The application of radioactive iodine scans in assessing and treating thyroid disorders, including thyroid cancer, is based on the specialized function of thyroid cells. These cells uniquely concentrate iodine, making it possible to create images of the thyroid gland to diagnose various conditions, such as hyperthyroidism, thyroid nodules, and thyroid cancer.
Thyroid hormone withdrawal in Thyroid cancer treatment
To treat differentiated thyroid cancer, it is necessary to stop thyroid hormone replacement treatment, primarily levothyroxine, for a certain period before the scan, typically 4-6 weeks. This approach is designed to increase thyroid-stimulating hormone (TSH) in the bloodstream. With high levels of TSH, typically greater than 30mIU/ml, the thyroid gland is stimulated to absorb more iodine, thus improving the effectiveness of the radioactive iodine scan.
During the scan, the patient receives a minor dose of radioactive iodine, I-123 or I-131, orally. The thyroid cells assimilate this radioactive iodine, and a gamma camera is then used to capture images that indicate the distribution of the radioactive material within the thyroid gland.
Role of Thyrogen in thyroid cancer treatment
Radioactive iodine (RAI) scans are used to assess patients with thyroid cancer, particularly differentiated thyroid cancer, to detect residual or recurrent disease after thyroidectomy. Thyrogen, also known as recombinant human thyrotropin (rhTSH), is a synthetic form of thyroid-stimulating hormone (TSH).
In patients who have undergone thyroidectomy, recombinant human TSH (thyrogen) administration stimulates any remaining thyroid tissue or metastatic thyroid cancer cells to increase iodine uptake. This is because TSH acts on thyroid cells to increase sodium iodide symporter (NIS) activity, which is responsible for the transport of iodine into the cells.
Recombinant human TSH (rhTSH) was created to stimulate TSH without the withdrawal of thyroid hormone and the associated risks of clinical hypothyroidism. Administered by two consecutive daily injections, rhTSH leads to short-term elevation of TSH, potentially reducing tumor stimulation risks.
Mechanism of Action of Radioactive Iodine
The use of iodine in the creation of thyroid hormones is crucial. Radioactive iodine isotopes, specifically Iodine-123 (I-123) and Iodine-131 (I-131), are used in nuclear medicine for both diagnostic and therapeutic purposes, with a specific focus on evaluating thyroid function.
I-131, a beta and gamma-emitting isotope with an eight-day half-life, is used predominantly to diagnose and treat hyperthyroidism and thyroid cancer. It is taken up by the thyroid gland in the same way as nonradioactive iodine, thus facilitating an analysis of thyroid function and a visual representation of the gland’s structure. However, due to the longer half-life and increased radiation exposure, the usage of I-131 has certain limitations for some diagnostic uses.
However, I-123, a gamma-emitting isotope with a half-life of 13 hours, is more suitable for some diagnostic purposes compared to I-131, mainly due to its lower radiation exposure. However, I-123 is not used for therapeutic interventions, as it lacks the beta particle emissions necessary to eradicate thyroid cells.
Interpretation of Radioactive Iodine Scans
| Imaging findings | Management plan |
| Absent radioiodine uptake in the thyroid bed | Suboptimal TSH stimulation Poor adherence to a low-iodine diet |
| Increased focus of uptake in the thyroid bed | Possible excess remnant tissue or persistent thyroid cancer. The patient may require repeat surgery or fine needle biopsy |
| Focus of uptake outside the thyroid bed | Additional testing is required. Evaluate metastatic disease and consider dosimetrically guided instead of empiric I-131 activities for treatment |
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