What is thyroid hormone replacement therapy?

Thyroid hormone replacement therapy is widely used to treat patients who develop different forms of hypothyroidism. Standard thyroid hormone replacement therapy focuses on administering levothyroxine in the form of pills to supplement thyroid hormone levels that may be deficient as a result of the destruction or removal of normal thyroid tissue. Levothyroxine is the major form of thyroid hormone produced by the thyroid gland. The pharmacologic preparation of levothyroxine is identical to the natural form that is produced by the thyroid gland. It is manufactured under a variety of different trade names including Synthroid, Levoxyl, Unithroid, and Levothroid. Levothyroxine is also available as a generic preparation.

How is a starting dose of levothyroxine chosen?

An starting dose of levothyroxine may be chosen based on a patient's body weight. Levothyroxine tablets are manufactured in units called micrograms. Currently available tablets contain doses that range from 25 micrograms to 200 micrograms. Most patients who are treated with thyroid hormone replacement therapy usually require a daily dose of approximately 0.8 micrograms per pound of body weight.

How is thyroid hormone replacement therapy monitored?

Patients who are treated with thyroid hormone replacement therapy are usually monitored with serial measurements of TSH levels. This approach to monitoring is based on idea that the amount of TSH secreted by the pituitary gland provides the most sensitive indication of whether there is an adequate amount of thyroid hormone present in the body. Individual therapy is usually targeted to identify a daily dose of levothyroxine that will supplement any T4 and T3 produced by remaining thyroid tissue to provide an adequate amount of thyroid hormone. Successful treatment is usually indicated by the measurement of TSH levels that fall within the limits of the reference range. The reference range for TSH is usually stated to be 0.5-5.0 mU/ml (microunits per milliliter), though there may be slight variation from laboratory to laboratory.

TSH levels that fall above the upper limit of the reference range may indicate that there is not enough thyroid hormone present in the body. This may signal the need for an increase in the daily dose of levothyroxine. TSH levels that fall below the lower limit of the reference range may indicate that there is an excess amount of thyroid hormone present in the body. This may signal the need for a decrease in the daily dose of levothyroxine. In general, a TSH level should be checked approximately six weeks after an initial dose of levothyroxine has been started. If it is checked any sooner, it may not truly reflect the impact of the daily dose on the production of TSH by the pituitary gland. If a patient is already taking levothyroxine and the daily dose has been increased or decreased, the TSH level should be rechecked after at least four weeks of compliance with the new daily dose.