What Are the Long-Term Side Effects of Radioactive Iodine?

Radioactive iodine (RAI) therapy is one of the most effective treatments for thyroid cancer and Graves’ disease, but it leaves a trail of potential long-term consequences that extend well beyond the thyroid gland. The most common lasting side effect is damage to the salivary glands, affecting roughly one in five to one in three patients depending on the dose received, but the list also includes increased risk of second cancers, tear duct obstruction, blood count changes, and shifts in fertility. Nearly all of these risks rise with higher cumulative doses, which makes the conversation between you and your doctor about how much iodine-131 to use more consequential than many patients realize.

Salivary Gland Damage

Your salivary glands concentrate iodine almost as avidly as your thyroid does. That makes them one of the most vulnerable bystanders during RAI therapy. The resulting inflammation, called sialadenitis, can become chronic and lead to persistent dry mouth, known clinically as xerostomia. In a study that followed patients for five years after a single RAI dose, about 16% still reported dry mouth, and roughly 20% of their salivary glands showed dysfunction on imaging.¹ For people treated as children, the numbers are worse: one study of pediatric thyroid cancer survivors found clinically significant salivary gland dysfunction in about 36%, with severity tied directly to the total amount of iodine-131 received over time.²

Dry mouth is more than a nuisance. Saliva protects your teeth, helps you taste food, and guards against oral infections. When production drops permanently, you face a higher risk of cavities, gum disease, and difficulty eating. Despite standard preventive measures, a substantial number of patients end up with lasting salivary damage that does not fully recover.³

Tear Duct Obstruction and Dry Eyes

The same iodine-concentrating mechanism that damages salivary glands can also affect your tear drainage system. Iodine-131 accumulates in the nasolacrimal duct, and in some patients this causes scarring that partially or completely blocks the duct. The result is persistent watery eyes, a condition called epiphora, which can seem paradoxical given that the underlying problem is actually damage to the gland and duct tissue. In one clinical series, 10 out of 390 patients who received RAI reported epiphora, with symptoms appearing an average of about six and a half months after the last dose. It then took an average of 18 months before the problem was correctly diagnosed, suggesting the complication is underrecognized.⁴

Prospective screening reveals these problems earlier. One study that actively looked for lacrimal system obstruction during the first six months after RAI found cases that would otherwise have gone undiagnosed until complications worsened.⁵ If you notice excessive tearing or eye irritation in the months following treatment, bringing it to your doctor’s attention promptly can prevent a more difficult surgical repair later. The obstruction itself forms over a period of months once the duct has absorbed enough radioactive iodine, and higher tear production may actually increase the duct’s exposure to the isotope.⁶

Second Cancers

The risk that has drawn the most research attention is whether RAI treatment increases the chance of developing an entirely new cancer later in life. The short answer is yes, but the risk scales with dose and the absolute numbers remain small for most patients. A large nationwide study found no meaningful increase in second cancer risk for patients who received 100 mCi or less in total. Above that threshold, risk climbed steadily: patients receiving 101–200 mCi had roughly a 21% higher risk, those receiving 201–300 mCi had about a 42% higher risk, and those receiving over 300 mCi had around a 69% higher risk compared to patients who did not receive RAI.⁷

The types of second cancers most strongly linked to RAI are not random. Myeloid leukemia leads the list among blood cancers, and among solid tumors the elevated risks concentrate in the salivary glands, lungs, uterus, and prostate.⁸ That pattern makes biological sense: these are organs that either concentrate iodine themselves or sit in the radiation field near the thyroid bed.

For leukemia specifically, a Korean population study of over 211,000 thyroid cancer patients found that the risk became significantly elevated only at higher cumulative doses, with hazard ratios of about 3.1 and 2.1 in the high- and very-high-dose groups compared to patients who received no RAI.⁹ Even in those groups, the absolute incidence of leukemia remained well under 1%, so the risk needs to be weighed against the benefit of treating an aggressive thyroid cancer. For low-risk thyroid cancers that could be managed with surgery alone, though, these numbers factor into whether RAI is worth adding.

