Drug Information

Summary of Interactions with Vitamins, Herbs, & Foods

Types of interactions:BeneficialAdverseCheck

Replenish Depleted Nutrients

  • none

Reduce Side Effects

  • none

Support Medicine

  • none

Reduces Effectiveness

  • none

Potential Negative Interaction

  • none

Explanation Required 

  • Calcium

    Thyroid hormones have been reported to increase urinary loss of calcium. However, recent research suggests that, under most circumstances, taking thyroid hormones may not be associated with reduced bone density. Calcium supplementation for people taking long-term thyroid medication has not yet been proven to be either helpful or necessary.

    Simultaneous ingestion of some calcium formulations with levothyroxine has been reported to reduce the effectiveness of levothyroxine. For example, 1,200 mg per day of calcium as calcium carbonate, taken along with levothyroxine, significantly reduced absorption of the thyroid hormone. Levothyroxine activity will not be blocked if it is taken in the morning and calcium carbonate is taken after lunch and dinner. Separating these medications by at least four hours is recommended.

    The interaction is supported by preliminary, weak, fragmentary, and/or contradictory scientific evidence.
  • Iron

    Iron deficiency has been reported to impair the body’s ability to make its own thyroid hormones, which could increase the need for thyroid medication. In a preliminary trial, iron supplementation given to iron-deficient women with low blood levels of thyroid hormones, partially normalized these levels. Diagnosing iron deficiency requires the help of a doctor. The body’s ability to make its own thyroid hormones is also reduced during low-calorie dieting. Iron supplementation (27 mg per day) was reported in a controlled study to help maintain normal thyroid hormone levels in obese patients despite a very low-calorie diet.

    However, iron supplements may decrease absorption of thyroid hormone medications. People taking thyroid hormone medications should talk with their doctor before taking iron-containing products. If advised to supplement, iron and the drug should not be taken within less than four hours of each other.

    The interaction is supported by preliminary, weak, fragmentary, and/or contradictory scientific evidence.
The Drug-Nutrient Interactions table may not include every possible interaction. Taking medicines with meals, on an empty stomach, or with alcohol may influence their effects. For details, refer to the manufacturers’ package information as these are not covered in this table. If you take medications, always discuss the potential risks and benefits of adding a new supplement with your doctor or pharmacist.

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