Calcium blocks thyroid medication absorption
Calcium binds to levothyroxine (Synthroid) in the gut, reducing absorption by up to 60%. This can lead to subtherapeutic thyroid levels and hypothyroid symptoms.
Take thyroid medication at least 4 hours before or after calcium supplements.
Separate by 4+ hours; thyroid med first thing in morning
Calcium blocks thyroid medication absorption. Take thyroid medication at least 4 hours before or after calcium supplements.
Separate by 4+ hours; thyroid med first thing in morning
This interaction is rated “Warning” by Formulate. Calcium binds to levothyroxine (Synthroid) in the gut, reducing absorption by up to 60%. This can lead to subtherapeutic thyroid levels and hypothyroid symptoms.
Studies from our registry that mention calcium or thyroid medication. Each links to the primary source and the other Formulate pages citing it.
Mentions thyroid medication · 2026
Memorial Sloan Kettering Integrative Medicine Monograph: Ashwagandha
Primary literature · Monograph
Documents thyrotoxicosis case reports associated with ashwagandha use. States ashwagandha may increase thyroxine levels. Also notes CYP3A4 induction (moderate) and sedative-additive effects. Does not give a hard avoid-with-levothyroxine warning but flags thyroid monitoring.
Mentions calcium · 2015
Calcium intake and risk of fracture: systematic review
Bolland et al. · BMJ
Systematic review finding little evidence that increasing calcium intake prevents fractures in community-dwelling adults.
Mentions calcium · 2010
Iron bioavailability and dietary reference values
Hurrell R, Egli I · American Journal of Clinical Nutrition
Reviews enhancers (vitamin C, meat) and inhibitors (phytate, polyphenols, calcium) of non-heme iron absorption, providing the mechanistic basis for pairing iron with vitamin C and separating it from tea/coffee.
Mentions calcium · 2000
Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability
Hallberg and Hulthén · American Journal of Clinical Nutrition
Foundational model for non-heme iron absorption factoring vitamin C, phytates, tannins, and calcium interactions.
Other interactions to know
Calcium significantly reduces iron absorption
Ashwagandha can raise thyroid hormone levels and stack additively with levothyroxine, risking thyrotoxicosis.
Iron reduces levothyroxine absorption
Iodine supplementation can destabilize thyroid
High calcium can impair magnesium absorption
Calcium may reduce zinc absorption
Magnesium may reduce thyroid med absorption
Soy may interfere with thyroid medication
Check your full stack in the free interaction checker.
The free checker lets you add any combination of supplements and medications at once. The free web app goes further — every stack change surfaces interaction alerts automatically.
Medical disclaimer. This page is for educational purposes only and does not replace advice from a qualified healthcare provider.
Last medically reviewed: