How to choose a nuclear medicine test for thyroid disease? Many patients suffer from thyroid disorders. A common feature of the patients I have contacted is a lack of understanding of the types of thyroid disorders and how to perform different nuclear medicine tests and treatments, so it is necessary to make some introductions. The thyroid gland is an important endocrine organ of the body, located in the middle of the neck in front of the trachea, divided into two lobes, left and right, shaped like a butterfly with open wings. It is responsible for the growth and metabolism of the human body through iodine intake, synthesis and release of thyroid hormones. Thyroid hormones exist in five forms: triiodothyronine (T3), thyroxine (T4), trans-triiodothyronine (rT3), free triiodothyronine (FT3) and free thyroxine (FT4), which are regulated by neurological and endocrine factors (e.g. TRH and TSH) at appropriate and constant concentrations in the blood. Once thyroid disease occurs, it can be broadly classified into three categories: functional, organic (occupational) and inflammatory. Functional diseases: Functional diseases of the thyroid gland refer to hyperthyroidism (hyperthyroidism) and hypothyroidism (hypothyroidism). In hyperthyroidism, the hormones synthesized by the thyroid gland increase, causing a series of symptoms of excessive metabolism in the body, such as panic, fatigue, easy to eat and hunger, accompanied by significant weight loss, fear of heat and sweating, several times a day, shaking hands and feet, and in severe cases, inability to hold pens and chopsticks, swollen eyes, protruding eyeballs, irritability, and periodic paralysis and lower limb paralysis in some cases with low blood potassium. Men may have impotence and women may have menstrual abnormalities. Hypothyroidism is the opposite, with insufficient levels of thyroid hormones causing low metabolism, swelling, coldness, drowsiness, lack of desire to eat, and constipation. Hypothyroidism in children can also cause growth retardation and mental retardation (cretinism or cretinism). The main tests for this type of disease are the concentration of thyroid hormones (T3, T4, rT3, FT3, FT4) in the blood and the concentration of thyrotropin-releasing hormone (TSH) secreted by the pituitary gland; the rate of iodine-131 absorption can be used as an auxiliary and differential diagnosis. If the concentration of T3, T4, rT3, FT3 and FT4 in the blood is higher than the normal range and the rate of iodine-131 absorption is high, hyperthyroidism can be diagnosed. Imaging tests (thyroid ECT imaging) are not significant for the diagnosis of hyperthyroidism, but can be used as an important reference indicator for calculating the weight of the thyroid gland during hyperthyroidism iodine-131 treatment. Sometimes, doctors may not have the right indications and prescribe thyroid ECT to patients who are suspected of having hyperthyroidism, but the patient may not be able to confirm the diagnosis of hyperthyroidism because they do not know the hormone level in their body. Radionuclide iodine-131 therapy is only indicated for patients with hyperfunction. It destroys and damages some of the thyroid cells through the biological effect of radiation, thus reducing the function to normal. There are, of course, other treatments available for hyperthyroidism. Hypothyroidism is mainly treated with exogenous thyroid hormone supplementation, which is known as “replacement therapy” in medical science. 2. Organic diseases: Thyroid tumors, thyroid cysts and thyroid cancer are among the diseases in this category. They mainly cause morphological changes and structural damage to the thyroid gland, and the symptoms are mostly a lump in the neck that can move up and down with swallowing. The main test is ECT imaging of the thyroid gland. The image of the thyroid gland can be obtained through in vitro detection and imaging, and then its morphology, size, location and radioactive distribution can be analyzed for the diagnosis of thyroid lesions. For images of the thyroid gland with a mass, the radioactivity of the mass is usually compared with the radioactivity of the normal thyroid gland in the adjacent area. If the intensity of radioactivity in the lump area is high, it is called “hot nodule”, and if it is reduced, it is called “cool nodule” or “cold nodule” (the degree of reduction varies). This combination of “temperature” and “nodules” is unique to nuclear medicine, but it is not a pathological diagnosis and cannot directly confirm a disease, but can only be indirectly determined by the nature of the nodules. In general, thyroid adenomas and cysts are usually warm or cool nodules; functional autonomic thyroid adenomas (also known as toxic thyroid adenomas) are hot nodules; and thyroid cancer is often cold nodules. In addition to thyroid cancer, which can be treated with radiation therapy using iodine-131, adenomas and cysts can only be treated surgically. It is worth mentioning that thyroglobulin (TG) measured by nuclear medicine in vitro analysis can be used as an important monitoring indicator for diagnosis, recurrence or metastasis of differentiated thyroid cancer. 3. Inflammatory diseases: With the improvement and development of detection technology, more and more patients with inflammatory thyroid diseases are being diagnosed correctly and timely. There are subacute thyroiditis and chronic lymphatic thyroiditis (Hashimoto’s disease). This inflammatory disease is different from ordinary bacterial inflammation in that it is an autoimmune disease in which antibodies against the thyroid’s own tissue (TGA, TMA) are present in the body. These antibodies stimulate or destroy the thyroid gland, resulting in corresponding changes in thyroid function and sometimes an inflammatory mass with significant pain in the mass area. The diagnosis is easily confused with the first two types of diseases. Therefore, in addition to the aforementioned T3, T4, rT3, FT3, FT4, TSH tests, iodine-131 absorption rate, and ECT imaging of the thyroid gland, special emphasis should be placed on the determination of anti-thyroid autoantibodies, which, when combined with a positive history and other tests, can confirm the diagnosis. Treatment of thyroiditis requires hormones to suppress the immune process supplemented by general anti-inflammatory drugs; for inflammatory masses, surgical treatment is contraindicated as it will cause hypothyroidism. Hashimoto’s thyroiditis combined with hyperthyroidism can also be treated with iodine-131 if the patient has high iodine-131 absorption. In addition, there is a simple goiter with normal thyroid function, which is especially common in adolescents entering puberty and in people in iodine-deficient areas. This is caused by the insufficient amount of iodine provided from food and drinking water, while the human body needs large amounts of iodine for growth and metabolism. This condition generally does not require treatment and should be treated with timely and scientific iodine supplementation. Iodine-131 treatment is also an option in cases of large goiter that affects life and aesthetics and is inoperable.