For a patient with hypothyroidism on levothyroxine (Synthroid) who does not use iodized salt, is a daily 150 µg iodine supplement necessary?

Taking Only 150 mcg Iodine Daily is Appropriate and Meets Standard Recommendations

For a patient with hypothyroidism on levothyroxine who does not use iodized salt, taking 150 mcg of iodine daily is the exact recommended amount and is appropriate, but only if you have confirmed iodine deficiency through 24-hour urinary iodine testing. 1

First: Determine If You Actually Need Iodine Supplementation

Before taking any iodine supplement, you must establish whether iodine deficiency is causing or contributing to your hypothyroidism:

• Measure 24-hour urinary iodine excretion (normal range: 100-300 mcg/24hr) combined with TSH, free T4, and thyroid antibodies (TPO and thyroglobulin antibodies) 1
• If your urinary iodine is >100 mcg/24hr, you are iodine-sufficient and should NOT supplement iodine, as excess iodine in non-deficient individuals worsens hypothyroidism by triggering thyroid autoimmunity, increasing TSH, and lowering thyroid hormone levels 1
• If your urinary iodine is <100 mcg/24hr AND thyroid antibodies are negative, then 150 mcg daily supplementation is appropriate 1

Why 150 mcg Daily is the Standard Dose

The 150 mcg daily dose you're considering is precisely the recommended amount:

• Clinical nutrition guidelines specify 150 mcg iodine daily as the minimum requirement for maintaining normal thyroid function, with an upper safe limit of 300 mcg 2, 3
• This dose matches the recommended daily intake for adolescents and adults who are not pregnant or lactating 4, 5
• The FDA and international standards (Codex Alimentarius) both use 150 mcg as the Daily Value for iodine 6

Critical Warning: Levothyroxine Contains Iodine

Your levothyroxine medication is already providing you with iodine:

• Levothyroxine contains 65.4% of its molecular weight in iodine 7
• Patients on levothyroxine substitution have significantly higher urinary iodine excretion (median 75 mcg/L) compared to those not on thyroid hormone (median 50 mcg/L), indicating an increased iodine pool from medication deiodination 7
• Adding 150 mcg supplemental iodine on top of levothyroxine may push you into excess territory, particularly if you consume any dietary iodine from seafood, dairy products, or other fortified foods 5

When NOT to Take Iodine Supplements

Do not supplement iodine if:

• You are iodine-sufficient (urinary iodine >100 mcg/24hr), as excess iodine induces hypothyroidism by triggering autoimmune thyroiditis—highly iodinated thyroglobulin is more immunogenic 2, 1
• You have positive thyroid antibodies (TPO or thyroglobulin antibodies), as excess iodine worsens autoimmune thyroid disease 1
• You have nodular thyroid disease, as excess iodine can trigger hyperthyroidism in autonomous nodules 4
• You are elderly or have pre-existing thyroid disease, as these are susceptible populations for iodine-induced dysfunction 5

Practical Algorithm for Your Situation

Step 1: Get 24-hour urinary iodine excretion measured along with TSH, free T4, and thyroid antibodies 1

Step 2: Interpret results:

• If urinary iodine <100 mcg/24hr AND antibodies negative: Take 150 mcg iodine daily through supplementation or dietary sources (iodized salt at 60 mcg/g, dairy, seafood) 1
• If urinary iodine >100 mcg/24hr: Do NOT supplement—you're already getting enough from levothyroxine and diet 1
• If antibodies positive: Do NOT supplement regardless of urinary iodine level 1

Step 3: If supplementing, monitor for signs of iodine toxicity including abdominal pain, metallic taste, nausea, diarrhea, and worsening thyroid function 2, 8

Common Pitfall to Avoid

The biggest mistake is supplementing iodine without first confirming deficiency. In iodine-sufficient individuals, excess iodine is most associated with elevated TSH, lower thyroid hormone levels, increased thyroid autoimmunity, and worsening hypothyroidism and goiter 2, 1. Since your levothyroxine already provides substantial iodine through deiodination 7, adding 150 mcg without testing could paradoxically worsen your hypothyroidism rather than improve it.