What is thyroid hormone? How does it interact with other drugs?

Calcium

Effect: 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 are taken at the same time.

A prospective cohort study has validated these case reports. Twenty persons with hypothyroidism stabilized on long-term levothyroxine therapy were included in the trial. Participants were given calcium carbonate, 1,200 milligrams (mg) daily of elemental calcium, for three months. During the period the calcium supplement was taken, thyroid hormone levels in the blood declined. However, 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, one should take thyroid hormone and calcium supplements as far apart as possible.

The thyroid is an important gland that produces thyroid hormone, the proper level of which is crucial to health. The inset shows the location of the thyroid gland.

Iron

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

Soy

Effect: Possible Harmful Interaction

Soy formula may interfere with the absorption of thyroid medication in infants. In addition, soy may directly interfere with thyroid function. The result may be a need to increase the infant’s dosage of thyroid medication. However, if one stops giving an infant soy formula, the thyroid dosage may need to be decreased. Of course, all changes related to thyroid treatment should be managed by a physician. Based on these findings, persons with impaired thyroid function should use soy (such as soybeans, soy milk, and tofu) with caution.

Carnitine

Effect: Supplementation Possibly Helpful

Persons 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.

Bibliography

Benvenga, S., et al. “Usefulness of L-carnitine, a Naturally Occurring Peripheral Antagonist of Thyroid Hormone Action, in Iatrogenic Hyperthyroidism.” Journal of Clinical Endocrinology and Metabolism 86 (2001): 3579-3594.

Butner, L. E., et al. “Calcium Carbonate-Induced Hypothyroidism.” Annals of Internal Medicine 132 (2000): 595.

Singh, N., et al. “Effect of Calcium Carbonate on the Absorption of Levothyroxine.” Journal of the American Medical Association 283 (2000): 2822-2825.