Radioactive Iodine: What is it? How does it work? History, Uses, Procedure, Safety, Side Effects and Precautions

It is a procedure that results in the intentional destruction of the thyroid gland.

This procedure is often used to treat serious medical conditions, such as cancer.

What exactly is radioactive iodine?

Radioactive iodine (often abbreviated RAI) is a therapy used to treat hyperthyroidism and thyroid cancer. In this procedure, you are intentionally given iodine, I-131, which gives off radiation .

Your body then absorbs this radioactive iodine in the thyroid gland, where the radiation proceeds to destroy the thyroid gland tissue. Radioactive iodine works by taking advantage of certain aspects of normal physiology.

You probably already know that iodine (not the radiated version) is needed for optimal thyroid hormone production. Each thyroid hormone has a specific amount of iodide (2, 3 or 4 residues depending on the type of thyroid hormone).

Also, your thyroid gland stores up to 15-20 grams of iodine as a reservoir for thyroid hormone production (stores that you can use when needed). Radioactive iodine takes advantage of this storage capacity of the thyroid gland.

Therefore, when you take radioactive iodine, your thyroid will absorb and store the irradiated iodine, causing local damage as it emits radiation and disrupts cell function.

History of radioactive iodine

Radioactive iodine, (I-131) first discovered in 1938, is a nuclear fission by-product of uranium. In the early 1940s, radioactive iodine was used to study the physiology of the thyroid.

In the late 1940s, it was used to treat hyperthyroidism and thyroid cancer, which is why I-131 was used clinically for the past 70 years.

How does it work?

Iodine is actively transported and incorporated into thyroglobulin , a precursor of thyroid hormone, thus trapping iodine in the gland.

With the exception of the salivary glands, very little iodine is absorbed elsewhere in the body, and unincorporated iodine is rapidly excreted in the urine, with much smaller amounts excreted in feces, saliva, and perspiration.

Two radioisotopes of iodine are used clinically: I-131 (half-life 8.02 days) which was used primarily for therapy, and I-123 (half-life 13 hours), which is a very low-energy emitter of gamma radiation useful for diagnosis and imaging studies.

The radioactive iodine uptake measurement (RAIU) determines how much iodine is absorbed by the thyroid gland over a period of several hours.

It is useful in determining the underlying cause of thyroid dysfunction and can be used to guide therapeutic management decisions based on the outcome.

Radioactive iodine procedure

The procedure is simple, the patient ingests a known amount of radioactive iodine and then returns 24 hours later to have their neck measured with a device similar to a Geiger counter.

The amount of radiation to the thyroid is then divided by the initial known amount of radioactive iodine, giving the% absorption. A normal uptake of radioiodine from the thyroid in adults is between 10-30% at 24 hours.

Patients with Graves’ disease often have markedly elevated radioactive iodine uptake, where thyroiditis patients typically have extremely low measurements of radioactive iodine uptake (due to a lack of iodine uptake in the injured thyroid tissue).

Radioactive iodine intake can be influenced by dietary iodine intake (e.g. vitamins, fish oil capsules, iodinated contrast dye) and antithyroid medications, making various radioactive iodine uptake measurements necessary days (sometimes weeks) after avoided

Radioactive iodine uptake measurements are inexpensive and can be done in the office. A radioactive iodine thyroid scan creates an image of actively trapped iodine in the thyroid, revealing areas of increased or decreased absorption.

In the past, thyroid scans were sometimes done to determine if a thyroid nodule was “hot,” a nodule that absorbed a lot of radioiodine) or “cold” (a nodule that did not absorb any radioiodine).

Most hot nodules are benign, but “cold” nodules have a 20% risk of malignancy and should be further evaluated.

The advent of ultrasound biopsy and fine needle aspiration (FNA) has avoided the need for some thyroid scans, except in the diagnosis of toxic multinodular goiter or toxic adenoma.

Another type of imaging study is a “whole body scan” that is obtained before or after I-131 therapy in patients undergoing total thyroidectomy for thyroid cancer.

