Dosage of thyroid must be carefully adjusted according to individual requirements and response. The age and general physical condition of the patient and the severity and duration of hypothyroid symptoms determine the initial dosage and the rate at which dosage may be increased to the eventual maintenance dosage. Dosage should be initiated at a lower level in geriatric patients; in patients with long-standing disease, other endocrinopathies, or functional or ECG evidence of cardiovascular disease; and in patients with severe hypothyroidism. Adjustment of thyroid replacement therapy should be determined mainly by the patient's clinical response and confirmed by appropriate laboratory tests. Because some commercially available thyroid preparations may be standardized according to their iodine content rather than the concentrations of levothyroxine and triiodothyronine and the ratio of these hormones, patients stabilized on a particular manufacturer's thyroid preparation should generally not be switched to another manufacturer's preparation unless under the direction and supervision of a physician. Because of differences in the levothyroxine:liothyronine ratio in thyroid preparations, replacement doses of thyroid that result in normalization of serum thyroxine concentrations may result in excessive serum triiodothyronine concentrations.
For the management of mild hypothyroidism in adults, the usual initial dosage of thyroid is 60 mg daily; dosage may be increased by increments of 60 mg daily at intervals of 30 days until the desired response is obtained. For the management of severe hypothyroidism in adults, the usual initial dosage is 15 mg daily; dosage may be increased to 30 mg daily after 2 weeks, and 2 weeks later increased to 60 mg daily. It is recommended that the patient's response be carefully assessed, including the use of appropriate laboratory tests, following administration of this dosage for 1 month, and again after an additional month of therapy at this dosage. If necessary, dosage may then be increased to 120 mg daily for 2 months, and the assessment repeated. If the clinical response is inadequate or if the values of the laboratory tests are low, dosage may be increased to 180 mg daily. Subsequent increases in dosage may be made in increments of 30 or 60 mg daily. The usual adult maintenance dosage of thyroid is 60-180 mg daily; however, dosage may vary in individual patients.
In infants and children, it is essential to achieve rapid and complete thyroid replacement because of the critical importance of thyroid hormone in sustaining growth and maturation. Slightly excessive dosages of thyroid agents were previously recommended for replacement therapy in congenital hypothyroidism, since it was thought that slight underdosage was harmful while slightly excessive dosage was not. However, it is currently recommended that excessive dosage be avoided since minimal brain damage has occurred in children with thyrotoxicosis during infancy and excessive dosage may accelerate bone age and cause craniosynostosis. For additional information on the use of thyroid agents in the treatment of congenital hypothyroidism, . In general, despite the smaller body size, the dosage (on a weight basis) required to sustain a full rate of growth, development, and general thriving is higher in children than in adults. Although levothyroxine sodium is considered the drug of choice for the treatment of congenital hypothyroidism (cretinism), thyroid has been used. For the treatment of congenital hypothyroidism or severe hypothyroidism in children, the dosage regimen of thyroid is the same as for adults with severe hypothyroidism (i.e., initiate therapy with 15 mg daily); however, in infants and childen, increases in dosage should be made at 2-week intervals. The eventual maintenance dosage of thyroid may be higher in growing children than in adults.