Thyroid Hormone
  • Calcium —Take at a Different Time of Day
  • Iron —Take at a Different Time of Day
  • Soy —Possible Harmful Interaction
  • Carnitine —Supplementation Possibly Helpful
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Thyroid hormone supplements are primarily used to treat hypothyroidism , a condition caused by deficient secretion of thyroid hormone by the thyroid gland. Forms of thyroid hormone include:
  • Dextrothyroxine (Choloxin)
  • Levothyroxine (Levoid, Levothroid, Levoxine, Levoxyl, Synthroid)
  • Liothyronine (Cytomel, Triostat)
  • Liotrix (Euthroid, Thyrolar)
  • Thyroglobulin (Proloid)
  • Thyroid (Armour Thyroid)

Take at a Different Time of Day
Two case reports suggest that calcium carbonate interferes with the body's absorption of thyroid hormone when both were taken at the same time. 1,2

A prospective cohort study has validated these case reports. 3 Twenty individuals with hypothyroidism stabilized on long-term levothyroxine therapy were included in the trial. Participants were given calcium carbonate (1,200 mg daily of elemental calcium) for 3 months. During the period the calcium supplement was taken, thyroid hormone blood levels declined. But after calcium supplementation was stopped, thyroid levels climbed back up, slightly surpassing the levels measured at the beginning of the study.

It is thought that calcium combines with thyroid hormone, thus reducing its absorption.

To prevent this interaction, take thyroid hormone and calcium supplements as far apart as possible.

Take at a Different Time of Day
Iron salts (including ferrous fumarate, ferrous gluconate, ferrous sulfate, and iron polysaccharide) may impair the effect of the thyroid hormone levothyroxine, probably by forming a complex with it and decreasing its absorption. 4

To prevent a problem, take iron supplements and thyroid hormones as far apart as possible.

Possible Harmful Interaction
Soy formula may interfere with the absorption of thyroid medication in infants. 5 In addition, soy may directly interfere with thyroid function. 6,7 The result may be a need to increase the infant's dosage of thyroid medication. However, if you stop giving an infant soy formula, the thyroid dosage may need to be decreased. Of course, all changes relating to thyroid treatment should be managed by a physician.

Based on these findings, individuals with impaired thyroid function should use soy (such as, soybeans, soy milk, tofu) with caution.

Supplementation Possibly Helpful
Individuals with an enlarged thyroid gland are sometimes given high doses of thyroid medication to shrink it. However, this treatment can cause unpleasant side effects, including bone loss, heart palpitations, and a feeling of malaise. A double-blind trial suggests that the supplement L-carnitine may safely reduce the adverse effects of this treatment. 8




Butner LE, Fulco PP, Feldman G. Calcium carbonate-induced hypothyroidism. Ann Intern Med. 2000;132:595. Schneyer CR. Calcium carbonate and reduction of levothyroxine efficacy [letter]. JAMA. 1998;279:750. Singh N, Singh PN, Hershman JM, et al. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283:2822-2825. Campbell NR, Hasinoff BB. Iron supplements: a common cause of drug interactions. Br J Clin Pharmacol . 1991;31:251-255. Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: the influence of soy-based formula. J Am Coll Nutr. 1997;16:280-282. Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action. Biochem Pharmacol. 1997;54:1087-1096. Chorazy PA, Himelhoch S, Hopwood NJ, et al. Persistent hypothyroidism in an infant receiving a soy formula: case report and review of the literature. Pediatrics. 1995;96(1 Pt 1):148-150. Benvenga S, Ruggeri RM, Russo A, et al. Usefulness of l-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial. J Clin Endocrinol Metab. 2001;86:3579-3594.
Last Reviewed August 2013



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