Express Levothyroxine Doses in Micrograms not Milligrams
This bulletin focuses on an analysis of recent incidents involving levothyroxine: the expression of medication strength and the error-prone process of converting between different units of measure.
Consistently express levothyroxine doses in micrograms (mcg), not milligrams (mg), in all written or computer-generated prescriptions and health records, pharmacy systems, medication administration records, provincial/territorial drug databases, drug information systems, and patient materials.
INTRODUCTION
A seemingly innocuous medication, levothyroxine, continues to be highlighted in medication incidents reported to ISMP Canada. The last ISMP Canada Safety Bulletin shared important learning from the preventable death of an individual on long-term levothyroxine therapy, whose excessive dose over several years may have contributed to her death.1 The emphasis of that bulletin was on establishing standardized processes to ensure that patients taking levothyroxine are regularly monitored and that test results are communicated and acted upon in a timely manner. The current bulletin focuses on a different aspect of this medication, as identified in an analysis of recent incidents: the expression of medication strength and the error-prone process of converting between different units of measure.
Multiple cases of errors and near misses involving levothyroxine dose conversions from milligrams (mg) to micrograms (mcg) and vice versa have been reported to ISMP Canada and in the literature.2 Canadian manufacturer labels express levothyroxine doses in micrograms (mcg) only. However, throughout the medication-use process (e.g., prescribing, dispensing, and administration), levothyroxine doses may be expressed in micrograms (mcg) or in milligrams (mg). As a result, patients and healthcare providers may need to convert doses from milligrams (mg) to micrograms (mcg), or vice versa, to match the prescribed dose to a particular product. Errors in the calculations required to convert between units are contributing to these errors and near misses. A common calculation error occurs when converting between 0.025 mg and 25 mcg, causing in a 10-fold error in dosing. The resultant dose, sometimes 250 mcg rather than 25 mcg, is considered a reasonable dose for some patients and, as such, does not raise a red flag for most practitioners.
FIGURE 1. Example of a levothyroxine strength expressed in micrograms (mcg) on the manufacturer label (far left), compared with dose expressed in milligrams (mg) on a typical pharmacy label.
RECOMMENDATION
It is strongly recommended that levothyroxine doses be expressed consistently in micrograms (mcg), not milligrams (mg), in all written or computer-generated prescriptions and health records, pharmacy systems, medication administration records, provincial/territorial drug databases, drug information systems, and patient materials.2 Using microgram units reduces the need for decimals (which can lead to errors), allows the dose to correspond directly to the manufacturer’s label (avoiding the need for conversion), and will standardize how levothyroxine information is communicated.
The Canadian Medication Incident Reporting and Prevention System (CMIRPS) is a collaborative pan-Canadian program of Health Canada, the Canadian Institute for Health Information (CIHI), the Institute for Safe Medication Practices Canada (ISMP Canada) and Healthcare Excellence Canada (HEC). The goal of CMIRPS is to reduce and prevent harmful medication incidents in Canada.
Funding support provided by Health Canada. The views expressed herein do not necessarily represent the views of Health Canada.
The Healthcare Insurance Reciprocal of Canada (HIROC) provides support for the bulletin and is a member owned expert provider of professional and general liability coverage and risk management support.
The Institute for Safe Medication Practices Canada (ISMP Canada) is an independent national not-for-profit organization committed to the advancement of medication safety in all healthcare settings. ISMP Canada’s mandate includes analyzing medication incidents, making recommendations for the prevention of harmful medication incidents, and facilitating quality improvement initiatives.
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