L-Thyroxine (L-T4) is very commonly used in clinical practice for replacement therapy of hypothyroidism, but the precautions for the use of L-T4 are less well known. For this reason, we have written an article on the 6 main precautions for the use of levothyroxine. L-T4 is a drug with a narrow therapeutic index, and a 12% reduction in dose can cause fluctuations in thyroid function. Therefore, it is recommended that the same brand of L-T4 be prescribed to the same patient. 2. Note that the dose of L-T4 needs to be increased in these cases (1) Gastrointestinal disorders. (1) Gastrointestinal disorders: gastric acid deficiency (e.g., atrophic gastritis), H. pylori infection, post-jejuno-ileal bypass, small bowel mucosal disease (e.g., Spirulina diarrhea), diabetic diarrhea; (2) Dietary: concomitant eating, high fiber foods, soy sauce, infant soy formula, espresso; (3) These drugs reduce L-T4 absorption: calcium salts (calcium carbonate, calcium citrate, calcium acetate), ferrous sulfate, oral di phosphates, H2 receptor antagonists such as ranitidine, proton pump inhibitors such as omeprazole, bile acid chelators (clofenamide, clofibrate, covalex), aluminum thioglycollate, cation exchange resins, phosphate binding agents (sevelamer, aluminum hydroxide), multivitamins containing ferrous sulfate or calcium carbonate, chromium pyridinecarboxylate, activated carbon, orlistat, ciprofloxacin; (4) these drugs increase cellular (4) these drugs increase the activity of the cellular pigmentase P450 (CYP3A4): rifampicin, carbamazepine, phenytoin sodium, sertraline; (5) drugs that block the conversion of T4 to T3: amiodarone; (6) factors that may block the synthesis of deiodinase: selenium deficiency, biliary cirrhosis; (7) factors that increase the concentration of serum thyroxine-binding globulin: pregnancy, estrogens, tamoxifen, raloxifene, hepatitis, porphyria (7) Factors that increase serum thyroxine binding globulin concentrations: pregnancy, estrogen, tamoxifen, raloxifene, hepatitis, porphyria, opiates, o-p-dichlorophenyl dichloroethane (mitotane), 5-fluorouracil, fenadine 3, note that the dose of L-T4 needs to be reduced in these cases: old age (≥ 65 years), androgens, anabolic steroids, high-dose glucocorticoids, L-asparaginase, nicotinic acid, active acromegaly, nephrotic syndrome, severe systemic disease, liver failure. It can be seen that there are many factors that affect the dose of L-T4 therapy. Before determining the dose of L-T4 therapy for a patient, it is necessary to carefully understand the patient’s past history, weight, medication and dietary habits. The starting dose of L-T4 for primary hypothyroidism and dose adjustment are related to the degree of hypothyroidism, age, gender, weight and comorbidities. (2) Regardless of the degree of hypothyroidism, patients over 50-60 years of age without evidence of coronary artery disease should start treatment with 50 ug/d; those with coronary artery disease should start treatment with 12.5 ug/d. (3) Generally, the first 3 months is the dose adjustment phase, requiring patients to have their thyroid function rechecked every 6 weeks. （The main criterion for the appropriate dose of L-T4 replacement therapy is that the TSH is within the normal range. 5, the time to take the medication has to be taken more than half an hour before breakfast or more than 3 hours after dinner, before bedtime. Recent studies have shown that L-T4 is more effective when taken after dinner. The treatment experience of our hospital proves that taking L-T4 before bedtime can avoid the influence of many factors, and the effect is especially good. Regular check of heart function and bone density Patients with thyroid cancer need to take L-T4 at higher doses to prevent side effects on heart and bones, and to check heart function and bone density regularly.