Iodine Blood Test: Levels, Ranges & Clinical Guide

What Is the Iodine Blood Test?

The serum iodine test measures the concentration of total iodine circulating in your bloodstream. Iodine is absorbed from food (seafood, dairy, eggs, iodised salt) and is rapidly taken up by the thyroid gland. The kidneys excrete excess iodine, so blood levels reflect recent intake and overall balance. LOINC code 2731-8 corresponds to iodine in serum or plasma. The test is typically ordered when there are signs of thyroid disease, suspected iodine deficiency, or to monitor patients receiving iodine-containing medications (e.g., amiodarone).

Why Is This Test Ordered?

• Evaluate thyroid function: Unexplained hypothyroidism or hyperthyroidism despite normal TSH and T4 can be due to abnormal iodine supply.
• Investigate goiter: An enlarged thyroid gland is a classic sign of iodine deficiency (endemic goiter) or excess (iodine-induced goiter).
• Amiodarone therapy monitoring: This heart medication contains high iodine and can disrupt thyroid function.
• Radioactive iodine therapy planning: Assessing baseline iodine helps predict treatment response in thyroid cancer.
• Investigating unexplained fatigue, weight changes, or cold intolerance.

Normal Reference Ranges for Serum Iodine

Reference intervals can vary slightly between laboratories. The table below shows typical values used in our practice (Mayo Clinic reference ranges adapted). Values are in micrograms per deciliter (mcg/dL).

Critical Values

Severe deficiency: < 2.0 mcg/dL — risk of cretinism in infants, myxedema coma in adults. Toxic excess: > 25.0 mcg/dL — may cause hyperthyroidism or goiter.

Low Iodine Levels: Causes and Symptoms

In my practice, the most common cause of low serum iodine is inadequate dietary intake, especially in regions without iodised salt or in people on restrictive diets (e.g., vegan, Paleo without kelp). Symptoms include fatigue, weight gain, cold hands and feet, dry skin, brittle nails, and a feeling of “brain fog.” Over time, the thyroid enlarges (goiter) to compensate. Pregnant women with low iodine risk giving birth to children with intellectual disabilities.

Iodine Deficiency During Pregnancy

This is a critical window. The fetal brain depends on maternal T4, which requires iodine. I recommend all pregnant and lactating women take a prenatal vitamin with 150 mcg of iodine, especially if they don’t eat fish or use iodised salt.

High Iodine Levels: When is it dangerous?

Excess iodine can trigger the Wolff-Chaikoff effect, causing transient hypothyroidism, or in susceptible individuals, iodine-induced hyperthyroidism (Jod-Basedow phenomenon). High levels are often seen in patients taking amiodarone, using iodine-containing expectorants, or consuming large amounts of kelp supplements. Symptoms of excess include palpitations, anxiety, weight loss, tremor, and heat intolerance. I once saw a patient who developed hyperthyroidism after drinking “detox” kelp tea daily for two months.

Amiodarone and Iodine Levels

Amiodarone contains about 37% iodine by weight. Up to 20% of patients on this drug develop thyroid dysfunction. I routinely check baseline and periodic serum iodine, TSH, and free T4 to catch abnormalities early.

How to Prepare for the Test

No special fasting is needed, but you should inform your doctor about any supplements, medications, or recent contrast dye (CT scans, X-rays) that contain iodine, as these can skew results. The test is a simple blood draw from your arm.

What Do My Results Mean?

I always interpret iodine levels in the context of clinical history and other thyroid labs (TSH, T4, T3, thyroid antibodies). Isolated low iodine with normal TSH may indicate early dietary insufficiency. High iodine with low TSH suggests hyperthyroidism; high iodine with high TSH suggests amiodarone-induced hypothyroidism. If you have borderline results, a 24-hour urine iodine test may provide a more accurate picture of whole-body iodine status.

Treatment and Next Steps

For mild to moderate deficiency, increasing dietary intake (fish, seaweed, dairy, iodised salt) or taking a low-dose supplement (150 mcg daily) usually corrects the problem within weeks. Severe deficiency may require higher doses under medical supervision. For excess, the first step is to discontinue any supplements or medications, and the doctor may prescribe antithyroid drugs if hyperthyroidism is present. In either case, I recommend repeat testing after three months.

Frequently Asked Questions

Can I get enough iodine from salt alone? Yes, half a teaspoon of iodised salt provides about 150 mcg — the daily recommended intake for most adults. But avoid overconsuming salt due to hypertension risks.

Is the iodine blood test covered by insurance? Typically yes, when ordered for a valid medical indication. Check with your provider.

How long does it take to get results? Usually 2–3 business days.