Does Fish Oil Interact with Thyroid Medications?
Fish oil does not interact with levothyroxine absorption or thyroid hormone replacement therapy, and can be taken together without timing separation or dose adjustment. 1
Evidence for No Pharmacokinetic Interaction
- A comprehensive systematic review of 107 articles examining substances that interfere with levothyroxine bioavailability did not identify fish oil or omega-3 fatty acids as interfering substances 1
- The review specifically catalogued medications and foods that impair levothyroxine absorption through mechanisms including direct complexing, alkalization, alteration of serum thyroxine-binding globulin levels, and acceleration of levothyroxine catabolism—none of which apply to fish oil 1
- Fish oil is rapidly incorporated into cell membrane phospholipids after oral intake regardless of food timing, indicating it does not chelate or bind other substances in the gastrointestinal tract 2
Potential Complementary Benefits in Thyroid Disease
While there is no interaction to avoid, fish oil may actually provide cardiovascular benefits for patients with thyroid disorders:
- For hypothyroid patients with cardiac manifestations: Hypothyroidism increases systemic vascular resistance by up to 50%, causes bradycardia, and promotes myocardial fibrosis 3
- Fish oil at 2-4 grams/day of EPA+DHA reduces cardiovascular risk factors including triglycerides (by 25-30%), improves endothelial function, and reduces inflammatory biomarkers 4, 2
- In experimental models of thyrotoxicosis, fish oil attenuated cardiac oxidative stress and fibrosis by normalizing antioxidant enzyme activities 5
Practical Administration Guidance
- No timing separation required: Fish oil and levothyroxine can be taken at the same time without concern for absorption interference 1
- Standard levothyroxine precautions still apply: Continue taking levothyroxine on an empty stomach (30-60 minutes before breakfast) to avoid interactions with calcium, iron, proton pump inhibitors, and other documented interfering substances 1
- Fish oil timing for tolerability: Take fish oil with meals to minimize gastrointestinal side effects (fishy aftertaste, belching), not for absorption reasons 2
Monitoring Considerations
- No additional thyroid monitoring needed: Fish oil supplementation does not require more frequent TSH monitoring beyond the standard 6-8 week intervals after dose changes 6
- If using therapeutic doses (≥2 grams/day): Monitor lipid panels and evaluate for atrial fibrillation risk, as high-dose omega-3 supplementation (>1g/day) has been associated with a 25% increase in atrial fibrillation risk 2
- For hypothyroid patients with cardiac disease: The American College of Cardiology recommends measuring TSH in all newly diagnosed hypertensive patients, as treatment with levothyroxine can normalize blood pressure in hypothyroid-induced hypertension 3
Common Pitfall to Avoid
- Do not unnecessarily separate fish oil from levothyroxine: Unlike calcium, iron, or proton pump inhibitors, fish oil does not impair levothyroxine absorption and does not require the typical 4-hour separation recommended for chelating substances 1