levothyroxine and calcium carbonate

Calcium carbonate binds to levothyroxine in the gastrointestinal tract, forming an insoluble complex that reduces levothyroxine absorption. This is a pharmacokinetic interaction at the level of intestinal absorption, likely involving chelation of the hormone by calcium ions.

The reduced absorption of levothyroxine can lead to elevated thyroid-stimulating hormone (TSH) levels and symptoms of hypothyroidism, such as fatigue, weight gain, and cold intolerance. The clinical impact is significant in patients on stable levothyroxine therapy who start or increase calcium carbonate, potentially requiring adjustment of the levothyroxine dose.

Administer levothyroxine and calcium carbonate at least 4 hours apart to avoid the interaction. This can be achieved by taking levothyroxine on an empty stomach and calcium carbonate with meals or at a different time of day. Monitor TSH levels approximately 6 to 8 weeks after any change in the dosing schedule or the introduction or discontinuation of calcium carbonate, and adjust the levothyroxine dose as needed.

This interaction is well established. Multiple pharmacokinetic studies and clinical case reports have demonstrated reduced levothyroxine absorption when co-administered with calcium supplements. It is widely recognised in clinical guidelines and product monographs, and management by dose separation is standard practice.