Radioactive iodine (131I) is one of the easiest and most effective of the many treatments for hyperthyroidism. It is usually an oral solution (some places use capsules) consisting of an appropriate amount of mineral water (cold boiled water is also acceptable) plus a certain dose of radioactive iodine. It usually requires only 1-2 treatments to cure and has a low recurrence rate. Strictly speaking, the oral radioactive iodine for hyperthyroidism is absorbed by the stomach and then enters the blood circulation. During the circulation process, an extremely small amount of radioactivity may pass through the reproductive organs or tissues adjacent to the reproductive organs, but this process is very short and the dose is very small, which is not enough to damage the reproductive system, let alone affect fertility. Scientific studies have shown that the reproductive organs of both men and women do not have the ability to concentrate radioactive iodine, but are only exposed to transient traces during the absorption, metabolism and excretion of iodine. In normal adults, the 24-hour iodine uptake rate of the thyroid gland is about 30%, while in hyperthyroidism, the 24-hour iodine uptake rate can be as high as 90% or more, with most of the iodine entering the thyroid gland and a small amount excreted from the body. Some scholars have investigated a large number of fertile women who received radioactive iodine treatment, and the incidence of infertility is no different from the incidence of natural infertility and genetic diseases. Of course, we should try to avoid the possible effects of radioactivity, and pregnancy is most appropriate six months after receiving radioactive iodine treatment. One issue that needs to be clarified is that a very small number of fertile women with hyperthyroidism can suffer from secondary amenorrhea or infertility. The reason is that the increase of thyroid hormones in the body of hyperthyroidism patients causes an imbalance in the functional status of many organs, among which the reproductive organs are no exception. At this time, the ovaries do not function normally and it is not easy to get pregnant, nor is it suitable for pregnancy. Some women with hyperthyroidism during pregnancy, or with pre-existing hyperthyroidism, may have a relapse after pregnancy. This kind of pregnancy with hyperthyroidism is prone to miscarriage, premature birth and gestational hypertension syndrome, and fetal growth retardation and intrauterine fetal death are also seen from time to time. Therefore, hyperthyroidism itself can cause infertility or affect fertility. If hyperthyroidism occurs during pregnancy, you should keep in touch with your doctor. In addition, impotence may occur in men with hyperthyroidism and should be taken seriously. Therefore, there is no reason to suspect that hyperthyroidism treated with radioactive iodine can cause infertility, and it is very clear that having hyperthyroidism can affect fertility or cause infertility. There are clear indications for the treatment of hyperthyroidism with radioactive iodine in the Code of Practice for Nuclear Medicine compiled by the Chinese Medical Association, in which there are no special requirements for age, and the main contraindications are pregnancy and lactating women. The main contraindication is pregnancy and lactation. The reason is that radioactive iodine in pregnant and lactating women may cause damage to the thyroid gland of the fetus and infants, resulting in hypothyroidism. If hypothyroidism occurs after radioactive iodine treatment, thyroid hormone replacement therapy can be used, which will not affect fertility or produce malformations while maintaining a balanced state of thyroid hormone levels in the body.