Thyroid hormones preparations available

Levothyroxine (L-thyroxine, T4) is the drug of choice for thyroid hormone replacement and suppressive therapy because it is chemically stable, relatively inexpensive, free of antigenicity, and has uniform potency however, any of the commercially available thyroid preparations can be used. Once a particular product is selected, therapeutic interchange is discouraged. 

Failure of the thyroid to produce sufficient thyroid hormone is the most common cause of hypothyroidism and is known as primary hypothyroidism. Secondary hypothyroidism occurs much less often and results from diminished release of TSH from the pituitary. Treatment of hypothyroidism is achieved by the replacement of thyroid hormone, primarily T4. A synthetic preparation of T4 is available, levothyroxine (Synthroid ), which has been a popular choice for hypothyroidism because of its consistent potency and prolonged duration of action. No toxicity occurs when given in physiological replacement doses. Desiccated animal thyroid is also available at a lesser cost. Overdoses cause symptoms of hyperthyroidism and can be used as a guide in clinical management. Hypothyroidism is not cured by the daily intake of thyroid hormone it is a life-long regimen. 

Other drugs that are useful in the rapid treatment of the severely thyrotoxic patient are agents that inhibit the peripheral conversion of thyroxine to triiodothyronine. Dexamethasone (0.5 to 1 mg two to four times daily), and the iodinated radiological contrast agents iopanoic acid (Telepaque, 500 to 1000 mg once daily), and sodium ipodate (Qragrafin, 500 to 1000 mg once daily) are effective in preoperative preparation. Neither iopanoic acid nor sodium ipodate is available in the United States. Cholestyramine has been used in severely toxic patients to bind thyroid hormones in the gut and thus block the enterohepatic circulation of the iodothyronines. 

The synthetic preparations used are the sodium salts of the natural isomers of the thyroid hormones. Levothyroxine sodium (L-T, synthroid, levoxyl, levothroid, unithroid, others) is available in tablets in a variety of doses and as a lyophilized powder for injection. L-T has a narrow therapeutic index, and the FDA has mandated demonstration of bioequivalence for brand and generic preparations by the various producers. Liothyronine sodium (L-Tft is available in tablets (cytomel) and in an injectable form (triostat). A mixture of L-T and L-T is marketed as liotrix (thyroiar). Desiccated thyroid preparations, derived from whole animal thyroids and containing both T and T have highly variable biologic activity and are much less desirable. 

Although radioiodinated antibodies have acceptable stability for in vitro applications, their in vivo stability is less ideal These tracers can act as substrates for the iodotransferase enzymes which are normally responsible for metabo lism of the thyroid hormones thyroxine and tri-iodothyronine which bear structural similarities. Although not present in the vascular circulation, these enzymes are widely distributed in many tissues and their action results in the intracellular release of the radionuclide which then either diffuses or is actively transported out of the cell and subsequently excreted in the urine. In order to try and increase the in vivo stability of antibodies labelled with radioiodine, alternative synthetic strategies have been pursued which result in molecules which are not substrates for the iodotransferases (11. 12). AU of these techniques are more complicated than the electrophilic substitution methods described above and require some synthetic chemistry in order to prepare the precursors which are not commercially available. 

Any of the commercially available thyroid preparations accomplish this goal (Table 73-8) however, levothyroxine (1-thyroxine) is considered to be drug of choice. The thyroid preparations are either natural (i.e., desiccated thyroid and thyroglobulin) or synthetic (levothyroxine, fiothyronine, and fiotrix) in origin. The availability of sensitive and specific assays for total and free hormone levels as well as TSH... 

---