Does taking fish oil supplements interfere with levothyroxine absorption or effectiveness?

Fish oil (omega-3) supplements are not known to significantly reduce levothyroxine (T4) absorption, and major prescribing information for levothyroxine does not list omega-3 as an interacting food or supplement that impairs absorption. Levothyroxine absorption is sensitive to several foods and minerals (like soy, high-fiber foods, calcium, iron, and certain medications), but omega-3 fish oil is not among the documented interferents. ^{[1] [2]}

What is known to affect levothyroxine absorption

• Certain foods can bind levothyroxine in the gut and lower absorption, including soybean flour, cottonseed meal, walnuts, and dietary fiber; grapefruit juice may delay absorption. ^[1]
• Administration guidance recommends taking levothyroxine on an empty stomach 30–60 minutes before breakfast and separating it by at least 4 hours from agents known to interfere with absorption (e.g., calcium, iron, antacids, bile acid sequestrants). ^{[2] [3]}

These instructions aim to maintain consistent and adequate absorption; omega-3 supplements are not included in this list. ^{[1] [3]}

Fish oil and thyroid hormones: what the evidence suggests

Human and animal research exploring omega-3 fatty acids and thyroid physiology does not show a direct impairment of levothyroxine absorption. In a randomized trial in adults, omega-3 supplementation modestly increased total T4 without significant changes in T3 or TSH, suggesting no reduction in thyroid hormone availability; this was a short-term finding and not specifically an absorption study, but it does not indicate antagonism. ^[4]
Animal studies indicate “cross-talk” between omega-3 effects on lipid metabolism and thyroid hormone signaling pathways, not a blocking of levothyroxine absorption; these findings suggest metabolic interplay rather than a harmful interaction with T4 therapy. ^[5]

Practical guidance if you take both

• Take levothyroxine correctly: with water, on an empty stomach, 30–60 minutes before breakfast, and separate by at least 4 hours from known binding agents like calcium and iron. ^{[2] [3]}
• Timing with fish oil: while not required, taking fish oil later in the day (with a meal) is a reasonable habit for consistency and to minimize any chance of minor, unstudied effects; this is precautionary rather than based on known risk. ^[2]
• Watch the bigger culprits: calcium supplements (including carbonate, citrate, acetate) can reduce levothyroxine absorption by roughly 20–25% when taken together; separate these by at least 4 hours. ^[6]
• Be consistent: keep your dosing routine stable and check TSH/T4 as recommended by your clinician; if labs shift unexpectedly, review all supplements and timing. ^[3]

Bottom line

Based on current drug-label guidance and available studies, fish oil supplements do not appear to meaningfully interfere with levothyroxine absorption or effectiveness, unlike calcium, iron, high-fiber foods, and certain other agents that are clearly documented to do so. Continuing levothyroxine on an empty stomach and spacing it from known interacting substances remains the key, and fish oil can be taken with a later meal if you prefer consistent routines. ^{[1] [2] [3] [6] [4] [5]}