Mechanism
Levothyroxine absorption is extremely sensitive — taken with food/drink reduces absorption 20-40%. Calcium, iron, and fiber chelate the hormone. Coffee reduces absorption 25-40%. Soy and high-fiber diets may increase the required dose.
Recommendation
Take levothyroxine on an empty stomach with water only. Wait 30-60 min before eating or drinking coffee. Wait 4 hours before taking calcium, iron, or fiber supplements. Take at the same time every day. Consistency in timing and fasting state is critical for stable TSH.