Drug Interactions:
Thyroid Hormone
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Calcium - Take at a Different Time of Day
Iron - Take at a Different Time of Day
Soy - Possible Harmful Interaction
Carnitine - Supplementation Possibly Helpful
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)
Calcium
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 chelates (combines) with thyroid hormone, thus reducing its absorption.
To prevent this interaction, take thyroid hormone and calcium supplements as far apart as possible.
Iron
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 as far apart as possible from thyroid hormones.
Soy
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 (e.g., soybeans, soy milk, tofu) with caution.
Carnitine
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
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Last reviewed March 2002 by Medical Review Board
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