In the lower part of the neck, there is a pair of endocrine organs shaped like a butterfly: the thyroid gland. The thyroid hormones they secrete are essential for human growth and development, and affect the function and metabolism of all organs in the body. If the synthesis and secretion of thyroid hormones are reduced, or if the tissues are underutilized, hypothyroidism, or hypothyroidism, will occur. There are two types of thyroid hormones, T4 (tetraiodothyronine) and T3 (triiodothyronine), which are often seen in laboratory tests. The main difference between the two is that the concentration of T4 in the blood is higher, tens of times higher than that of T3, while the duration of action of T3 is shorter and its biological activity is stronger. At present, the main drug for the treatment of hypothyroidism is a synthetic product of T4: levothyroxine tablets (LT4), which will be converted into T3 to exert physiological effects after entering the body, and the common trade names are: Eugenol, Gahon, and Raitis. LT4 is the best choice for hypothyroidism treatment because of its long-term efficacy, low side effects, easy dose adjustment, good intestinal absorption, long serum half-life and low drug cost, which beat other similar drugs. According to the principle of “make up what is lacking” and “make up how much is lacking”, the treatment of hypothyroidism is very simple, as long as the appropriate dose of LT4 is provided, there is almost no difference between hypothyroid patients and healthy people. Simple as it is, there are still many common misunderstandings and precautions for hypothyroidism treatment, the following knowledge, you deserve to have. 1.When should I take the medication every day? Food may affect the absorption of LT4. To ensure the best utilization of the medication, it is recommended to take the medication 60 minutes before breakfast, or at bedtime (3 hours or more after dinner). 2. Can LT4 be taken with other medications? Some drugs, such as calcium carbonate, ferrous sulfate, aluminum thioglycollate, orlistat, and koleenamide, may have an effect on the absorption of LT4. Therefore, when it is necessary to apply these drugs, it is recommended to take LT4 after an interval of 4 hours. 3.What other factors may affect the absorption of drugs? In addition to the drugs mentioned above, other factors that affect the absorption of LT4 include: gastrointestinal diseases and surgery, high dietary fiber foods, pregnancy, heart failure, etc. This suggests that if the actual demand for LT4 in hypothyroid patients is significantly different from the expected value by the physician, these factors need to be analyzed and the LT4 dose needs to be re-evaluated after making appropriate interventions. 4. How to start LT4 therapy? It is important to determine the appropriate individualized dose for the patient based on various aspects such as weight, pregnancy status, etiology of hypothyroidism, serum thyrotropin (TSH) values, age, and clinical symptoms. In general, patients with loss of thyroid function require a daily LT4 dose of approximately 1.6 μg/kg, where “per kilogram of body weight” refers to the patient’s ideal body weight rather than actual body weight. If the patient’s serum TSH level is significantly elevated, a full replacement dose should be used, while if the TSH is only slightly elevated (<10 mU/L), a lower starting dose (25-50 μg) should be used. In addition, the starting dose should be lower for the elderly, patients with coronary heart disease and other people, and even if the dose is insufficient, it needs to be increased slowly; while for patients with hypothyroidism after isotope therapy, the starting dose can be slightly higher and the rate of increase should be faster. 5.How should the LT4 dose be adjusted during the treatment? Except for some special cases, for most hypothyroid patients, serum TSH level is the reference scale for LT4 dose adjustment. It is recommended to stabilize the TSH value within the range of 0.5~3.5 mU/L. For senior citizens over 70 years old, the target should be relaxed to 4~6 mU/L. After 4~6 weeks of LT4 treatment, the LT4 dose should be increased or decreased according to the level of TSH. Thereafter, the dose should be measured and adjusted in cycles every 4~6 weeks until the TSH standard is reached, and then the frequency of testing can be reduced to once every 4~6 months, and then gradually reduced to once a year. For patients with hypothyroidism who are old, pregnant or have large changes in weight, the frequency of review should be moderately increased. 6.Do I need to take medication every day? However, for patients with poor compliance, due to the long half-life of the drug, consider taking oral LT4 once a week or half a weekly dose twice a week. 7.Is there any significant difference between different brands of LT4? There is no particular difference between different brands of LT4. However, it is important to avoid switching between brands for frail, pregnant, young children, and thyroid cancer patients, as small changes in dosing may adversely affect this sensitive population. 8.What happens in case of LT4 overdose? Excessive doses of LT4 can cause "medical thyrotoxicosis", which can lead to atrial fibrillation and osteoporosis. It is important to avoid this, especially in the elderly and postmenopausal women, and to ensure that TSH is not less than 0.1 mU/L. 9. Can I take LT4 when my thyroid function is normal? LT4 is not recommended for individuals with normal thyroid function and it has been used for the treatment of obesity, depression, urticaria and other diseases or for the treatment of suspected hypothyroidism, which is a misconception that needs to be corrected. 10.How to adjust LT4 treatment in pregnancy status? It is recommended that hypothyroid patients should control TSH to <2.5mIU/L before starting to conceive. After pregnancy, the drug dose needs to be increased by 25%-30% from the original one, and the easiest way to adjust is to take two extra days of dose per week. During the course of pregnancy, the dose of LT4 should also be adjusted in a timely manner according to the TSH target value at different stages: 0.1~2.5 mIU/L in early pregnancy, 0.2~3.0 mIU/L in mid pregnancy and 0.3~3.0 mIU/L in mid and late pregnancy. 11.How should hypothyroidism in infants and children be treated? Infants and children with a clear diagnosis of hypothyroidism should be treated with LT4 as soon as possible at a starting dose of 10-15 μg/kg per day, and aim to achieve near normal serum thyroxine within 2-4 weeks. The goal of treatment is to maintain thyroxine values in the upper middle of the reference range and TSH values in the lower middle of the reference range. 12. Do children with subclinical hypothyroidism need to take L-T4? Subclinical hypothyroidism refers to a state in which the serum TSH level is higher than the upper limit of normal, while the FT4 level remains normal. If the anti-thyroid antibody is positive and the serum TSH value is higher, the higher the possibility of subclinical hypothyroidism progressing to overt hypothyroidism later. For children with subclinical hypothyroidism, when the TSH level is in the range of 5-10 mIU/L, treatment is generally not recommended; when the TSH is >10 mIU/L and the signs and symptoms are consistent with primary thyroid disease, L-T4 therapy should be given. 13. Has LT4 completely reigned supreme? LT4 has become the best choice for hypothyroidism treatment with many advantages, but there are still a few patients who respond poorly to LT4. Therefore, drugs such as dry thyroid tablets (T4+T3) and T3 have not completely retired from the historical stage, and more clinical studies are needed to scientifically evaluate different treatment options. Compared with other diseases, the treatment of hypothyroidism can indeed be called “simple”, basically realizing the great situation of “one drug for one disease”. However, when we zoom in and dig deeper, we realize that there are still too many problems that deserve attention and attention to be optimized.