Magnesium in Kiwi and Levothyroxine Absorption
Yes, the magnesium in kiwi can reduce levothyroxine absorption and should be separated from your thyroid medication by at least 4 hours.
The Core Issue: Magnesium Interferes with Levothyroxine
The FDA-approved prescribing information for Synthroid explicitly lists magnesium hydroxide-containing antacids among substances that decrease levothyroxine absorption, potentially causing hypothyroidism 1. The mechanism is straightforward: divalent cations like magnesium form complexes with levothyroxine in the gastrointestinal tract, binding the medication and preventing its absorption 1.
The Evidence on Magnesium-Levothyroxine Interaction
A 2025 randomized crossover trial directly tested this interaction in healthy adults 2. When magnesium aspartate was co-administered with levothyroxine:
- Thyroxine absorption decreased by 12% (statistically significant)
- Peak levels (Cmax) dropped by 7%
- Time to peak (Tmax) increased by 17%
Magnesium citrate showed a smaller but still concerning 7% reduction in absorption 2.
An earlier 1999 study documented two real-world cases where patients on stable levothyroxine doses developed elevated TSH and low thyroxine levels after starting magnesium-containing antacids. Their thyroid function normalized only after discontinuing the magnesium products 3.
Kiwi's Magnesium Content
Kiwifruit contains notable amounts of magnesium and is recognized as exceeding most other fruits in key micronutrient content, including magnesium 4. While the absolute magnesium content in a single kiwi is lower than a magnesium supplement, the interaction principle remains the same.
Clinical Recommendations
Take levothyroxine at least 4 hours apart from kiwi or any magnesium-containing foods or supplements 1. This is the FDA-mandated separation time for substances known to interfere with levothyroxine absorption.
Practical Implementation:
- Take levothyroxine on an empty stomach, 30-60 minutes before breakfast 1
- Consume kiwi at lunch, dinner, or as an afternoon snack—well separated from your morning dose
- If you take magnesium supplements, separate them by at least 4 hours from levothyroxine
Important Caveats:
This interaction is particularly critical for patients who:
- Require tight TSH control (thyroid cancer patients on suppressive therapy)
- Are elderly or have cardiac disease (where even small fluctuations matter)
- Have difficulty achieving target TSH levels despite dose adjustments
The interaction may be less pronounced with liquid or softgel levothyroxine formulations compared to tablets 2, 5, though separation is still recommended.
Monitor Your Thyroid Function
If you've been regularly consuming kiwi close to your levothyroxine dose, you may be chronically under-replaced. Watch for:
- Rising TSH levels on routine monitoring
- Return of hypothyroid symptoms (fatigue, weight gain, cold intolerance)
- Need for unexplained dose increases
After separating kiwi from your levothyroxine, recheck TSH in 6-8 weeks to ensure you're not becoming over-replaced 1.
The Broader Context
Magnesium joins a list of divalent cations that interfere with levothyroxine, including calcium and iron 1, 6. The 4-hour separation rule applies to all of these. Coffee, tea, and soy products also reduce absorption through different mechanisms 6, 7.