Is it Safe to Take Vitamin D with Synthroid?
Yes, it is safe to take vitamin D with Synthroid (levothyroxine), and there are no contraindications or significant drug interactions between these two medications. 1
Safety and Compatibility
Vitamin D supplementation can be safely administered to patients taking levothyroxine for hypothyroidism, as demonstrated in clinical studies where patients with Hashimoto's thyroiditis on stable levothyroxine therapy received vitamin D without adverse effects. 2
In a study of levothyroxine-treated women with Hashimoto's thyroiditis, daily vitamin D supplementation (2000 IU) for 6 months showed no interference with thyroid hormone levels or hypothalamic-pituitary-thyroid axis function. 2
Guidelines for managing congenital nephrotic syndrome explicitly recommend supplementing with levothyroxine for hypothyroidism while simultaneously monitoring and supplementing vitamin D and calcium, indicating these medications are routinely used together. 1
Potential Benefits of Combined Use
Vitamin D supplementation may actually reduce thyroid autoimmunity in levothyroxine-treated women with Hashimoto's thyroiditis, as evidenced by decreased titers of thyroid peroxidase and thyroglobulin antibodies. 2
The reduction in thyroid antibodies was more pronounced for thyroid peroxidase antibodies and correlated with baseline antibody titers, suggesting a potential immunomodulatory benefit. 2
Adequate levothyroxine replacement in Hashimoto's thyroiditis may be an essential factor in maintaining vitamin D at sufficient levels, as free T4 levels positively correlate with vitamin D concentrations. 3
Recommended Vitamin D Dosing
For general supplementation in adults taking levothyroxine, 800-2000 IU daily is recommended to maintain optimal serum 25(OH)D levels of 30-80 ng/mL. 4, 5
For patients with documented vitamin D deficiency, initiate with 50,000 IU weekly for 8 weeks, followed by maintenance therapy of 800-2000 IU daily. 4
Most international authorities consider 2000 IU daily as absolutely safe, with doses up to 10,000 IU per day over several months showing no adverse events in studies. 1
Timing Considerations
While there is no specific requirement to separate vitamin D from levothyroxine, it is generally advisable to take levothyroxine on an empty stomach in the morning, 30-60 minutes before breakfast, for optimal absorption.
Vitamin D can be taken at any time of day, with or without food, making it convenient to take separately from levothyroxine if desired, though this is not medically necessary.
Monitoring Recommendations
Measure baseline 25(OH)D levels before starting supplementation if possible, particularly in patients with autoimmune thyroid disease. 4
Wait at least 3 months after starting vitamin D supplementation before measuring 25(OH)D levels to assess response and adjust dosing if needed. 4, 5
Continue routine monitoring of thyroid function (TSH, free T4) as clinically indicated, typically every 6-12 months once stable on levothyroxine.
Common Pitfalls to Avoid
Do not assume vitamin D supplementation will reduce levothyroxine requirements—thyroid hormone dosing should be based on TSH and free T4 levels, not vitamin D status. 2
Avoid single annual mega-doses of vitamin D (≥500,000 IU), as these have been associated with adverse outcomes including increased falls and fractures. 4
Do not neglect calcium intake—ensure adequate calcium supplementation (1000-1200 mg daily) alongside vitamin D, particularly in patients with thyroid disease who may be at risk for bone loss. 1