MPA | Pharmacy News

Priya Verma, Pharm.D., PGY-1 pharmacy resident, Sparrow Health System,
Lansing, Mich.

Levothyroxine (Synthroid®) is a synthetic form of thyroxine, which is an endogenous hormone secreted by the thyroid gland. Thyroxine (T4) is converted to the active thyroid hormone, L-triiodothyronine (T3). The thyroid hormones exert the metabolic effects involving normal metabolism, promoting gluconeogenesis and stimulating protein synthesis.1 The 2014 American Thyroid Association (ATA) Guidelines recommend initiating levothyroxine as the first line treatment for primary hypothyroidism.2 Levothyroxine is available in both an oral and intravenous (IV) formulation. For the treatment of hypothyroidism, it is recommended to initiate oral levothyroxine, whereas the IV formulation is often reserved for patients who are temporarily unable to receive oral therapy. Historically, when converting from a parenteral to enteral route, the dose conversion was 1:2. The dose conversion has been updated in the ATA Guidelines, which recommend a dose conversion of 0.75:1 when converting from a parenteral to enteral route. This updated dose conversion is based on recent literature which has shown that the bioavailability of levothyroxine is estimated to be around 79-81 percent.3 Based on this data, the ATA Guidelines and Lexicomp recommend administering an IV dose that is 75 percent of the oral dose when converting from a parenteral to enteral route. At Sparrow, we have updated the parenteral to enteral drug conversion policy to include the new levothyroxine conversion to match the updated guidelines.
 
References

1. Levothyroxine. Lexi-Drugs. Lexicomp. Wolters Kluwer Health, Inc. Riverwoods, IL. Available at: http://online.lexi.com. Accessed February 7, 2019.
2. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid 2014; 24: 1670-751.
3. Dickerson RN, Maish GO, Minard G, et al. Clinical Relevancy of the Levothyroxine-Continuous Enteral Nutrition Interaction. Nutr Clin Pract 2010; 25: 646-52.

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