Blood Cell Changes

RAI can suppress the bone marrow, at least temporarily. A systematic review and meta-analysis of hematological effects found that platelet counts dropped in the first few months after treatment and remained slightly reduced at follow-up, though the changes were modest in most patients.¹⁰ In a separate study looking at longer-term outcomes, patients over 60 had significantly higher rates of low platelet and lymphocyte counts, and cumulative doses above 175 mCi were a risk factor for persistent white blood cell and platelet reductions. Repeated RAI courses made the problem worse.¹¹

For most people receiving a single standard dose, blood counts recover without incident. The concern grows when someone needs multiple rounds of treatment or starts with a high cumulative exposure. Regular blood work monitoring after RAI, particularly for patients who have received repeated doses, is standard practice for good reason.

Effects on Fertility

Fertility is one of the first concerns patients of reproductive age raise, and the evidence paints a nuanced picture that differs between men and women.

In women, menstrual irregularities are common in the first year after RAI, occurring in roughly 12% to 31% of patients, with about 8–16% experiencing temporary loss of periods.¹² One finding that gets less attention is the effect on menopause timing. A meta-analysis found that women who received RAI entered menopause about one and a half years earlier than women who did not, at around 49.5 years versus 51 years. Anti-Müllerian hormone, a marker of ovarian reserve, also drops significantly in the year following treatment. In a smaller study, 40% of women exposed to iodine-131 went through early menopause, compared to none in a control group.¹³ Despite all this, the overall ability to become pregnant does not appear significantly impaired at a population level, though some individuals clearly experience reduced fertility.

In men, the most consistent finding is a temporary dip in sperm count in the first three months after treatment, followed by recovery around the 12-month mark. One cohort study documented sperm counts falling from about 38 million per milliliter to 32 million, then rebounding to 37 million by one year.¹⁴ Another study found that FSH levels rose after treatment but testosterone, sperm DNA fragmentation, and most other reproductive hormones stayed stable.¹⁵ The takeaway for men: expect a transient disruption, but lasting damage from a single treatment is uncommon. Men planning to father children are still typically advised to wait at least a few months after RAI.

What About Offspring Health

A question that haunts many patients is whether radiation exposure could affect future children. The data here is largely reassuring, with one important caveat about timing. A study of children fathered by RAI-treated men found no significant differences in any adverse outcome compared to children of untreated fathers.¹⁶

For women, the story has one alarming footnote. In a long-term follow-up of females treated with RAI during childhood and adolescence, two children conceived within one year of therapy were born with fatal birth defects. No other children in the study had birth defects, and 61 children alive for follow-up had no major health problems.¹⁷ Two cases from a single study cannot establish causation, but they reinforce why doctors universally recommend waiting at least 6–12 months after RAI before attempting conception.

Hypothyroidism After Graves’ Disease Treatment

For patients receiving RAI for Graves’ disease rather than cancer, the most predictable long-term outcome is hypothyroidism itself. In many clinics, this is actually the goal: destroy enough overactive thyroid tissue to stop the hyperthyroidism, knowing the patient will need lifelong thyroid hormone replacement. One long-term follow-up found that 99% of RAI-treated Graves’ patients became hypothyroid.¹⁸ The cumulative incidence of hypothyroidism across studies is around 50% after the first dose, with most cases developing within the first year.¹⁹

Several factors influence how quickly hypothyroidism sets in, including thyroid size, how long you took anti-thyroid drugs beforehand, and individual radiosensitivity. In the first three to six months, patients with smaller thyroids and higher levels of thyroglobulin antibody are at greatest risk. After a year, the dominant predictor shifts to lower iodine uptake.²⁰ For most Graves’ patients, hypothyroidism is less a side effect than a planned trade: swapping an unpredictable, difficult-to-control overactive thyroid for a stable underactive one that is managed with a daily pill.

Parathyroid Gland Effects

The parathyroid glands sit right next to (or sometimes within) the thyroid, so they inevitably catch some radiation during RAI therapy. This has raised questions about whether RAI could cause overactive parathyroid glands years later. An older study of children and adolescents treated with RAI for Graves’ disease found elevated calcium levels in about 6% of patients, suggesting a rate of hyperparathyroidism several times higher than normal.²¹ A surgical series found that among nearly 2,000 patients undergoing parathyroid surgery, about 2.5% had a history of prior RAI, with an average gap of 24 years between RAI treatment and developing hyperparathyroidism.²²

On the reassuring side, a more recent study looking specifically at differentiated thyroid cancer patients found that while high-dose RAI transiently lowered calcium and parathyroid hormone levels in the first year, parathyroid function appeared normal at the five-year mark regardless of dose received.²³ The connection between RAI and hyperparathyroidism decades later remains plausible but not firmly established, and it would take very long follow-up studies to settle the question.

