Radioiodine is one of the cornerstone therapies of thyroid endocrinology and is based on a unique principle of biological selectivity: the thyroid gland’s ability to take up and concentrate iodine through the sodium-iodide symporter (NIS). The therapeutic use of radioactive iodine exploits this property to induce functional and structural destruction of hyperfunctioning or residual thyroid tissue, producing a targeted effect that limits systemic exposure. In clinical practice, radioiodine is used primarily to treat hyperthyroidism and as ablative or adjuvant therapy in differentiated thyroid carcinoma.
Unlike antithyroid medications, which reversibly modulate hormone synthesis, radioiodine produces a definitive or semi-definitive effect, with a time course of response extending over weeks or months. This characteristic makes it a highly effective strategy, but requires careful patient selection, appropriate preparation and informed management of long-term endocrine consequences, particularly iatrogenic hypothyroidism.
The radioiodine used in clinical practice is primarily iodine-131, a radionuclide that emits beta and gamma radiation. Beta particles are responsible for the local therapeutic effect because they deposit energy over a short range within iodine-avid thyroid tissue, inducing direct cellular damage and progressive follicular destruction. Gamma radiation, by contrast, allows monitoring and assessment of radioisotope distribution, but contributes only minimally to the therapeutic effect.
The selectivity of radioiodine derives from the expression of the NIS on the basolateral membrane of thyrocytes, which enables substantially greater intracellular accumulation of iodide than occurs in other tissues. Once taken up, radioactive iodine is organified and retained, allowing prolonged exposure of thyroid tissue to beta radiation. Radiobiological damage results in apoptosis, focal necrosis and progressive fibrosis, with a reduction in functioning tissue mass and secretory capacity.
The magnitude of the effect depends on multiple factors, including the administered activity, the volume and distribution of thyroid tissue, the degree of uptake, iodine turnover and the underlying disease. In hyperfunctioning tissues, such as those found in Graves’ disease or autonomous nodules, high uptake makes radioiodine particularly effective. In differentiated thyroid carcinoma, persistence of NIS expression in differentiated tumour cells enables the ablation of residual tissue or micrometastases.
The underlying biological principle is therefore that of targeted internal radiation, in which the target is not anatomically defined from outside the body, but biologically identified by its ability to take up iodine. This makes radioiodine a conceptually elegant therapy, but also one whose effectiveness depends on the biology of the disease being treated.
In the treatment of hyperthyroidism, radioiodine is indicated primarily for Graves’ disease, solitary toxic nodules and toxic multinodular goitre. In Graves’ disease, it represents an effective alternative to antithyroid medications and surgery, particularly in patients with recurrence after pharmacological therapy, intolerance or contraindications to thionamides, poor adherence or a preference for a definitive treatment. The objective is to eliminate hyperfunctioning tissue and interrupt the hormonal and immunological stimulation that sustains the disease.
In autonomous nodules and toxic multinodular goitre, radioiodine is often preferred to chronic pharmacological therapy because it selectively targets autonomous tissue and reduces the risk of recurrence. However, in large goitres its effectiveness may be incomplete and the reduction in volume is gradual, requiring careful assessment of the balance between the expected benefit and the time needed to achieve a response.
In differentiated thyroid carcinoma, radioiodine has a distinct role as ablative therapy for postoperative thyroid remnants and as adjuvant or therapeutic treatment for persistent or metastatic iodine-avid disease. Indications depend on oncological risk, staging, histology and the initial response to surgery. In this setting, the objective is not functional control, but reduction of the disease burden and improvement in the sensitivity of follow-up markers such as thyroglobulin.
It is essential to emphasise that radioiodine is not indicated in destructive thyrotoxicosis or in forms of hyperthyroidism that are not sustained by increased hormone synthesis, because thyroid uptake is low or absent in these conditions. Appropriate use therefore depends on accurate aetiological diagnosis and functional assessment of iodine uptake.
Preparation for radioiodine treatment is a critical step in maximising therapeutic effectiveness. In the treatment of hyperthyroidism, antithyroid medications are generally withdrawn before administration to avoid reducing the uptake and organification of radioactive iodine. The timing of withdrawal depends on the medication used and the clinical severity of the disease, balancing the risk of a transient worsening of hyperthyroidism against the need to ensure adequate radioiodine effectiveness.
Another fundamental factor is the patient’s iodine status. Excess iodine from iodinated contrast media, medications or supplements can saturate the NIS and markedly reduce radioisotope uptake. For this reason, a low-iodine diet is recommended during the weeks preceding treatment, particularly in oncological protocols in which ablative effectiveness is crucial. Recent exposure to iodinated contrast media also requires an adequate interval before therapy.
In differentiated thyroid carcinoma, radioiodine uptake is enhanced by stimulating thyroid-stimulating hormone (TSH), either through levothyroxine withdrawal or through administration of recombinant TSH. Both strategies increase NIS expression and radioiodine retention in the target tissue. The choice between hormone withdrawal and recombinant TSH depends on the clinical setting, oncological risk and the impact on the patient’s quality of life.
