I. About the use of dried thyroid tablets: The first use of animal thyroid tissue extracts by humans was in 1892, 100 years ago. The advantages of using animal thyroid tissue extract (dry thyroid tablets) are that the drug is widely available, easy to obtain, and inexpensive. However, dried thyroid tablets have the following disadvantages: 1. The dried thyroid tablets are crude in preparation, low in purity, and high in impurities. In addition to T4 and T3, they contain more animal protein fragments and other macromolecular substances. 2. The ratio of T4 and T3 in the thyroid gland of animals is 3:1 to 4:1, which is also affected by the type of animals, the iodine content of feed and seasonal changes. The T3 content is significantly higher than that of humans, and the bioavailability of T3 is high, with an oral absorption rate of 95%, while the bioavailability of T4 is low, with an oral absorption rate of only 40%, and a small pool of T3 iodine in humans, which can easily cause transient medically induced hyper-T3emia. It is easy to cause hyperthyroidism or induce atrial fibrillation in elderly patients and patients with cardiac disorders. 3. The unstable biological potency of T4 in dry thyroid tablets is mainly reflected in the inaccuracy of T4 content. China’s pharmacopoeia stipulates that the content of organic iodine in a 40 mg dry thyroid tablet should be between 0.255% and 0.345%, and the pharmacopoeia itself allows for a dose error of 30%. Due to the above-mentioned shortcomings of dry thyroid tablets, they tended to be eliminated abroad in the 1960s. In the past, due to economic constraints in China, dry thyroid tablets have been used as an effective alternative treatment for hypothyroid patients. In recent years, with the development of domestic economy, chemically synthesized levothyroxine has been gradually used in clinical practice. Second, about the use of levothyroxine: Compared with dry thyroid tablets, levothyroxine has the following advantages: 1. levothyroxine (L-T4) is a chemically synthesized drug, and its active ingredient is levothyroxine sodium, with stable, precise and high purity content. 2. The bioavailability of levothyroxine (L-T4) is high, with an oral absorption rate of 80%. 3.The biological adaptability of L-T4 is good, and the physiological balance of T4 and T3 is achieved through the in vivo conversion mechanism. 4. L-T4 has stable efficacy and long duration of efficacy, with a half-life of about 8 days. For the above reasons, it is the historical trend for levothyroxine to gradually replace dry thyroid tablets.