THYROID HORMONES

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Safetychecker Summary for Thyroid Hormones
(for details about the summarized interactions, read the full article)

May be Beneficial: Depletion or interference—The medication may deplete or interfere with the absorption or function of the nutrient. Taking these nutrients may help replenish them.	Calcium
Avoid: Reduced drug absorption/bioavailability—Avoid these supplements when taking this medication since the supplement may decrease the absorption and/or activity of the medication in the body.	Soy Calcium
Avoid: Adverse interaction—Avoid these supplements when taking this medication because taking them together may cause undesirable or dangerous results.	Bugleweed* Lemon balm*
Check: Other—Before taking any of these supplements or eating any of these foods with your medication, read this article in full for details.	Iron
Side effect reduction/prevention	None known
Supportive interaction	None known

Interactions with Dietary Supplements

Calcium
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
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.

Soy
Ingestion of soy products simultaneously with thyroid hormones appears to reduce the absorption of the hormones. To be safe, people taking thyroid medication 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

Interactions with Herbs

Bugleweed (Lycopus virginicus, Lycopus europaeus) and lemon balm (Melissa officinalis) may interfere with the action of thyroid hormones and should not be used during treatment with thyroid hormones.¹²

Interactions with Foods and Other Compounds

Food
Taking levothyroxine with food may decrease its absorption.¹³ Levothyroxine absorption is increased when taken on an empty stomach.¹⁴ High-fiber diets have been shown to decrease levothyroxine absorption.¹⁵ Thyroid hormones should be taken an hour before eating, at the same time very day.¹⁶

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. Brinker F. Herb Contraindications and Drug Interactions. Sandy, OR: Eclectic Institute, 1997, 21, 29–30.

13. Benvenga S, Bartolone L, Squadrito S, et al. Delayed intestinal absorption of levothyroxine. Thyroid 1995;5:249–53.

14. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.

15. 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.

16. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.

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The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over-the-counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires December 2003.

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