DRUGS USED IN THE TREATMENT OF HYPOTHYROIDISM
Levothyroxine sodium (Levothroid, Synthroid, Levoxine) is the sodium salt of the naturally occurring levorota-tory isomer of T4. It is the preparation of choice for maintenance of plasma T4 and T3 concentrations for thyroid hormone replacement therapy in hypothyroid patients. It is absorbed intact from the gastrointestinal tract, and its long half-life allows for convenient once-daily administration. Since much of the T4 is deiodi-nated to T3, it is usually unnecessary to use more ex-pensive preparations containing both T4 and T3. The aim is to establish euthyroidism with measured serum con-centrations of T4, T3, and TSH within the normal range.
The TSH-suppressive effects of exogenous T4 also prove useful in removing the stimulatory effects of TSH on the thyroid gland in the management of simple nonendemic goiter, chronic thyroiditis, and TSH-dependent thyroid carcinoma.
Liothyronine sodium (Cytomel) is the sodium salt of the naturally occurring levorotatory isomer of T3. Liothy-ronine is generally not used for maintenance thyroid hormone replacement therapy because of its short plasma half-life and duration of action. The use of T3 alone is recommended only in special situations, such as in the initial therapy of myxedema and myxedema coma and the short-term suppression of TSH in patients un-dergoing surgery for thyroid cancer. The use of T3 alone may also be useful in patients with the rare condition of 5 -deiodinase deficiency who cannot convert T4 to T3.
Liotrix (Euthroid, Thyrolar) is a 4:1 mixture of levothy-roxine sodium and liothyronine sodium. Like levothy-roxine, liotrix is used for thyroid hormone replacement therapy in hypothyroid patients. Although the idea of combining T4 and T3 in replacement therapy so as to mimic the normal ratio secreted by the thyroid gland is not new, it does not appear that liotrix offers any thera-peutic advantage over levothyroxine alone.
Thyroid USP (Thyrar, Thyroid Strong, S-P-T) is derived from dried and defatted thyroid glands of domestic animals (bovine, ovine, or porcine), while Tg (Proloid) is a partially purified extract of frozen porcine thyroid glands. Although used extensively in the past, these preparations are rarely used today.
The total thyroid hormone content of thyroid glands and the ratio of T3 to T4 vary somewhat from one species to another. Thyroid USP preparations are therefore standardized on the basis of their iodine con-tent. Much of the iodine in these preparations is in the metabolically inactive form of iodotyrosines. Thus, a given preparation may satisfy the USP iodine assay re-quirements and yet contain low amounts of T4 and T3. Thyrar (a beef extract) and Armour Thyroid tablets (a pork extract) are evaluated by additional biological as-says to ensure consistent potency from one batch to an-other.
The production of Proloid, which is a partially pu-rified frozen porcine Tg preparation, is an attempt to avoid the variability in desiccated thyroid prepara-tions. It is also assayed and standardized for biological potency. Thyroglobulin is slightly more expensive and offers no particular therapeutic advantage over Thyroid USP. These two preparations have a higher ra-tio of T3 to T4 than that found in human thyroid secre-tion, so supraphysiological levels of T3 may occur in the immediate postabsorptive period because of the rapid release of T3 from ingested Tg, its immediate ab-sorption, and the relatively long period (1 day) required for T3 to equilibrate in its volume of distribution.