What interferes with Synthroid (levothyroxine) absorption?

What Interferes with Synthroid (Levothyroxine) Absorption

Many substances can interfere with levothyroxine absorption, including medications, supplements, and food, requiring separation of administration times to maintain optimal thyroid hormone levels.

Medications That Interfere with Levothyroxine

Binding Agents

• Bile Acid Sequestrants:

  • Cholestyramine, colestipol, colesevelam
  • Mechanism: Bind to levothyroxine in the GI tract
  • Recommendation: Take levothyroxine at least 4 hours before these medications 1

• Phosphate Binders:

  • Calcium carbonate, ferrous sulfate, sevelamer, lanthanum
  • Mechanism: Bind to levothyroxine, preventing absorption
  • Recommendation: Administer levothyroxine at least 4 hours apart from these agents 1

• Ion Exchange Resins:

  • Kayexalate
  • Mechanism: Decreases levothyroxine absorption
  • Recommendation: Take levothyroxine at least 4 hours prior to these drugs 1

Gastrointestinal Agents

• Proton Pump Inhibitors:

  • Mechanism: Reduce gastric acidity needed for levothyroxine absorption
  • Evidence: Even 6 weeks of pantoprazole use leads to significant TSH elevation regardless of timing 2

• Antacids:

  • Aluminum & magnesium hydroxides, simethicone
  • Mechanism: Reduce gastric acidity needed for absorption
  • Recommendation: Separate administration times 1

• Sucralfate:

  • Mechanism: Affects intragastric pH and reduces levothyroxine absorption
  • Recommendation: Monitor TSH levels when used concurrently 1

Supplements

• Calcium Supplements:

  • Mechanism: Forms insoluble complex with levothyroxine
  • Evidence: Reduces free T4 and total T4 levels, increases TSH 3
  • Recommendation: Separate administration by at least 4 hours 1

• Iron Supplements:

  • Mechanism: Binds to levothyroxine
  • Recommendation: Separate administration by at least 4 hours 1

• Chromium Picolinate:

  • Mechanism: Decreases thyroxine absorption
  • Recommendation: Separate administration by several hours 4

Other Medications

• Orlistat:

  • Mechanism: Interferes with absorption
  • Recommendation: Monitor thyroid function when used concurrently 1

• Tyrosine Kinase Inhibitors:

  • Mechanism: Unclear, but can cause hypothyroidism
  • Recommendation: Evaluate thyroid function in patients on these medications 5

Food and Beverages

• Food: Taking levothyroxine with food significantly reduces absorption
• Dietary Fiber: Interferes with absorption
• Espresso Coffee: Reduces absorption 6

Medical Conditions Affecting Absorption

• Celiac disease
• Inflammatory bowel disease
• Lactose intolerance
• H. pylori infection
• Atrophic gastritis 6

Best Practices for Levothyroxine Administration

1. Timing: Take levothyroxine on an empty stomach, 30-60 minutes before breakfast

2. Consistency: Take at the same time each day

3. Separation: When taking medications that interfere with levothyroxine:

   • Take levothyroxine first thing in the morning
   • Wait at least 4 hours before taking binding agents, calcium, or iron supplements
   • For antacids and PPIs, maximize separation time when possible

4. Monitoring: Regular TSH monitoring is essential when starting medications known to interfere with levothyroxine

Common Pitfalls to Avoid

• Failing to separate levothyroxine from interfering medications
• Taking levothyroxine with food or coffee
• Not recognizing that certain supplements (calcium, iron, chromium) can significantly impact absorption
• Overlooking the impact of PPIs and antacids on levothyroxine effectiveness
• Not considering alternative formulations (liquid or soft-gel) when malabsorption is an issue

Remember that proper timing of levothyroxine administration relative to other medications and food is crucial for maintaining optimal thyroid hormone levels and preventing unnecessary dose adjustments.