Radioactive iodine (131I) is one of the simpler and more effective methods of treating hyperthyroidism. It is usually an oral solution (some places use capsules) consisting of suitable cold boiled water (mineral water is fine) plus a certain amount of radioactive iodine. In my daily work, I often encounter various questions from patients or their family members. One of the most frequently asked questions is: Will the treatment of hyperthyroidism with radioactive iodine affect the fertility of young women? My answer is definitely not. However, many people did not fully understand and accept this question, and some of them expressed doubts. Only after I fully explained the reasoning, they accepted it sincerely. The thyroid gland has a high degree of selective uptake of iodine, and most of the iodine that enters the body goes into the thyroid gland, while a small percentage is excreted. The concentration of iodine in the thyroid gland is 30-50 times higher than the concentration of iodine in the blood, and when hyperthyroidism occurs in patients with hyperthyroidism, the ability to concentrate iodine increases several to more than ten times. Therefore, almost all of the radioactive iodine taken orally in hyperthyroidism enters the thyroid tissue, and the amount of exposure to the whole body is very small, which does not cause any health effects, let alone damage to the reproductive system. Strictly speaking, after oral radioactive iodine in hyperthyroidism is absorbed by the stomach and then enters the blood circulation, a tiny amount of radioactive iodine may pass through the reproductive organs or the tissues adjacent to the reproductive organs during the circulation process, 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. It is like a person who is sleeping and there is an airplane flying overhead, the noise generated by the airplane will not affect his sleep. Some scholars have investigated a large number of fertile women who received radioactive iodine treatment, and the incidence of infertility is not different from the incidence of natural infertility and genetic diseases. Of course, we should also try to avoid the possible effects of radioactive iodine, and pregnancy is most appropriate six months after receiving radioactive iodine treatment. 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, including the reproductive organs, and women may experience menstrual disorders or even amenorrhea. 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 fetal death in utero may also occur. Therefore, we should be clear that hyperthyroidism itself can cause infertility or affect fertility. If hyperthyroidism occurs during pregnancy, you should keep in touch with your doctor. In addition, men with hyperthyroidism can develop impotence, which can also affect pregnancy and should be taken seriously. According to the above reasoning, there is no reason to suspect that hyperthyroidism can cause infertility after taking radioactive iodine treatment, but it is very clear that having hyperthyroidism can affect fertility or cause infertility. Therefore, hyperthyroidism patients should be actively treated for early recovery in order to have healthy little ones. Radioactive iodine can cure hyperthyroidism more quickly, so patients who want to have children and have hyperthyroidism can safely use this treatment. It is important to note that this treatment is contraindicated in pregnant and lactating women. The reason for this is that radioactive iodine may cause damage to the thyroid gland of the fetus and infant during pregnancy and lactation, 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.