Drug General Information (ID: DDI2FPVBOT)
Drug Name			Levothyroxine		Drug Info		Calcium carbonate		Drug Info
Drug Type		Small molecule				Small molecule
Therapeutic Class		Antithyroid Agents				Antacids/Minerals And Electrolytes
Structure

Mechanism of Levothyroxine-Calcium carbonate Interaction (Severity Level: Moderate)
Complex formation Click to Show/Hide Mechanism Graph
Drug Name		Levothyroxine				Calcium carbonate
Mechanism		Binds to polyvalent cations				Polyvalent cations
Key Mechanism Factor 1
		Factor Name		Chelation
		Factor Description		Chelation is a direct drug interaction that usually involves the formation of dimers or trimers, resulting in larger complexes and poorer absorption.
Mechanism Description		Decreased absorption of Calcium carbonate due to formation of complexes caused by Levothyroxine

Recommended Action
Management		Some experts recommend separating the times of administration of levothyroxine and calcium-containing preparations by at least 4 hours. Monitoring of serum TSH levels is recommended. Patients with gastrointestinal or malabsorption disorders may be at a greater risk of developing clinical or subclinical hypothyroidism due to this interaction.

References
1	Csako G, McGriff NJ, Rotman-Pikielny P, Sarlis NJ, Pucino F "Exaggerated levothyroxine malabsorption due to calcium carbonate supplementation in gastrointestinal disorders." Ann Pharmacother 35 (2001): 1578-83. [PMID: 11793625]
2	Neafsey PJ "Levothyroxine and calcium interaction: timing is everything." Home Healthc Nurse 22 (2004): 338-9. [PMID: 15131426]
3	Schneyer CR "Calcium carbonate and reduction of levothyroxine efficacy." JAMA 279 (1998): 750. [PMID: 9508149]
4	Singh N, Singh PN, Hershman JM "Effect of calcium carbonate on the absorption of levothyroxine." JAMA 283 (2000): 2822-5. [PMID: 10838651]