Although liothyronine (T3) is three to four times more potent than levothyroxine, it is not recommended for routine replacement therapy because of its shorter half-life (24 hours), requiring multiple daily doses, and difficulty in monitoring its adequacy of
TABLE 38–3 Drug effects and thyroid function.
Inhibition of TRH or TSH secretion without induction of Bexarotene, dopamine, bromocriptine, cabergoline, levodopa, corticosteroids, hypothyroidism or hyperthyroidism somatostatin, octreotide, metformin, interleukin-6, heroin
Inhibition of thyroid hormone synthesis or release Iodides (including amiodarone), lithium, aminoglutethimide, thioamides, ethionamide, with the induction of hypothyroidism (or occasionally tyrosine kinase inhibitors (eg, sunitinib, sorafenib, imatinib), HIV protease inhibitors hyperthyroidism)
Alteration of thyroid hormone transport and serum total T3 and T4 levels, but usually no modification of FT4 or TSH
Increased TBG Estrogens, tamoxifen, raloxifene, heroin, methadone, mitotane, 5-fluorouracil, perphenazine
Decreased TBG Androgens, anabolic steroids, glucocorticoids, danazol, L-asparaginase, nicotinic acid
Displacement of T3 and T4 from TBG with transient Salicylates, fenclofenac, mefenamic acid, intravenous furosemide, heparin
Alteration of T4 and T3 metabolism with modified serum T3 and T4 levels but not TSH levels (unless receiving thyroxine replacement therapy)
Increased hepatic metabolism, enhanced degradation Nicardipine, phenytoin, carbamazepine, primidone, phenobarbital, rifampin, rifabutin, of thyroid hormone tyrosine kinase inhibitors (eg, sunitinib, sorafenib, imatinib), sertraline, quetiapine replacement by conventional laboratory tests. T3 should also be avoided in patients with cardiac disease due to significant elevations in peak levels and a greater risk of cardiotoxicity. Using the more expensive thyroxine and liothyronine fixed-dose combination (liotrix) and desiccated thyroid has not been shown to be more effective than T4 administration alone. T3 is best reserved for short-term TSH suppression. Research is ongoing to clarify whether T3 might be more appropriate in patients with a polymorphism in the D2 gene or in those who continue to report fatigue, weight gain, and mental impairment while on T4 alone.
The use of desiccated thyroid rather than synthetic preparations is never justified, since the disadvantages of protein antigenicity, product instability, variable hormone concentrations, and difficulty in laboratory monitoring far outweigh the advantage of lower cost. Significant amounts of T3 found in some thyroid extracts may produce significant elevations in T3 levels and toxicity. Exact equi-effective doses have not been determined. Approximate equivalence of desiccated thyroid 60 mg (1 gr) to 80 to 100 mcg of levothyroxine, and approximately 37.5 mcg of liothyronine has been reported. Any dosage conversions should be re-titrated based on laboratory and clinical response.
The shelf life of synthetic hormone preparations is about 2 years, particularly if they are stored in dark bottles to minimize spontaneous deiodination. The shelf life of desiccated thyroid is not known with certainty, but its potency is better preserved if it is kept dry.