MajorMineral supplements. Calcium×Thyroid hormones
Updated · Methodology
Mechanism
Calcium carbonate and citrate bind levothyroxine in the gut lumen into a complex that is not absorbed. The AEMPS-approved label names calcium among the substances that reduce absorption of the hormone. The effect depends on timing rather than dose: four hours apart, both are absorbed normally.
Symptoms
Hypothyroid symptoms return on a dose that used to work: fatigue, feeling cold, constipation, weight gain, dry skin, hair loss. TSH climbs at the 6–8 week check.
Management
Space the doses at least 4–5 hours apart, as the label states. In practice: levothyroxine on an empty stomach on waking, calcium with lunch or dinner. If calcium was started recently and TSH has risen, fix the timing first and repeat the test after 6–8 weeks before changing the hormone dose.
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.
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Sources
- AEMPS: Ficha técnica Levotiroxina Aristo 100 microgramos comprimidos EFG (2023)– AEMPS, CIMA, nº registro 84484, sección 4.5
- NIH Office of Dietary Supplements: Calcium – Health Professional Fact Sheet (2025)– NIH ODS, Calcium, Interactions with Medications