Iodine, Serum or Plasma

Iodine is a mineral your body needs to make thyroid hormones, which control your metabolism and energy levels. This blood test measures how much iodine is currently circulating in your bloodstream, which can be affected by what you've eaten recently, certain medications, or medical procedures involving contrast dye. Too little iodine can cause your thyroid to become underactive or enlarged, while too much—often from medications like amiodarone or contrast agents—can also disrupt thyroid function. Your doctor will typically look at this result alongside thyroid hormone tests to get a complete picture of your thyroid health. Eating a balanced diet that includes iodized salt, seafood, or dairy products usually provides adequate iodine for most people.

When elevated: Elevated serum iodine may indicate excessive dietary intake, use of iodine-containing medications (notably amiodarone), recent administration of iodinated contrast agents, or topical iodine exposure. High iodine levels can suppress thyroid function (Wolff-Chaikoff effect) causing hypothyroidism, or paradoxically trigger hyperthyroidism (Jod-Basedow phenomenon) in susceptible individuals with underlying thyroid nodules or latent Graves' disease. When low: Low serum iodine suggests insufficient dietary iodine intake, which is the leading cause of preventable hypothyroidism and goiter worldwide. Iodine deficiency during pregnancy is associated with fetal neurodevelopmental impairment and cretinism. Mild-to-moderate deficiency may present with subclinical hypothyroidism, fatigue, weight gain, and thyroid enlargement.

Serum iodine is moderately relevant to athletes because adequate iodine supports thyroid hormone production, which regulates metabolic rate, energy availability, and recovery. While not a direct performance marker, chronic iodine insufficiency can impair thyroid function, potentially affecting endurance capacity and training tolerance—though this is typically only a concern in athletes with poor dietary iodine intake or those restricting processed foods (a major iodine source).