Other nuclear medicine scans also use I-131 to image other tissues (eg, MIBG; iodine-131-meta-iodobenzylguanidine, an agent that targets adrenal medullary tissue [chromaffin], to identify pheochromocytomas).


Higher doses of radioiodine (30-200 mCi) are often used to treat high-risk papillary or follicular thyroid cancer; there is no evidence that radioactive iodine treatment decreases the risk of recurrence or improves longevity in low-risk patients.

Radioactive iodine therapy is given to outpatients when thyroid stimulating hormone (TSH) levels are elevated (either through the use of recombinant human thyroid stimulating hormone or hormone withdrawal thyroid).

In the last decade, there has been a marked reduction in the doses of radioactive iodine used to treat thyroid cancer, with fewer side effects, but no apparent difference in results.

Uses of radioactive iodine

Radioactive iodine is often used to treat medical conditions that cause hyperthyroidism or the production of too much thyroid hormone from the thyroid gland.

Conditions such as Graves’ disease or toxic multinodular goiter can lead to excessive thyroid hormone production.

This production can be stopped or mitigated by either taking medications (antithyroid drugs), removing the thyroid gland (thyroidectomy), or destroying it completely with radioactive iodine.

In this way, radioactive iodine is a powerful therapy that results in intentional damage to the tissue of the thyroid gland. As you may suspect, the destruction of your gland with radioactive iodine is associated with some side effects and is not without risks.

It may seem like a barbaric treatment, but it has been around since the 1940s and is often used to treat hyperthyroidism and thyroid cancer. It is considered a “definitive” treatment for hyperthyroidism, as is thyroidectomy.

It is safe?

For the most part, if done correctly and if proper precautions are taken after the procedure, it is safe.

However, there are some groups of people who should not use radioactive iodine, and this group includes pregnant women, women who are breastfeeding, and patients who are unstable or in a thyroid crisis.

Radioactive iodine has been studied extensively in a variety of different thyroid diseases, situations, doses, and ages, studies dating back to the 1950s.

For “low” doses of radioactive iodine used to treat hyperthyroidism (eg, 10-30 mCi), the risk of malignancies appears negligible in most studies and meta-analyzes.

However, there is increased concern about the routine use of radioactive iodine in preadolescents, so caution is advised with the use of radioactive iodine in the pediatric population, and it is never used in pregnancy.

I-131 therapy is generally for outpatients, and recipients are instructed to follow some basic hygiene and safety precautions for varying periods of time after therapy.

How is radioactive iodine used for individual conditions?

Radioactive iodine treatment of hyperthyroidism : Perhaps one of the most common uses for radioactive iodine is as a therapy to treat hyperthyroidism. As we discussed, radioactive iodine produces complete and irreversible damage to the thyroid gland.

Because of this, you should consider all the options and therapies available to you before undergoing this procedure.

In the case of hyperthyroidism (caused by Graves’ disease, toxic multinodular goiter, or any other cause), radioactive iodine is considered a “definitive” therapy.

It is definitive because after the procedure your thyroid gland will no longer function optimally and your hyperthyroidism will have been treated.

You can compare this to temporary hyperthyroidism treatments, such as thyroid-blocking medication.

Thyroid blocker medication is not permanent because once you stop taking the medication, your condition will likely return.

Is radioactive iodine effective in treating hyperthyroidism?

The answer is yes. Up to 60-80% of patients with hyperthyroidism have Graves disease, which is an autoimmune disease.

Patients with this condition often experience excessive doses of thyroid hormone as the antibodies stimulate the thyroid gland. Almost 90% of patients with Graves’ disease experience a reduction in serum thyroid hormone levels after the procedure.

Because of this, it is considered a safe alternative to complete thyroid removal (thyroidectomy) for hyperthyroidism. It is important to realize that radioactive iodine is not the only option available for the treatment of hyperthyroidism.

