1. How is iodine-131 used to treat hyperthyroidism?
Iodine is an important raw material for the synthesis of thyroid hormones. Thyroid cells have a special affinity for iodide and concentrate iodine from the blood circulation through a sodium/iodine cotransport factor (NIS) that overcomes the electrochemical gradient. NIS is overexpressed in thyroid follicular cells of hyperthyroid patients and aggregation of iodine is significantly higher than in normal thyroid tissue. The beta rays emitted by the decay of iodine-131 (99%) have an average range of 1 mm (0.5-2 mm) in the tissues, so the beta particles are released almost entirely in the thyroid gland, with minimal effect on the tissues and organs surrounding the thyroid. Therefore, when the patient is given the right amount of iodine-131, it can effectively destroy a significant portion of the thyroid tissue without affecting the adjacent tissues, resulting in a shrinking of the thyroid gland with a return to normal thyroid function, achieving a purpose similar to subtotal thyroidectomy. Therefore, some people call iodine-131 treatment for hyperthyroidism “internal thyroid surgery” or “surgical treatment without surgery”. Two to four weeks after oral administration of iodine-131, the thyroid tissue becomes edematous, degenerative, with swelling of epithelial cells, vacuole formation and follicular destruction, and the damage in the central part of the gland is more obvious than in the periphery. 2 to 3 months later, the symptoms gradually disappear, and the size and function of the thyroid gland return to normal.
2. Which patients are suitable for iodine-131 treatment?
Radioactive iodine-131 treatment for hyperthyroidism is the most prominent, widely used and mature method of nuclear therapy. For indications, the traditional view is that iodine-131 therapy is only appropriate in the following cases.
(1) Patients aged 25 years or older with diffuse goiter with mild to moderate hyperfunction.
(2) Patients with poor results of long-term antithyroid drug (ATD) therapy, drug allergy, aggravation or relapse after discontinuation or reduction of drugs, and relapse after surgery.
(3) Those with hyperthyroidism with obvious protrusion or combined with hyperthyroid heart disease.
(4) Those who cannot adhere to the medication for hyperthyroidism and do not want to undergo surgery.
In recent years, the Nuclear Medicine Branch of the Chinese Medical Association has organized extensive discussions on the indications for iodine-131 treatment of hyperthyroidism, and has proposed new operating guidelines and clarified the relaxation of indications. The main bases are as follows.
1) Half century of clinical research and application have fully proved the safety of iodine-131 treatment without the risk of carcinogenesis, leukemogenesis, genetic damage and infertility.
2) There is a new understanding of hypothyroidism, the incidence of spontaneous hypothyroidism in Graves’ disease can reach 16-20%, any method of treatment can produce hypothyroidism, replacement therapy can yield satisfactory results, and rapid and timely control of hyperthyroidism can help growth and development and improve quality of life.
(3) It is simple, easy and reliable, and the majority of patients and physicians are sure of its efficacy. Therefore, the latest Chinese Medical Association’s “Clinical Technical Practice Guidelines? Nuclear Medicine Sub-volume” has made new revisions to the indications.
Indications for the latest hyperthyroidism iodine-131 treatment.
(1) Patients with diffuse goiter with hyperthyroidism (Graves’ disease).
(2) Adolescent patients with Graves’ disease who are allergic to ATD, or who have poor ATD efficacy, or who have had multiple relapses of ATD therapy, or who have significant goiter.
(3) Patients with Graves’ hyperthyroidism with leukocytopenia or thrombocytopenia.
(4) Patients with Graves’ disease with atrial fibrillation.
(5) Patients with Graves’ disease combined with chronic lymphocytic thyroiditis with an increased rate of iodine-131 uptake, but in cautious dosage.
Iodine-131 therapy is not recommended for adolescent patients under 18 years of age, but it may be considered for adolescent patients if the following conditions are met
(1) Patients with marked enlargement of the thyroid gland, moderate to severe hyperthyroidism or with severe complications (hyperthyroid heart or periodic paralysis).
(2) The patient’s family strongly prefers iodine-131 therapy.
(3) The patient refuses surgery or is not suitable for surgical treatment.
Iodine-131 treatment for hyperthyroidism is particularly indicated for women of childbearing age who want to plan a pregnancy in the near future (4-6 months) (recommendation 4 of the American Thyroid Association and American Association of Clinical Endocrinologists’ Guidelines for the diagnosis and treatment of hyperthyroidism and other causes of thyrotoxicosis). This is because timely treatment of hyperthyroidism is beneficial in choosing a reasonable time to prepare for pregnancy so that recurrent episodes of hyperthyroidism do not delay the optimal childbearing age.
3. Can nodular goiter with hyperthyroidism be treated with iodine-131?
Nodular goiter with hyperthyroidism is a type of hyperthyroidism that occurs on the basis of multinodular goiter, mostly in patients over 50 years old. Unlike Graves, most have a slow onset without anterior tibial mucinous edema and proptosis. They are prone to myocardial damage, including tachycardia, atrial fibrillation, angina pectoris, and heart failure. Most goiter is severe and mostly asymmetric, often extending posteriorly to the sternum causing compression symptoms.
In general, radioactive iodine-131 therapy should be chosen for most patients. Because of the large size of the thyroid gland in this disease and the lack of significant increase in the uptake rate of iodine-131, the required dose of iodine-131 is significantly higher than that for Graves’ disease, and it is difficult to destroy all of the multiple nodules with a single treatment, so it is often necessary to repeat the treatment several times. Therefore, subtotal thyroidectomy is recommended for patients who do not want to undergo multiple treatments.
4. Can autonomous functional thyroid nodules be treated with iodine-131?
Autonomously functioning thyroid nodules secrete excessive thyroid hormone, which inhibits TSH secretion from the pituitary gland, and the decrease in TSH levels reduces the uptake of iodine-131 by normal thyroid tissue outside the nodule. When a therapeutic dose of iodine-131 is given to a patient, the suppressed normal thyroid tissue does not take up iodine-131 or takes up very little, but is mainly taken up by the autonomous functional nodules and is treated, but a larger dose is required and a single cure is less likely. It is indicated for those who have contraindications to surgery or who do not want to undergo surgery. It is contraindicated during pregnancy or lactation and in cases of suspected nodule malignancy.
5. Absolute contraindications to radioactive iodine treatment for hyperthyroidism and other issues that should be noted.
(1) Absolute contraindications for radioactive iodine treatment of hyperthyroidism
(1) Pregnant and lactating women: When iodine-131 is ingested by the mother, it can enter the thyroid gland of the fetus or infant through the placenta and breast milk, causing cretinism in the fetus and infant.
②Patients with acute myocardial infarction.
(iii) Patients with severe liver and kidney diseases.
(2) Points to note when selecting patients for indications
(1) Patients with hyperthyroidism with thyroid adenoma and cold nodules confirmed by scan are generally not treated with iodine-131. Because of the difference in iodine uptake ability, the dosage of treatment is larger, the effect is slightly poorer, and repeated treatment is often required. In such cases, surgical removal should be considered first.
②Iodine-131 should be used only for those whose total peripheral blood leukocyte count is below 3×109/liter, after improvement by leukocyte-raising therapy.
③In severe hyperthyroidism, thyroid crisis and heart failure can often be induced, so it is best to control the symptoms with anti-thyroid drugs before iodine-131 treatment.
④Patients with hyperthyroidism with huge goiter that has produced tracheal compression symptoms are often not easy to eliminate the compression symptoms after taking iodine-131, and some of them are aggravated, so surgery is recommended.