Dosing regimen. The starting dose and the time needed to reach the full replacement dose are determined by age, weight, and cardiac status. Patients younger than 50 years of age with no prior history of heart disease can reach a full replacement dose as soon as possible. Patients older than 50 years of age should have their cardiac status routinely checked before taking L-T4. Generally start at 25-50 μg/day and increase by 25?g every 1-2 weeks until therapeutic goals are reached. Smaller starting doses and slower dose adjustments are appropriate for those with ischemic heart disease to prevent triggering and exacerbating heart disease. The ideal way to take L-T4 is to take it before meals, and the interval between taking it and some medications should be more than 4 hours, because some medications and foods can affect the absorption and metabolism of T4, such as intestinal malabsorption, aluminum hydroxide, calcium carbonate, anticholinergic amines, aluminum thioglycollate, ferrous sulphate, and dietary fibre additives, which can affect the absorption of L-T4 in the small intestine; phenobarbital, phenytoin sodium, carbamazepine, Phenobarbital, phenytoin sodium, carbamazepine, rifampicin, remifentan, lovastatin, amiodarone, sertraline, chloroquine and other drugs can accelerate the clearance of L-T4. Patients with hypothyroidism need to increase their L-T4 dosage when taking these medications concurrently. Monitoring Indicators. Supplementation of thyroid hormones to re-establish the balance of the hypothalamic-pituitary-thyroid axis generally takes 4-6 weeks, so hormonal indicators are measured at 4-6 week intervals at the beginning of treatment. The L-T4 dose is then adjusted according to the test results until the goal of treatment is reached. After the treatment reaches the target, hormone indicators need to be rechecked every 6-12 months.