4
• The child has a severely restrictive diet lacking iodized salt, dairy, bread, and seafood 4
• Hypothyroidism is directly attributable to iodine deficiency rather than autoimmune thyroiditis, congenital hypothyroidism, or other causes 1, 4

Iodine Dosing Recommendations

For children with documented iodine deficiency causing hypothyroidism:

• Standard dose: 150 mcg/day via oral or enteral route 1
• Upper limit: 300 mcg/day in enteral nutrition formulations 1
• Alternative dosing for severe deficiency: 300-600 mcg/day orally 1
• IV route (severe deficiency only): 131 mcg/24hr as sodium iodide solution when oral/enteral routes unavailable 1

Route of Administration

• Oral/enteral route is preferred as iodine is well absorbed 1
• IV sodium iodide can be given in acute severe deficiency (distinct from multi-trace element vials which contain ~130 mcg per dose) 1
• IM injection is an alternative option 1

Standard Treatment: Levothyroxine Dosing

Since levothyroxine is the actual treatment for hypothyroidism, age-based dosing is:

Pediatric Levothyroxine Doses (by age):

• 0-3 months: 10-15 mcg/kg/day 2
• 3-6 months: 8-10 mcg/kg/day 2
• 6-12 months: 6-8 mcg/kg/day 2
• 1-5 years: 5-6 mcg/kg/day 2
• 6-12 years: 4-5 mcg/kg/day 2
• >12 years (growth incomplete): 2-3 mcg/kg/day 2
• Growth complete: 1.6 mcg/kg/day 2

For autoimmune thyroiditis-induced hypothyroidism, lower doses are required:

• 6-10 years: 2.0 ± 0.4 mcg/kg/day 5
• 10-12 years: 1.6 ± 0.4 mcg/kg/day 5
• 12-14 years: 1.5 ± 0.6 mcg/kg/day 5
• ≥14 years: 1.4 ± 0.6 mcg/kg/day 5

Clinical Approach Algorithm

Step 1: Diagnose the etiology of hypothyroidism

• Measure TSH, free T4, and thyroid antibodies 1
• Obtain 24-hour urinary iodine excretion if deficiency suspected 1
• Take detailed dietary history focusing on iodized salt, dairy, bread, and seafood intake 4

Step 2: Initiate appropriate treatment

• If iodine deficiency confirmed: Start iodine 150 mcg/day PLUS levothyroxine at age-appropriate dose 1, 2
• If other etiology: Start levothyroxine alone at age-appropriate dose 2, 3

Step 3: Monitor response

• With iodine supplementation: Visible goiter improvement within 2 weeks, thyroid function normalization within 4 weeks 4
• Titrate levothyroxine every 2 weeks based on TSH and free T4 until euthyroid 2
• Target TSH in normal range; free T4 in upper half of normal range for secondary/tertiary hypothyroidism 2

Critical Pitfalls to Avoid

Excess iodine causes harm:

• Doses >300 mcg/day can induce autoimmune thyroiditis and paradoxical hypothyroidism 1
• Chronic excess causes elevated TSH, lower thyroid hormones, and increased autoimmunity 1
• Toxicity signs include abdominal pain, metallic taste, coughing, fever, delirium, diarrhea, and vomiting 1

Overtreatment with levothyroxine impairs cognition:

• Episodes of overtreatment during the first 6 months correlate with deficits in sustained attention at school age 6
• Avoid excessive initial doses; titrate carefully 6

Co-nutrient deficiencies must be addressed:

• Thyroid function requires adequate selenium and iron 1
• Iron deficiency impairs thyroid metabolism 1
• Screen and correct these deficiencies concurrently 1

Special populations require dose adjustments:

• Infants 0-3 months at risk for cardiac failure: start lower and increase every 4-6 weeks 2
• Children at risk for hyperactivity: start at one-fourth dose, increase weekly by one-fourth until full dose reached 2