Appropriate preparation is not merely a technical detail, but a direct determinant of treatment effectiveness and safety. Errors during this phase may result in therapeutic failure or the need for repeated administration, increasing cumulative radiation exposure.
Radioiodine is administered orally, generally as a capsule, and does not require invasive procedures. From a dosimetric perspective, two main approaches are available: fixed activity and calculated activity. Fixed activity uses standardised amounts based on the clinical indication and established experience, and is widely adopted because of its simplicity and reproducibility.
The personalised dosimetric approach, by contrast, takes thyroid volume, uptake and iodine turnover into account, with the aim of delivering an optimal absorbed dose to the target tissue. This method is more complex and is used primarily in oncology or in selected cases of hyperthyroidism in which optimisation of the balance between effectiveness and the risk of excessive tissue damage is desired.
Regardless of the approach used, the clinical result is not immediate. In the treatment of hyperthyroidism, functional control develops progressively, with a latency period during which symptoms may persist or transient worsening may occur. This requires active management with beta-blockers and, in some cases, temporary resumption of antithyroid medications until the full effect of radioiodine is achieved.
In thyroid carcinoma, treatment effectiveness is assessed through post-therapy imaging, thyroglobulin trends and medium- to long-term follow-up. Administration therefore marks the beginning of a therapeutic pathway rather than an isolated intervention.
In the treatment of hyperthyroidism, the most common long-term outcome of radioiodine therapy is hypothyroidism, which may develop gradually over months or years. This outcome is not considered therapeutic failure, but an expected and often desirable result, because it allows stable and safe control through levothyroxine replacement therapy. Spontaneous euthyroidism may occur in a proportion of patients, particularly in Graves’ disease, but it is less predictable and less stable over time.
In autonomous nodules and toxic multinodular goitre, radioiodine may selectively reduce the activity of autonomous tissue, with a generally lower, although not negligible, risk of global hypothyroidism than in Graves’ disease. Reduction in goitre volume is progressive and may continue for many months, with a clinical impact that depends on the initial size and distribution of iodine uptake.
In differentiated thyroid carcinoma, the response to radioiodine is assessed in terms of remnant ablation and disease control. An effective response improves the sensitivity of follow-up markers and may reduce the risk of recurrence in selected patients. However, not all tumours retain sufficient iodine avidity, and loss of differentiation represents a biological limitation of therapy.
Endocrine outcomes must therefore be interpreted in relation to the original therapeutic objective. In the treatment of hyperthyroidism, iatrogenic hypothyroidism is a manageable consequence; in oncology, ablation is a tool for improving disease control and monitoring.
Radioiodine is generally safe when used according to established indications and protocols, but it is associated with specific adverse effects. In the short term, patients may develop neck pain, radiation thyroiditis, transient worsening of thyrotoxicosis and mild gastrointestinal symptoms. In Graves’ disease, there is a documented risk of worsening thyroid eye disease, particularly in smokers and patients with pre-existing active ocular involvement; in these cases, corticosteroid prophylaxis may reduce the risk.
In the long term, the most relevant adverse effect is permanent hypothyroidism. The risk of radiation-induced secondary malignancies is considered low at the activities used in endocrinology, but remains a matter of discussion, particularly in young patients and after repeated treatments. Available epidemiological evidence supports a favourable safety profile when radioiodine is used appropriately.
Radiation protection measures are an integral part of treatment. After administration, patients must follow instructions designed to limit radiation exposure to family members and household contacts, particularly children and pregnant women. These precautions are temporary and proportionate to the administered activity, but constitute an essential educational component of the therapeutic pathway.
The safety of radioiodine therefore depends not only on the administered activity, but also on patient selection, appropriate preparation and informed management of the post-treatment period. When these elements are properly addressed, the risk-benefit profile remains favourable for most endocrinological indications.
In the treatment of hyperthyroidism, radioiodine occupies a position between pharmacological therapy and surgery as a definitive, non-invasive option. Compared with antithyroid medications, it offers a higher probability of durable disease control, at the cost of accepting iatrogenic hypothyroidism. Compared with surgery, it avoids operative and anaesthetic risks, but has a longer response latency and requires radiation protection precautions.
The choice among these options is never purely technical. It must take into account age, comorbidities, disease severity, thyroid size, the presence of thyroid eye disease, reproductive plans and the patient’s informed preferences. In many cases, radioiodine represents an optimal balance between effectiveness, safety and acceptability.
In differentiated thyroid carcinoma, radioiodine is not an alternative to surgery, but a complementary treatment in selected cases. Its role is determined by oncological risk and the initial response to treatment, and it should not be used indiscriminately. More recent guidelines emphasise increasingly refined patient selection to avoid unnecessary treatment and reduce radiation exposure in the absence of clinical benefit.
In summary, radioiodine is a powerful therapeutic tool that achieves its greatest effectiveness when incorporated into a personalised strategy, integrated with other treatment options and guided by sound endocrinological principles.