Can You Take Magnesium Glycinate 3 Hours After Synthroid?
No, three hours is not sufficient separation—you should wait at least 4 hours between taking levothyroxine (Synthroid) and magnesium glycinate to avoid reduced thyroid hormone absorption.
The Evidence-Based Reasoning
The FDA-approved Synthroid label explicitly states that "levothyroxine sodium tablets should not be administered within 4 hours of these agents" when referring to substances like iron, calcium supplements, and antacids that can decrease levothyroxine absorption 1. While the label doesn't specifically list magnesium glycinate, it clearly establishes the 4-hour separation rule for divalent cations that interfere with absorption.
The 2012 AACE/ATA clinical practice guidelines reinforce this, stating that "L-thyroxine should not be taken with substances or medications that interfere with its absorption" 2. The guidelines emphasize taking levothyroxine consistently 30-60 minutes before breakfast or at bedtime 4 hours after the last meal 2.
Recent Research Confirms the Interaction
A 2025 randomized crossover trial (the ThyroMag Trial) provides the most direct evidence: magnesium aspartate significantly reduced levothyroxine absorption by 12%, and magnesium citrate reduced it by 7% 3. The study concluded that "hypothyroid patients should take levothyroxine separated from magnesium-containing formulations, especially if TSH levels are desired to be within a narrow range."
Additional research from 1999 demonstrated that aluminum hydroxide and magnesium oxide caused marked increases in TSH and low serum thyroxine in hypothyroid patients, with in vitro studies showing dose-related increased adsorption of levothyroxine by magnesium-containing compounds 4.
Practical Recommendations
Optimal Timing Strategy:
- Take Synthroid first thing in the morning on an empty stomach, 30-60 minutes before breakfast
- Take magnesium glycinate at least 4 hours later (e.g., lunchtime or later in the day)
- Alternatively, take Synthroid at bedtime (4 hours after your last meal) and magnesium in the morning
Why This Matters:
The clinical consequences of inadequate separation can be significant. Even a 7-12% reduction in levothyroxine absorption may lead to:
- Elevated TSH levels
- Persistent hypothyroid symptoms despite "adequate" dosing
- Need for unnecessary dose increases
- Difficulty achieving therapeutic goals
Common Pitfall to Avoid:
Many patients don't realize that magnesium supplements (including glycinate, citrate, oxide, and aspartate forms) are divalent cations that bind to levothyroxine in the gastrointestinal tract, forming complexes that reduce absorption. The 4-hour rule applies to all magnesium formulations, though liquid levothyroxine formulations may be less affected 5, 6.
If You've Already Taken Them Together:
Don't panic—occasional lapses won't cause immediate harm. However, if you consistently take them too close together, your TSH should be rechecked in 4-8 weeks to ensure adequate thyroid hormone levels 1.