Before undergoing radioactive iodine, make sure you have evaluated all the options available to you. There are 3 main ways to treat hyperthyroidism:

  1. Thyroid blocking drugs, usually temporary.
  2. Radioactive iodine, more definitive than medication.
  3.  Thyroidectomy, more definitive than medication.

Treatment of thyroid cancer with radioactive iodine

In addition to being used to treat hyperthyroidism, radioactive iodine can also be used to treat thyroid cancer. But it is often used in a different way that is worth discussing.

Thyroid cancer represents a different medical condition, compared to hyperthyroidism, and requires slightly different treatment. The recommended standard treatment for thyroid cancer is thyroidectomy or complete removal of the thyroid.

This procedure allows the almost complete removal of the thyroid gland and, in general, 99% of the thyroid cancer cells. But what you may not realize is that it is nearly impossible to remove 100% of the thyroid gland tissue even with thyroidectomy.

This leaves a small percentage of thyroid gland tissue in the body. The remaining tissue of the thyroid gland slightly increases the risk of thyroid cancer recurrence.

Also, it is possible that thyroid cancer cells may exist outside the thyroid gland during the operation (for metastatic disease).

Due to these two main reasons, radioactive iodine is often used as an additional therapy in combination with thyroidectomy to completely remove the thyroid gland tissue in the body.

This combination of therapy reduces the risk of thyroid cancer recurrence and increases the likelihood that it will kill any thyroid gland tissue or cancer cells that may be outside the thyroid gland.

Is there a problem with this approach?

Remember that each procedure has risks and possible side effects. These side effects must be weighed against the possible positive outcome of therapy.

Because of this, not every patient with thyroid cancer will have both procedures. In fact, current recommendations state that the addition of radioactive iodine to thyroidectomy can be determined by the physician and in agreement with the patient.

Before obtaining radioactive iodine in addition to thyroidectomy, these two factors should be considered:

# 1. Thyroid cancer has a low risk of recurrence after thyroidectomy.

Even without radioactive iodine, the risk of thyroid cancer recurrence is very low, so adding radioactive iodine cannot further reduce that risk in all patients.

# 2. There are always health risks associated with radioactive iodine.

Radioactive iodine is a procedure in which radioactive iodine is administered with the intention of destroying the tissue of the thyroid gland.

About 50% of thyroid cancer patients receive thyroidectomy and radioactive iodine. This leaves another 50% who only receive thyroidectomy.

Each individual thyroid cancer patient must weigh the risks and benefits before adding radioactive iodine to their treatment plan. Your doctor should be able to help you determine whether radioactive iodine makes sense for you.

Radioactive Iodine Side Effects and What to Expect After Therapy

Although radioactive iodine is considered a “safe” procedure, it is not without risks. The side effects of low-dose radioactive iodine are minimal – some patients experience mild thyroid pain that usually responds to pain relievers.

Remember, when you take radioactive iodine, you intentionally put radioactive substances into your body. Yes, it is true that this radioactivity is mainly limited to the thyroid gland, but there is still a small risk that you can transmit some of this radiation to others.

Because of this, certain precautions must be taken to avoid this transfer to others. But beyond the side effects, what should you expect after treatment? Remember that the goal of radioactive iodine is to completely destroy your thyroid gland.

This destruction of the thyroid gland tissue will permanently alter the thyroid hormone status in your body. After the procedure, you will be dependent on thyroid medications for the rest of your life.

In reality, you are becoming a hypothyroid patient, which can be confusing for many patients, especially those who are undergoing radioactive iodine for the treatment of hyperthyroidism.

Rates of post-procedure hypothyroidism vary in terms of its onset, depending on the dose of radioactive iodine and the underlying cause of the hyperthyroidism: most patients with Graves’ disease develop hypothyroidism within several months.

While a smaller percentage of patients with toxic multinodular goiter or toxic adenomas develop hypothyroidism (10-25%) for a longer period of time. Once hypothyroidism develops, it usually requires life-long thyroid hormone replacement therapy.

Many of the side effects associated with radioactive iodine have to do with the hypothyroid state that occurs after the procedure and not with the side effects directly associated with the procedure itself.

