The treatment of hypothyroidism with thyroid hormone-containing medications has entered its second century. The success of treatment for this common disorder stands out among therapies for chronic disease. Due to the slow metabolism and onset of action of thyroxine (T4), it is possible to reproduce physiologic serum levels of thyroid hormone and TSH by once daily oral administration of T4. Modern assays for serum TSH enable precise regulation of T4 replacement so that subtle "undertreatment" or "overtreatment" can be prevented. The goals of therapy depend on age, coexisting diseases, and general health of the patient. The initial T4 dose, its adjustment, and the frequency of follow-up evaluation are also affected by these variables. Adverse effects of overtreatment, such as worsening coronary artery disease or the accelerated loss of bone mineral, can be avoided by following simple management guidelines. Health status changes such as pregnancy, systemic disease, or new medications should prompt a review of the management plan.
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