Is it safe to take Synthroid (levothyroxine) and vitamin D supplements at the same time?

Can You Take Synthroid and Vitamin D Together?

Yes, you can take Synthroid (levothyroxine) and vitamin D supplements together, but they should be separated by at least 4 hours to avoid any potential interaction that could reduce levothyroxine absorption.

Timing Strategy for Optimal Absorption

The key issue is not safety but rather ensuring both medications work effectively:

• Take levothyroxine on an empty stomach, ideally 30-60 minutes before breakfast or at bedtime (at least 3 hours after the last meal), as food and supplements can interfere with its absorption 1
• Take vitamin D with a meal later in the day, as it is a fat-soluble vitamin that absorbs better with dietary fat 2
• Maintain at least a 4-hour separation between levothyroxine and vitamin D (or any other supplements, particularly calcium or iron) 1

Why Separation Matters

While the evidence specifically examining levothyroxine-vitamin D interactions is limited, we know that:

• Levothyroxine absorption can be significantly reduced when taken with various supplements, including calcium and iron 1
• Many vitamin D supplements are combined with calcium, which definitively interferes with levothyroxine absorption 1
• Even standalone supplements may affect levothyroxine efficacy when taken simultaneously 1

Practical Dosing Schedule

Morning dosing option:

• Take levothyroxine immediately upon waking (6-7 AM)
• Eat breakfast 30-60 minutes later
• Take vitamin D with lunch or dinner 2, 1

Bedtime dosing option:

• Take levothyroxine at bedtime (at least 3 hours after dinner)
• Take vitamin D with breakfast or lunch 2, 1

Both morning and bedtime levothyroxine dosing are equally effective when properly timed 1

Vitamin D Dosing Recommendations

For most adults taking levothyroxine who need vitamin D supplementation:

• Standard maintenance dose: 800-2000 IU daily 2, 3
• Higher-risk populations (obesity, dark skin, limited sun exposure, malabsorption): 1500-4000 IU daily 2
• Target serum 25(OH)D level: 30-50 ng/mL 3

Critical Pitfalls to Avoid

• Never take levothyroxine with coffee, even without vitamin D, as coffee significantly reduces levothyroxine absorption 1
• Avoid calcium-containing supplements or antacids within 4 hours of levothyroxine 1
• Do not take iron supplements within 4 hours of levothyroxine 1
• If using enteral nutrition, coordinate timing carefully with your healthcare provider, as continuous feeding can interfere with levothyroxine absorption 1

Monitoring Recommendations

• Check thyroid function (TSH) 6-8 weeks after starting vitamin D supplementation to ensure levothyroxine remains effective 1
• Measure vitamin D levels (25(OH)D) after 3 months of supplementation to confirm adequate dosing 2, 3
• Report any symptoms of hypothyroidism (fatigue, weight gain, cold intolerance) that might suggest reduced levothyroxine effectiveness 1

Novel Levothyroxine Formulations

If maintaining proper timing is difficult, newer levothyroxine formulations (liquid or soft gel capsules) may be less affected by food and supplement interactions than traditional tablets, though maintaining separation remains the safest approach 1