A physician's prescription reads Levothyroxine 150 mcg daily orally. The drug is available as 0.1 mg tablet. How many tablets are required daily?

Appeared in: SCTIMST Staff Nurse - 2015 (Set-B)

Explanation

The goal is to calculate the number of tablets needed to deliver a 150 mcg dose of Levothyroxine.
The available tablets have a strength of 0.1 mg. This must be converted to micrograms to match the prescribed dose's unit.
Since 1 mg equals 1000 mcg, the available tablet strength is 0.1 mg * 1000 = 100 mcg.
The calculation is the desired dose (150 mcg) divided by the available dose per tablet (100 mcg), which equals 1.5 tablets.

Option A: Administering 2 tablets would result in a dose of 200 mcg (2 x 100 mcg), which is a 50 mcg overdose.
Option C: Administering 1 tablet would provide only 100 mcg, which is a 50 mcg underdose.
Option D: Administering 3 tablets would result in a dose of 300 mcg (3 x 100 mcg), which is double the prescribed dose and a significant overdose.

Clinical Relevance

Nursing practice connection: Knowing Dosage calculation and unit conversion (milligrams to micrograms) helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Accurate dosage calculation is a fundamental nursing responsibility to prevent medication errors, which can lead to patient harm from either underdosing (ineffective treatment) or overdosing (toxicity).
A common medication error with Levothyroxine is confusing milligrams (mg) and micrograms (mcg). Doses are typically in mcg, and a dose written in mg should be questioned as it could represent a 1000-fold overdose.
What if the available tablet was 50 mcg? In that case, the nurse would need to administer 3 tablets to achieve the prescribed 150 mcg dose (150 mcg / 50 mcg = 3).

How to Approach the Question

First, identify the question as a dosage calculation problem.
Identify the key pieces of information: the 'Desired' dose (150 mcg) and the 'Available' or 'On Hand' dose (0.1 mg).
Notice that the units are different (mcg vs. mg). This is a critical clue that a unit conversion is required before you can calculate.
Recall the conversion factor: 1 mg = 1000 mcg.
Convert the available dose to the same unit as the desired dose: 0.1 mg becomes 100 mcg.
Apply the standard formula: Desired ÷ Available = 150 mcg ÷ 100 mcg = 1.5 tablets.

Concept Tested & Keywords

Concept Tested: Dosage calculation and unit conversion (milligrams to micrograms).
Stem keywords: Levothyroxine, 150 mcg, 0.1 mg, dosage calculation
Lead-in keywords: How many tablets
Negative lead-in flag: false

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E6 Pharmacology Nursing Lilley 11e Part 2 p. 187-189

Practise the full SCTIMST Staff Nurse - 2015 (Set-B)

Attempt every question from this paper in a timed mock, then review the full solution for each one.