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 severe 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. You are intentionally given iodine, I-131, which gives off radiation in this procedure.

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, so I-131 has been 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.

Except for the salivary glands, very little iodine is absorbed elsewhere in the body. Unincorporated iodine is rapidly excreted in the urine, with much smaller amounts in feces, saliva, and sweat.

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

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

It helps determine 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 radioiodine uptake from the thyroid in adults is between 10-30% at 24 hours.

Patients with Graves’ disease often have markedly elevated radioactive iodine uptake. In contrast, thyroiditis patients typically have shallow 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 avoiding.

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 increased or decreased absorption areas.

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 drink 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 diagnosing toxic multinodular goiter or toxic adenoma.

Another type of imaging study is a “whole-body scan” 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 (e.g., 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 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 product can be stopped or mitigated by either taking medications (antithyroid drugs), removing the thyroid gland (thyroidectomy), or destroying it entirely with radioactive iodine.

In this way, radioactive iodine is a powerful therapy that results in intentional damage to the thyroid gland tissue. 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 brutal 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 a thyroidectomy.

Is it safe?

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

However, some groups of people 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 various thyroid diseases, situations, doses, and ages, studies dating back to the 1950s.

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

However, there is increased concern about the routine use of radioactive iodine in preadolescents, so caution is advised with the help 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 after treatment.

How is radioactive iodine used for individual conditions?

Radioactive iodine treatment of hyperthyroidism: Perhaps one of the most common uses for radioactive iodine is a therapy to treat hyperthyroidism. As 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 your thyroid gland will no longer function optimally after the procedure, 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 your condition will likely return once you stop taking the drug.

Is radioactive iodine effective in treating hyperthyroidism?

The answer is yes. Up to 60-80% of patients with hyperthyroidism have Graves disease, 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 essential to realize that radioactive iodine is not the only option available for treating hyperthyroidism.

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

  1. Thyroid blocking drugs are usually temporary.
  2. Radioactive iodine is more definitive than medication.
  3.  Thyroidectomy is more classic 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 than hyperthyroidism and requires slightly different treatment. The recommended standard treatment for thyroid cancer is thyroidectomy or complete thyroid removal.

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, 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 combined with thyroidectomy to remove the thyroid gland tissue in the body.

This combination of therapy reduces the risk of thyroid cancer recurrence. It increases the likelihood that it will kill any thyroid gland tissue or cancer cells 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 potential positive outcome of therapy.

Because of this, not every patient with thyroid cancer will have both procedures. 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 shallow, 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 to destroy 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 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, 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 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 undergoing radioactive iodine to treat hyperthyroidism.

Rates of post-procedure hypothyroidism vary in terms of 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 an extended period, it usually requires lifelong thyroid hormone replacement therapy once hypothyroidism develops.

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

Weight gain after radioactive iodine: It is well known that it 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. Still, 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 results from 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 administering 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 one 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 essential 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.

Unique 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, leading 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 system can prevent these adverse 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, consider this side effect and take necessary precautions, such as banking sperm.

Special side effects for women

Specifically for women, radioactive iodine poses a severe 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, breastfeeding, 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 six months. These precautions and recommendations from your doctor may vary.

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

Radioactive iodine permanently damages the thyroid gland, leaving you dependent on thyroid medications for the rest of your life.

If not done correctly, this thyroid hormone supplementation 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.