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A popular but unreliable way to check iodine status. Here's what it tells us and what it doesn't.
Paint a 2-inch square of iodine (usually Lugol's) on the inner forearm. The idea is that if the stain disappears quickly (within a few hours), the body is deficient — "soaking up" the iodine. If it lingers, sufficiency is supposedly indicated.
Paint a patch
Apply 2-3 drops of Lugol's on the inner forearm in a small square.
Wait and observe
Note how long the orange-brown stain remains visible on the skin.
Interpret (loosely)
Fast disappearance suggests higher absorption; slow disappearance suggests lower absorption.
The disappearance rate depends on many factors beyond iodine status: skin thickness, temperature, humidity, blood flow to the area, and even clothing friction. The iodine also evaporates — it doesn't all get absorbed. This makes the test too variable to draw medical conclusions from.
The 24-hour urine loading test is considered more reliable for assessing iodine status. It measures how much iodine the body retains vs. excretes after a measured dose.
While it can't diagnose deficiency, the patch test is a gentle way to start transdermal iodine supplementation. Painting a small amount on the skin is a very low-dose introduction that allows observing the body's initial response before taking iodine orally.
Learn about the 24-hour urine loading test and other assessment methods.