Weight gain after radioactive iodine: It is well known that radioactive iodine poses a potentially serious threat to your current weight.

The risk of being overweight and obese has been reported (and shown) to increase dramatically after radioactive iodine ablation therapy.

This risk increases two to three times compared to normal healthy euthyroid controls (those without thyroid damage).

It was previously thought that this weight gain was likely due to changes in metabolism and food intake after treatment with hyperthyroidism, but the same effect is seen in those with normal thyroid status who undergo radioactive iodine (such as those with cancer thyroid).

Other theories suggest that weight gain is the result of a hypothyroid state that can occur immediately after the procedure and before thyroid hormone levels are optimized with medication.

The exact cause of the weight gain is not well understood, but it is most likely related to the administration of thyroid medications after the procedure.

Your thyroid is responsible for about 50-60% of your basal metabolism. Small changes in thyroid hormone can disrupt the normal regulation of your metabolism and can lead to weight gain.

It seems unlikely that doctors will be able to perfectly balance the amount of thyroid hormone you need after your procedure with 1 pill taken once a day.

This weight gain is also seen after thyroidectomy and with antithyroid medications, indicating that the weight gain is likely secondary to thyroid hormone levels and thyroid dose.

It is important to know that not all patients who undergo radioactive iodine will experience dramatic weight gain. Patients who enter the procedure overweight or obese are much more likely to experience weight gain after the procedure.

There are more side effects with higher doses of radioactive iodine therapy, including nausea, vomiting, and sialadenitis (swelling of the salivary glands) that can lead to pain and dry mouth (xerostomia), as well as changes in test sensation and tearing excessive (epiphora).

At high doses of radioactive iodine, there is an increased risk of secondary malignancies, including salivary glands, stomach, bladder, and hematologic malignancies, although these are very rare.

Special side effects for men

Radioactive iodine can pose a threat to fertility, especially in men.

Radioactivity associated with radioactive iodine has been shown to reduce sperm count, which can lead to infertility for up to 2 years after the procedure.

This side effect is not necessarily a contraindication to the procedure, as depositing your sperm before the procedure can prevent these negative effects.

Long-term infertility is generally not a problem unless several rounds of radioactive iodine are needed (as for metastatic cancer).

Before undergoing radioactive iodine, be sure to consider this side effect and take necessary precautions, such as banking sperm, if necessary.

Special side effects for women

Specifically for women, radioactive iodine poses a serious risk to fetal development in pregnant women.

Radioactive iodine is easily transferred through the placenta, which means it can seek out and destroy the developing baby’s thyroid gland.

Thyroid function is necessary for brain development, and damage to the thyroid can lead to mental retardation in the developing fetus. Radioactive iodine should also be avoided in women who are breastfeeding.


Because radioactive iodine uses radioactivity, certain precautions should be taken after the procedure. These precautions help reduce exposure to radioactivity to loved ones or the people you live with.

Because of this risk, doctors often make recommendations to avoid sexual intercourse, avoid contact with children, breastfeed, avoid sleeping next to people, and avoid conception for days or months after the procedure.

Certain activities, such as sleeping near someone, returning to work, preparing food for others, being in public places, traveling (traveling by plane and traveling by car), sharing utensils with other people, and sitting in a shared bathroom should only be avoided, for up to 5-7 days.

While others, such as attempting conception, should be avoided for at least 6 months. These precautions and recommendations from your doctor may vary.

Radioactive iodine is a relatively safe and very effective treatment for hyperthyroidism and thyroid cancer. Like any procedure, be sure to weigh the potential benefits against the risks because this procedure is not without side effects.

Radioactive iodine results in permanent damage to your thyroid gland that will leave you dependent on thyroid medications for the rest of your life.

This thyroid hormone supplementation, if not done correctly, can increase your risk of weight gain, cardiovascular disease, and other side effects.

Radioactive iodine should always be avoided if you are pregnant or breastfeeding, but other than that, it doesn’t have many absolute contraindications.