Magnesium Glycinate and T4 Levels
Magnesium glycinate can reduce your thyroxine (T4) absorption and potentially lower your T4 levels if taken together with levothyroxine, so you should separate them by at least 4 hours. 1
The Evidence on Magnesium-Levothyroxine Interaction
The FDA drug label for levothyroxine explicitly lists magnesium hydroxides in antacids as substances that may decrease T4 absorption, potentially resulting in hypothyroidism 1. The mechanism involves binding and delaying or preventing levothyroxine absorption, similar to calcium carbonate and ferrous sulfate.
Recent Clinical Data
A 2025 randomized crossover trial specifically examined magnesium-levothyroxine interactions 2. This study found:
- Magnesium aspartate significantly reduced thyroxine AUC by 12% (p=0.002) and decreased Cmax by 7%
- Magnesium citrate reduced thyroxine AUC by 7% (though not statistically significant, p=0.076)
- The effects were smaller than other divalent cations, possibly due to liquid formulation used in the study
While magnesium glycinate wasn't specifically tested, all magnesium salts contain the divalent magnesium cation that can chelate with levothyroxine, reducing absorption.
Practical Management
Take your levothyroxine and magnesium glycinate at least 4 hours apart 1. The standard recommendation from guidelines is:
- Take levothyroxine on an empty stomach, 30-60 minutes before breakfast 3
- Take magnesium glycinate at a different time of day (e.g., with lunch, dinner, or bedtime if levothyroxine is taken in morning)
When Separation Matters Most
This separation is particularly critical if 3:
- Your TSH needs to be maintained within a narrow range
- You're already on high doses of levothyroxine (>200 µg/day)
- You have difficulty achieving target TSH levels
Monitoring Recommendations
If you start magnesium supplementation while on levothyroxine 3:
- Check TSH and free T4 within 4-8 weeks
- Continue monitoring every 4-8 weeks until stable on the same levothyroxine dose
- Adjust levothyroxine dose if TSH becomes elevated
Important Caveats
The interaction affects absorption, not your body's T4 production. If you're not taking levothyroxine medication, magnesium supplementation alone does not lower your natural T4 levels. In fact, some research suggests magnesium supplementation may support thyroid function in certain contexts 4, 5, though this doesn't apply to the drug interaction scenario.
The bottom line: Magnesium glycinate won't directly lower your T4, but it can interfere with levothyroxine absorption if taken together, leading to inadequate thyroid hormone replacement and potentially elevated TSH.