Lung Damage in Patients With Pulmonary Metastases

Pulmonary fibrosis from RAI is rare but can be serious. It occurs almost exclusively in patients who have thyroid cancer that has already spread to the lungs, because those metastatic deposits absorb iodine-131 and expose surrounding lung tissue to high local radiation. A study of young patients treated with RAI after the Chernobyl disaster found that five out of a group with lung metastases developed advanced pulmonary fibrosis, with one eventually dying from it. These patients tended to be younger at diagnosis and had received more intensive treatment.²⁴ A pediatric case report further illustrates the pattern: progressive respiratory symptoms and imaging changes consistent with radiation-induced lung injury, eventually including fibrosis.²⁵ If you do not have lung metastases, this complication is essentially not a concern.

Younger Patients Face Amplified Risks

Children and young adults treated with RAI deserve special mention because their risks are consistently higher than those of older patients. A large study of over 27,000 survivors of pediatric and young adult thyroid cancer followed for a median of 15 years found that RAI treatment was associated with a 23% increased risk of developing a new solid cancer and a 51% increased risk of a hematologic cancer compared to non-RAI-treated survivors. The risk of salivary gland cancer stood out with a standardized incidence ratio of 34 in one analysis, meaning it was 34 times more common than expected.²⁶ Beyond 20 years of follow-up, the risks climbed further: the relative risk for solid cancers overall was about 1.47, and for breast cancer it was about 1.46.²⁷

These amplified risks reflect basic radiation biology. Younger tissues are more radiosensitive, and younger patients simply have more remaining years during which a radiation-induced cancer could develop. This is one reason pediatric endocrinologists are especially cautious about RAI doses and increasingly weigh surgery-only approaches for low-risk pediatric thyroid cancers.

The Dose Question

If there is one theme running through nearly every long-term side effect, it is that cumulative dose matters enormously. A systematic review covering salivary dysfunction, lacrimal problems, gonadal effects, and second cancers found that the prevalence and severity of all of these rose with increasing total iodine-131 exposure.²⁸ This is not a binary on/off switch. At 100 mCi or below, the risks for second cancer are barely distinguishable from background. At 200–300 mCi, they become measurable. Above 300 mCi, they become hard to ignore.

This dose-response relationship is why many thyroid cancer centers have moved toward using the lowest effective dose of RAI, reserving higher doses for aggressive or metastatic disease. If your doctor recommends RAI, asking about the planned dose and how it was chosen is entirely reasonable.

Strategies to Reduce Salivary and Gland Damage

The most studied preventive measure is the use of sialogogues, substances that stimulate saliva production and theoretically flush radioactive iodine through the salivary glands more quickly. Lemon juice and sour candies are the classic examples. Some practitioners recommend using them continuously, every 30 to 60 minutes, for the first 24 hours after RAI administration.²⁹ Other proposed approaches include salivary gland massage, vitamin E, and the radioprotective drug amifostine. There is no strong consensus on which strategy works best, though the use of sialogogues is the most commonly recommended.³⁰

The honest reality is that rigorous studies proving any of these interventions prevent long-term salivary damage are lacking.³¹ They are low-risk and make physiological sense, so there is little downside to trying them, but you should not assume they guarantee protection.

Psychosocial and Quality-of-Life Effects

The physical side effects get most of the clinical attention, but the psychological burden of RAI treatment is real and underappreciated. A review of thyroid cancer survivors found that beyond the salivary dysfunction, dry eyes, altered taste, and chronic fatigue, patients commonly reported anxiety, insecurity about recurrence, changes in body image, social isolation, and a feeling that their lives had become permanently medicalized.³² Surviving thyroid cancer does not automatically restore a person’s sense of well-being, and the isolation protocols required during RAI treatment can intensify feelings of being “radioactive” in a social sense long after the physical risk has passed.

For patients treated for hyperthyroidism rather than cancer, the trajectory looks more hopeful. A study using patient-reported outcome questionnaires found significant improvements in quality of life after RAI in both men and women, with the biggest predictor of improvement being how symptomatic you were before treatment rather than the dose received.³³ In other words, if hyperthyroidism was making your life miserable, RAI tends to make it better on balance, even accounting for the shift to hypothyroidism and lifelong medication.