A Look at Iodine's Two Isotopes You'll Hear About: I-127 and I-131

Not all iodine is chemically identical — isotopes matter. This post distinguishes the stable, naturally occurring I-127 (used in nutrition and supplements) from I-131, a radioactive isotope used in unrelated medical contexts, to clear up a common point of public confusion.

What an Isotope Is

Every iodine atom has 53 protons; that's what makes it iodine. Isotopes are versions of the same element with different numbers of neutrons. They behave almost identically in chemical reactions, but they differ in mass, and some are stable while others are radioactive, meaning they break down over time and give off radiation.

Scientists know of dozens of iodine isotopes. Only one of them is stable.

Iodine-127: The Iodine in Food and Supplements

• Makeup: 53 protons and 74 neutrons.
• Stable: it does not decay and is not radioactive.
• Natural: essentially all naturally occurring iodine on Earth is iodine-127.
• Where you find it: seawater, seaweed, seafood, dairy, eggs, iodized salt and every iodine supplement.

When the thyroid uses iodine to make hormones, it's using iodine-127. See How Iodine Supports Thyroid Hormone Production.

Iodine-131: A Radioactive Isotope

• Makeup: 53 protons and 78 neutrons.
• Radioactive: it decays, with a half-life of about eight days. After eight days, half of a given amount has decayed; after a few months, very little remains.
• Not found in nature in meaningful amounts: it's produced in nuclear reactors.
• Uses: it's used in specialized medical settings, administered only by physicians in controlled conditions.

Other radioactive iodine isotopes, such as iodine-123 and iodine-125, are also used in medical imaging and research.

Why the Confusion Happens

Because both are called "iodine," news stories about radioactive iodine sometimes leave people wondering whether iodine supplements are radioactive. They are not. Supplements, iodized salt and iodine-rich foods contain only stable iodine-127. The name is shared; the behavior is completely different.

Why the Body Treats Them Alike

Because isotopes of the same element behave the same way chemically, the thyroid can't tell them apart. It takes up whatever iodine is available. That's the scientific reason radioactive iodine is useful in specialized medicine, and it's also why the thyroid's iodine supply is a subject health agencies pay attention to.

The Bottom Line

The iodine in a bottle of Lugol's Solution, in a serving of cod or in a pinch of iodized salt is stable iodine-127, the same iodine the body has always relied on. For more on iodine's place on the periodic table, see Iodine and the Periodic Table.

Frequently Asked Questions

Are iodine supplements radioactive?
No. Iodine supplements contain stable iodine-127, which is not radioactive.

Do I need to understand the chemistry to use this safely?
No — you don't need a chemistry background to use a liquid iodine product as directed. Understanding the basics just helps you read labels and compare products more critically.

What does stable I-127 actually do once it's in the body?
See Iodine's Role in Metabolism and Cellular Energy.

Shop Lugol's Solution — the traditional, trusted way people have supported their iodine intake for over 200 years.

Ryan Whitfield covers the chemistry and wellness-routine side of iodine supplementation for the Iodine Journal.

This article is for general educational purposes only and has not been evaluated by the FDA. It is not intended to diagnose, treat, cure, or prevent any disease, and is not a substitute for advice from a qualified healthcare provider — especially if you are pregnant, nursing, taking thyroid medication, or managing a thyroid condition.