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Levothyroxine-calcium interaction — GPhC CRA MCQ

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EasyEndocrine TherapeuticsLevothyroxine-calcium interactionGPhC CRA

A 60-year-old woman with hypothyroidism takes levothyroxine 100 micrograms daily. She has recently started calcium carbonate 1.25g twice daily for osteoporosis. She reports feeling more tired and gaining weight over the past month. What is the most likely explanation?

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Correct answer: A — Calcium carbonate is reducing the absorption of levothyroxine

Calcium salts significantly reduce the gastrointestinal absorption of levothyroxine by forming insoluble complexes. This can lead to a rise in TSH and return of hypothyroid symptoms. The BNF recommends separating levothyroxine from calcium supplements by at least 4 hours. This temporal separation is essential to ensure adequate levothyroxine absorption. TSH should be rechecked after implementing the separation.

Reference: BNF Levothyroxine; Drug Interactions