Common Registration Assessment, Part 1
Levothyroxine dose by weight calculations
Three original questions on weight-based levothyroxine starting doses, with the working shown.
Published 14 October 2026. Independent revision material, not GPhC questions, not for patient care.
NICE guideline NG145 says to consider starting levothyroxine at 1.6 micrograms per kilogram of body weight per day, rounded to the nearest 25 micrograms, for adults under 65 with primary hypothyroidism and no history of cardiovascular disease.
For adults aged 65 and over, and adults with a history of cardiovascular disease, the guideline suggests considering a start of 25 to 50 micrograms per day with titration. The weight-based rule is the trap in those questions.
The method
- Check the age and any cardiovascular history first.
- If the weight-based rule applies, multiply the weight in kg by 1.6 micrograms.
- Round to the nearest 25 micrograms by finding the two multiples of 25 either side and picking the closer one.
- If the patient is 65 or over or has cardiovascular disease, use the 25 to 50 micrograms range instead.
- State the answer in micrograms per day.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A 40-year-old woman with primary hypothyroidism and no cardiovascular disease weighs 70 kg. What starting dose is suggested?
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Answer: 100 micrograms daily
Working: 70 × 1.6 = 112 micrograms. The nearest multiples of 25 are 100 and 125. 112 is 12 from 100 and 13 from 125, so the nearest is 100 micrograms.
Question 2
A 52-year-old man with primary hypothyroidism and no cardiovascular disease weighs 82 kg. What starting dose is suggested?
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Answer: 125 micrograms daily
Working: 82 × 1.6 = 131.2 micrograms. The nearest multiples of 25 are 125 and 150. 131.2 is 6.2 from 125 and 18.8 from 150, so the nearest is 125 micrograms.
Question 3
A 72-year-old man with primary hypothyroidism weighs 80 kg. A student works out 1.6 × 80 = 128 micrograms and rounds to 125. What does NICE suggest instead?
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Answer: 25 to 50 micrograms daily, with titration
Working: The weight-based rule is for adults under 65 with no history of cardiovascular disease. At 72 the guideline suggests considering 25 to 50 micrograms a day with titration, so 125 micrograms is not the answer.
Exam approach
Check age and cardiovascular history before doing any weight calculation.
Where marks are lost
- Applying the weight-based rule to a patient aged 65 or over.
- Rounding to the nearest 10 instead of the nearest 25.
- Forgetting to check for cardiovascular history.
- Using pounds or stones without converting to kg.
Frequently asked questions
What starting dose does NG145 suggest by weight?
Consider 1.6 micrograms per kg per day, rounded to the nearest 25 micrograms, for adults under 65 with primary hypothyroidism and no history of cardiovascular disease.
What about older patients?
NG145 says to consider 25 to 50 micrograms a day with titration for adults aged 65 and over and adults with a history of cardiovascular disease.
Sources
- NICE, NG145 thyroid disease: assessment and management
- NICE NG145 recommendations 1.3.6 and 1.3.7 (NCBI Bookshelf copy)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
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See also the formula sheet, the eight sample questions and the approved calculator page.