A 46-year-old woman on stable levothyroxine replacement has a free T4 at the lower limit of normal despite no dose change for two years. She recently started daily oral calcium carbonate for osteopenia. Which advice will correct the problem?

• A Take levothyroxine with the calcium tablet to improve adherence
• B Double the levothyroxine dose while continuing both drugs together
• C Switch to intramuscular levothyroxine permanently
• D Take levothyroxine on an empty stomach and separate it from calcium by at least four hours ✓

Explanation

Levothyroxine absorption is maximal in the fasted state and is reduced by divalent cations such as calcium and iron, which chelate the hormone in the gut lumen. Separating doses by at least four hours restores full absorption without any dose increase. Doubling the dose while co-administering produces erratic levels and risks toxicity on days the patient forgets calcium, so option B is unsafe practice rather than a solution.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.