Summary of Interactions with Vitamins, Herbs, & Foods
|May Be Beneficial: Depletion or interference—This medication may deplete these substances from the body or interfere with how they work; extra intake may help replenish them.||
|May Be Beneficial: Side effect reduction and/or prevention—These substances may help reduce the likelihood and/or severity of a potential side effect caused by the medication.||
|May Be Beneficial: Supportive interaction—These substances may help this medication work better.||
|Avoid: Reduces drug effectiveness—When taking this medication, avoid these substances as they may decrease the medication's absorption and/or activity in the body.||
|Avoid: Adverse interaction—When taking this medication, avoid these substances, as the combination may cause undesirable or dangerous interactions.||
|Check: Explanation needed—When taking this medication, read the article details and discuss them with your doctor or pharmacist before taking these substances.||
Interactions with Vitamins
Thyroid hormones have been reported to increase urinary loss of calcium.1 However, recent research suggests that, under most circumstances, taking thyroid hormones may not be associated with reduced bone density.2 , 3 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.4 For example, 1,200 mg per day of calcium as calcium carbonate, taken along with levothyroxine, significantly reduced absorption of the thyroid hormone.5 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.
Iron deficiency has been reported to impair the body’s ability to make its own thyroid hormones,6 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.7 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.8
However, iron supplements may decrease absorption of thyroid hormone medications.9 , 10 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.
Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.11
The normal stomach produces hydrochloric acid, which influences the absorption of thyroid hormones. Research has shown that taking drugs that inhibit the production of stomach acid reduces the absorption of thyroid hormones.12 Conversely, people with low stomach acid who take betaine hydrochloride for "acid-replacement therapy" would be expected to have an increase in their absorption of thyroid hormones. For that reason, people taking thyroid hormones should not take betaine hydrochloride without the supervision of a doctor, who can determine whether a change in thyroid hormone dose is necessary.
Interactions with Herbs
Interactions with Foods & Other Compounds
Taking levothyroxine with food may decrease its absorption.14 Levothyroxine absorption is increased when taken on an empty stomach.15 High-fiber diets have been shown to decrease levothyroxine absorption.16 Thyroid hormones should be taken an hour before eating, at the same time very day.17
1. Kung AWC, Pun KK. Bone mineral density in premenopausal women receiving long-term physiological doses of levothyroxine. JAMA 1991;265:2688–91.
2. Schneider DL, Barrett-Connor EL, Morton DJ. Thyroid hormone use and bone mineral density in elderly men. Arch Intern Med 1995;155:2005–7.
3. Franklyn JA, Betteridge J, Daykin J, et al. Long-term thyroxine treatment and bone mineral density. Lancet 1992;340:9–13.
4. Schneyer CR. Calcium carbonate and reduction of levothyroxine efficacy. JAMA 1998;279:750.
5. Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA 2000;283:2822–5.
6. Beard JL, Borel MJ, Derr J. Impaired thermoregulation and thyroid function in iron-deficiency anemia. Am J Clin Nutr 1990;52:813–9.
7. Beard JL, Borel MJ, Derr J. Impaired thermoregulation and thyroid function in iron deficiency anemia. Am J Clin Nutr 1990;52:813–9.
8. Beard J, Borel M, Peterson FJ. Changes in iron status during weight loss with very-low-energy diets. Am J Clin Nutr 1997;66:104–10.
9. Campbell NR, Hasinoff BB. Iron supplements: A common cause of drug interactions. Brit J Clin Pharmacol 1991;31:251–5.
10. Campbell NR, Hasinoff BB, Stalts H, et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med 1992;117:1010–3.
11. Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: The influence of soy-based formulas. J Am Coll Nutr 1997;16:280–2.
12. Centanni M, Gargano L, Canettieri G, et al. Thyroxine in goiter, Helicobacter pylori infection, and chronic gastritis. N Engl J Med2006;354:1787–95.
13. Brinker F. Herb Contraindications and Drug Interactions. Sandy, OR: Eclectic Institute, 1997, 21, 29–30.
14. Benvenga S, Bartolone L, Squadrito S, et al. Delayed intestinal absorption of levothyroxine. Thyroid 1995;5:249–53.
15. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.
16. Liel Y, Harman-Boehm I, Shany S. Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients. J Clin Endocrinol Metab 1996;81:857–9.
17. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.
Last Review: 08-17-2011
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